Chapter IV: Environmental ConsequencesIntroductionThe National Environmental Policy Act requires that environmental documents disclose the environmental impacts of a proposed federal action, reasonable alternatives to that action, and any adverse environmental effects that cannot be avoided should the proposed action be implemented. This chapter of the Cascades Diversion Dam Removal Project Environmental Assessment analyzes the environmental impacts of the three project alternatives on natural resources, cultural resources, and social resources. This analysis provides the basis for comparing the beneficial and adverse effects of the alternatives. Following this introduction, the chapter presents the methodologies used in the environmental impact analysis. The impact analyses sections are organized by alternative. The first section analyzes Alternative 1 (the No Action Alternative), including impacts on natural resources, cultural resources, and social resources, and presents cumulative impacts and impact conclusions. The same framework is applied to Alternatives 2 and 3 in subsequent sections. Environmental impacts are summarized in table II-2: Summary of Environmental Consequences, located at the end of Chapter II, Alternatives. Cumulative ImpactsA cumulative impact is described in regulations developed by the Council on Environmental Quality, Regulation 1508.7, as follows: A “cumulative impact” is the impact on the environment which results from the incremental impact of the action when added to other past, present, and reasonably foreseeable future actions regardless of what agency (Federal or non-Federal) or person undertakes such other actions. Cumulative impacts can result from individually minor but collectively significant actions taking place over a period of time. To determine potential cumulative impacts, projects within
the Cascades Diversion Dam region were identified, including projects within The impacts of these cumulative actions are evaluated in conjunction with the impacts of each alternative to determine any additive effects on a particular natural, cultural, or social resource. Because most of these cumulative actions are in the early planning stages, the evaluation of cumulative impacts was based on a general description of the project. Context, Duration, Intensity, and Type of ImpactContextThe context of the impact considers whether the impact would be local or regional. For the purposes of this analysis, local impacts would generally be those that occur within the immediate vicinity of Cascades Diversion Dam and downstream to the Cascades Picnic Area. DurationThe duration of the impact considers whether the impact would occur in the short term or the long term. Short-term impacts are temporary, transitional, or removal-related impacts associated with project activities. Long-term impacts are typically those effects that would last 10 years or more or would be permanent. IntensityThe intensity of the impact considers whether the effect would be negligible, minor, moderate, or major. Negligible impacts would not be detectable and would have no discernible effect. Minor impacts would be slightly detectable, but would not be expected to have an overall effect. Moderate impacts would be clearly detectable and could have an appreciable effect. Major impacts would have a substantial, highly noticeable effect. Type of ImpactImpacts were evaluated in terms of whether they would be beneficial or adverse. Beneficial impacts would improve resource conditions. Adverse impacts would deplete or negatively alter resources. ImpairmentPursuant to the 1916 Organic Act, the National Park Service has a management responsibility “to conserve the scenery and the natural and historic objects and the wildlife therein and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of future generations.” As a result, the National Park Service cannot take an action that would “impair” park resources. National Park Service Management Policies 2001 provide guidance on addressing impairment. Impairment is an impact that, in the professional judgement of the responsible National Park Service manager, would harm the integrity of park resources or values, including the opportunities that otherwise would be present for the enjoyment of those resources or values. Impairment of park resources and values was evaluated on the basis of duration and intensity of impacts. Director’s Order #12 requires that impairment be addressed in all environmental assessments and draft and final environmental impact statements, as well as in the decision documents (Finding of No Significant Impact, Record of Decision). In this environmental assessment, impairment is addressed in the conclusion section of each impact topic under each alternative. MethodologiesThis section presents the methodologies and assumptions used to conduct the environmental impact analysis for each resource topic. Natural ResourcesGeology, Geologic Hazards, and SoilsThis impact assessment focuses on effects that geologic
processes in Several assumptions regarding facility placement, geologic design parameters, and public safety were integrated into this assessment, as summarized below. §
It is not possible to avoid risks due to
geologic processes such as earthquakes and rockfalls. Considering this, some
facilities located within the park, including those in the § In emergency situations, the National Park Service may mechanically trigger a rockfall, but in most cases the National Park Service will allow natural processes to occur unimpeded. § The National Park Service is currently revising its management policies pertaining to geologic resources and hazards. The focus of these guidelines will be to protect visitors, employees, and infrastructure from geologic hazards and to locate facilities out of geologically hazardous areas. § In the event of a rockfall, the National Park Service would close the affected area to protect visitor and employee safety. Rocks on roads would be removed, but rockfall talus in rivers would not be removed, unless the river is dammed and flooding threatens utilities or facilities. § Geologic risks to public safety are rarely predictable, and the extent of potential harm to people and property cannot be quantified. The analysis of effects was qualitative, and professional judgment was used to reach reasonable conclusions as to the context, intensity, and duration of potential impacts. § Project activities would remove and/or cover the soil surface and result in significant changes to the basic soil properties of the topsoil. Excavation and removal of soil would result in a long-term impact because the basic soil properties, which have taken thousands of years to develop, would be altered. Capping the surface would reduce water movement and minimize the opportunity for the normal processes of physical transport and chemical transformations, such as illuviation, eluviation, and nutrient cycling. § Soil excavation and redistribution would result in removal or mixing of the soil profile and disrupt soil structural characteristics, interrupting the chemical, physical, and biological processes that naturally occur in the soil. The level of change would be dependent on the level of the alteration. It could take many years for the soil profile to redevelop. § Soil compaction could occur as a result of project activities or in areas of intensive use such as trails. Wetland soils are very susceptible to compaction effects. Soil compaction reduces infiltration rates, thereby increasing surface runoff and the potential for erosion. Deep compaction of soils could impede subsurface flow. In turn, these effects could alter soil chemical processes such as nutrient transfer, biological processes such as root development and microbial patterns, and physical processes such as soil structure. Vegetation growth on compacted soils is often limited due to low infiltration and poor root penetration. § Removal of vegetation through project activities or pedestrian use could result in accelerated erosion of the soil surface. Soils on steep slopes and along watercourses are especially susceptible to erosion. § The addition of chemical constituents into the soils as a result of pavement installation, untreated runoff from paved surfaces, or from incidental spills could alter micro- or macro-organism populations, diversity, and dynamics. Machinery involved with project activities could deposit small amounts of natural and synthetic petrohydrocarbons onto soils through equipment failure and normal operations. § Ecological restoration that would minimize erosion potential and increase organic matter in the soil would be considered a beneficial effect. Short-term adverse effects could occur during site restoration activities where work equipment could compact soils, temporarily eliminate groundcover vegetation, and cause potential erosion from surface water runoff over the exposed soils. Hydrology, Floodplains, and Water QualityThis assessment focuses on the physical and chemical
processes of the Channel MorphologyThe analysis examines potential changes to channel morphology (channel depth, position, and streamflow) as a result of the three alternatives. This section addresses existing and potential future restrictions to streamflow, potential repositioning of the channel bed, potential channel bed scour and bank erosion or instability, flow rates, and sediment transport mechanics. FloodplainNational Park Service policy is to protect natural floodplain values and functions, and to minimize risk to life or property by avoiding the use of the regulatory floodplain whenever there is a feasible alternative. Impacts are evaluated in this section based on the potential to avoid loss of life and property during major floods. The National Park Service manages floodplains in accordance with Executive Order 11988, Floodplain Management, and the National Park Service Special Directive 93-4 (Floodplain Management Guidelines [NPS 1993a]). The regulatory floodplain is defined as the 100-year, 500‑year, or maximum possible flood, depending on the type of activity and the amount of risk inherent in the nature of flooding at a location. Generally, the regulatory flood is the 100-year flood for most park functions in non-flash-flood environments. For critical facilities such as schools, hospitals, and large fuel-storage facilities, the regulatory floodplain is defined as the 500‑year floodplain in non-flash-flood areas. Facilities such as picnic areas and day-visitor parking are exempt from the National Park Service guidelines because they are often located near water for the enjoyment of visitors and do not involve overnight occupation. When there is no practicable alternative to placing facilities in a floodplain, National Park Service policy permits the use of the floodplain when there are compelling reasons for doing so, when the level of impact to natural floodplain processes is acceptable, and when mitigation is provided to protect human life and property. A statement of findings must be written to document a decision to place facilities within a floodplain. This section qualitatively analyzes the impacts or benefits to the river’s floodplain due to removal of Cascades Diversion Dam. For this qualitative assessment, the removal of streamflow impediments was determined to be beneficial to the floodplain and protection of the river channel. Water QualityThis section identifies potential effects on water quality
associated with project activities, such as the location of staging areas near the
WetlandsWetlands and riparian areas are relatively rare in the context of the entire landscape. Modification of even small wetland areas induces effects that are proportionally greater than elsewhere in an ecosystem (Graber 1996). The National Park Service is committed to minimizing
wetland loss. The wetland protection mechanisms used by the National Park
Service include Executive Order 11990, Protection of Wetlands; the National
Park Service’s Director’s Order #77-1, Wetland Protection, and its accompanying
Procedural Manual #77-1; Clean Water Act Section 404; and the “no net loss”
goal outlined by the White House Office on Environmental Policy in 1993.
Executive Order 11990 requires that leadership be provided by involved agencies
to minimize the destruction, loss, or degradation of wetlands. Director’s Order
#77-1 and Procedural Manual #77-1 provide specific procedures for carrying out
the executive order. Section 404 of the Clean Water Act authorizes the U.S.
Army Corps of Engineers to grant permits for construction and disposal of
dredged material in waters of the VegetationImpacts on vegetation communities are assessed in terms of duration, type, and intensity in site-specific, parkwide, and regional contexts. Two primary parameters are used to evaluate the intensity of impacts on vegetation: (1) the size and continuity of the plant community, and (2) the natural structure, productivity, diversity (integrity), and rarity of the plant community. WildlifeThis section addresses the effects of alternatives on wildlife and their habitat. Nearly all wildlife concerns can be addressed by considering the effects of the alternatives on wildlife habitat as represented by general vegetation types. The relationship between vegetation impacts and effects on wildlife is described within this section. Impacts on wildlife are assessed in terms of changes in the amount and distribution of wildlife habitat, the size and connectivity of habitat, the integrity of the site (including past disturbance), the potential for habituation of wildlife to humans, and the relative importance of habitats. Special-Status SpeciesWildlifeThe impact evaluation for special-status wildlife species for each alternative is based on the following: (1) the possibility of a species or its preferred habitat types to occur in areas expected to be affected; (2) the direct loss of habitat or individuals; (3) the partial loss of habitat due to habitat modification; and (4) the species’ sensitivity to disturbance resulting from human activities that could cause species to abandon currently occupied habitat or deter them from occupying suitable habitat. VegetationThe assessment of potential impacts to special-status plant species is based on comparisons between the No Action Alternative and each of the action alternatives. Impacts are evaluated considering species’ sensitivity to impacts (based on rarity, resilience, size of population, and extent of species throughout the park); location of species in proximity to new disturbance; and mitigation measures applied as appropriate for the species and the site. Air QualityThe air quality impact assessment herein evaluates the effect of project activities on air pollutant emissions and concentrations. Air quality impacts are evaluated in terms of their context, intensity, and duration, and whether the impacts are considered to be beneficial or adverse. The air quality impact assessment involves the identification and qualitative description of the types of activities associated with the Cascades Diversion Dam Removal Project that could affect air quality, corresponding emissions sources and pollutants, and relative source strengths. Based on the relative source strengths, this qualitative assessment was performed to determine the potential for higher pollutant emissions or concentrations, taking into account the frequency, magnitude, duration, location, and reversibility of the potential impact. In addition, regional pollutant transport issues are evaluated in the context of regional cumulative impacts. Neither the National Park Service nor the Mariposa County Air Pollution Control District has established emissions-based criteria for evaluating the significance of project implementation impacts. Without such recommendations, the typical approach is to qualitatively evaluate the significance of temporary project implementation impacts. The analysis of effects herein is qualitative, and professional judgment has been applied to reach reasonable conclusions as to the context, intensity, and duration of potential impacts. When possible, mitigation measure(s) are incorporated into the project to reduce the intensity of adverse effects. NoiseThe noise impact assessment involves the identification and qualitative description of the types of actions that could affect the ambient noise environment, corresponding noise sources, relative source strengths, and other characteristics. Based on the relative source strengths, a qualitative assessment was performed to determine the potential for a substantial increase in ambient noise levels in areas where there is natural quiet. Assessments were also performed where noise-sensitive uses are located or would expose persons to excessive noise levels, taking into account the frequency, magnitude, duration, location, and reversibility of the potential impact. Cultural ResourcesThe method of impact analysis described below applies to three types of cultural resources: archeological sites, ethnographic resources, and cultural landscape resources (including individually significant historic structures). Section 106 of the National Historic Preservation Act of
1966, as amended, requires a federal agency to take into account the effects of
undertakings on properties included in, eligible for inclusion in, or
potentially eligible for inclusion in the National Register of Historic Places,
and provides the Advisory Council on Historic Preservation the reasonable
opportunity to comment. A Programmatic Agreement was developed among the
National Park Service at The methodology for assessing impacts to historic resources is based on stipulations of the Programmatic Agreement. This methodology includes: (1) establishing an Area of Potential Effect; (2) assessing the background information regarding historic properties within this area and conducting any necessary surveys, inventories, and resource evaluations; (3) comparing the location of the impact area with that of resources listed, eligible, or potentially eligible for listing in the National Register of Historic Places; (4) identifying the extent and type of effects; (5) assessing those effects according to procedures established in the Advisory Council on Historic Preservation’s regulations; and (6) considering ways to avoid, reduce, or mitigate adverse effects. Cultural resource impacts in this document are described in terminology consistent with the regulations of the Council on Environmental Quality, and in compliance with the requirements of both the National Environmental Policy Act and Section 106 of the National Historic Preservation Act. Mitigation of ImpactsThe National Environmental Policy Act also calls for a discussion of the appropriateness of mitigation and an analysis of the effectiveness of mitigation. A reduction in the intensity of an impact due to mitigation is an estimate of the effectiveness of this mitigation under the National Environmental Policy Act, but does not suggest that the level of effect, as defined by implementing regulations for Section 106 of the National Historic Preservation Act, is similarly reduced. Although adverse effects under Section 106 may be mitigated, and the National Park Service may satisfy the Section 106 obligation, the effects would still be considered adverse under the National Historic Preservation Act. Treatment for National Environmental Policy Act purposes in this document is based on the Programmatic Agreement and includes the avoidance of adverse effects or the application of one or more standard professional practices, as described in Stipulations VII (C) and VIII of the Programmatic Agreement. Avoidance strategies may include the application of the Secretary of the Interior’s Standards and Guidelines for Archeology and Historic Preservation (USDOI 1983), design methods such as vegetation screening when placing new facilities in a historic district, and the development of guidelines to ensure compatibility between new and existing facilities. Stipulation VIII of the Programmatic Agreement requires the National Park Service notify the State Historic Preservation Officer, American Indian tribes, and certain members of the public of its decision to implement standard mitigation measures, as described in Stipulation VIII (A) for individual actions having an adverse effect on historic properties. Archeological ResourcesArcheological resources are typically considered eligible for inclusion in the National Register of Historic Places under criterion d of 36 Code of Federal Regulations Part 60, for the information they have or may be likely to yield. Any change in the physical attributes of an archeological site is considered irreparable, adverse, and permanent. Adverse impacts to archeological resources most often occur as a result of earthmoving activities within an archeological site, soil compaction or increased erosion, unauthorized surface collection, or vandalism. Beneficial impacts to archeological resources can occur when ongoing impacts, which would otherwise continue to degrade archeological resources, are reduced or arrested due to changes in visitor use patterns or management practices in the vicinity of archeological resources. Direct impacts can occur as a result of grading, trenching, or other activities that damage the structure of an archeological site. Indirect impacts can occur as a result of increasing visitor activity or management actions in the vicinity of an archeological site, leading to such occurrences as artifact collection, accelerated soil compaction, or erosion. The intensity of impact to an archeological resource would depend upon the potential of the resource to yield important information, as well as the extent of the physical disturbance or degradation. For example, major earthmoving at an archeological site with low data potential might result in a minor adverse impact. Negligible impacts would be barely perceptible and not measurable and would usually be confined to archeological sites with low data potential. Minor impacts would be perceptible and measurable and would remain localized and confined to archeological site(s) with low to moderate data potential. Moderate impacts would be sufficient to cause a noticeable change and would generally involve one or more archeological sites with moderate to high data potential. Major impacts would result in substantial and highly noticeable changes, involving archeological site(s) with high data potential. For archeological resources, mitigation includes avoiding sites through project design or recovering information that makes the sites eligible for inclusion in the National Register of Historic Places. According to Stipulation VII (C) of the Programmatic Agreement, impacts to archeological resources are not considered adverse for purposes of Section 106 of the National Historic Preservation Act if data recovery is carried out in accordance with the Archeological Synthesis and Research Design (Hull and Moratto 1999).[2] Ethnographic ResourcesEthnographic resources are considered eligible for inclusion in the National Register of Historic Places as traditional cultural properties (or places) when: (1) a district, site, building, structure, or object is rooted in a community’s history and is important for maintaining the continuing cultural identity of the community; and (2) the property(ies) meet National Register criteria for significance and integrity. Impacts to ethnographic resources occur as a result of changes in the physical characteristics, access to, or use of resources, such that the cultural traditions associated with those resources are changed or lost. Beneficial impacts can occur when intrusive facilities or visitor or management activities are removed from a traditional use area; when ecological conditions are improved at a gathering area such that the traditionally used resource is enhanced; or when access for American Indian people is enhanced. Adverse impacts occur when physical changes to a traditionally used resource or its setting degrade the resource itself, or degrade access to or use of a resource. Impacts are considered short term if they represent a temporary change in important vegetation or temporarily restrict access to an important resource, and do not disrupt the cultural traditions associated with that resource for a noticeable period of time, or alter the characteristics for which they are eligible for listing in the National Register. They are considered long term if they involve a change in important vegetation or cultural features, add a new facility or increase visitor use in a way that would change the physical character of or access to a resource for a noticeable period of time, or alter the characteristics for which the resources are eligible for listing in the National Register. This period of time would vary by resource type and traditional practitioners. These long-term changes would disrupt cultural tradition(s) associated with the affected resource, but the disruption would not alter traditional activities to the extent that the important cultural traditions associated with the resource are lost. Permanent impacts to ethnographic resources would involve irreversible changes in important resources such that the ongoing cultural traditions associated with those resources are lost. The intensity of impacts to an ethnographic resource would depend on the importance of the resource to an ongoing cultural tradition, as well as the extent of physical damage or change. Negligible impacts would be barely perceptible and not measurable and would be confined to a small area or single contributing element of a larger National Register district (such as the ethnographic landscape). Minor impacts would be perceptible and measurable and would remain localized and confined to a single contributing element of a larger National Register district. Moderate impacts would be sufficient to cause a change in a significant characteristic of a National Register district or property and/or would generally involve a small group of contributing elements in a larger National Register district. Major impacts would result in substantial and highly noticeable changes in significant characteristics of a National Register district or property and/or would involve a large group of contributing elements in a larger National Register district and/or an individually significant property. The National Park Service would continue to consult with culturally associated American Indian tribes according to stipulations of the Programmatic Agreement, as well as specific agreements such as the October 17, 1997 “Agreement Between the National Park Service, Yosemite National Park, and the American Indian Council of Mariposa County, Inc. for Conducting Traditional Activities” to develop appropriate strategies to mitigate impacts on ethnographic resources. Such strategies could include identification of and assistance in providing access to alternative resource gathering areas, continuing to provide access to traditional use or spiritual areas, and screening new development from traditional use areas. Cultural Landscape Resources, Including Individually Significant Historic Sites and StructuresImpacts to cultural landscape resources result from physical changes to significant characteristics of a resource or its setting. Beneficial impacts can occur as a result of restoration or rehabilitation of resources, or removal of incompatible or noncontributing facilities. Direct adverse impacts generally occur as a result of modifying a significant characteristic of a historic structure or landscape resource, removing a significant structure or landscape resource, or adding new, incompatible facilities in proximity to a historic site or structure. Indirect adverse impacts can also occur following project completion and are generally associated with changes in historic vegetation or continued deterioration of historic structures. They are considered indirect impacts as they are not directly associated with project construction, but rather result from increased visitor use or changes in the management of resources fostered by the project. Impacts to historic structures and cultural landscape resources are considered short term if they involve activities such as temporary removal of vegetation or other contributing resources, road closures, or prescribed burns, where the impacts are noticeable for a period of from one to five years. Other examples of short-term impacts to historic structures include constructing scaffolding surrounding a building during rehabilitation work, or minor deterioration in historic fabric that is repairable as part of routine maintenance and upkeep. Impacts are considered long term if they involve a reversible change, lasting from five to 20 years, in a significant characteristic of a historic structure or landscape. These changes could include such actions as alteration of contributing resources or construction of an incompatible building addition or adjacent facility. Permanent impacts to a historic structure or landscape resources would include irreversible changes in significant characteristics, such as removal of contributing resources, restoration of natural systems and features, irreversible removal of historic fabric that changes the historic character of a property, or demolition of a historic structure. Negligible impacts would be barely perceptible and not measurable and would be confined to small areas or a single contributing element of a larger National Register district. Minor impacts would be perceptible and measurable but remain localized and confined to a single contributing element of a larger National Register district. Moderate impacts would be sufficient to cause a change in a significant characteristic of an individually significant historic structure or would generally involve a single or small group of contributing elements in a larger National Register district. Major impacts would result from substantial and highly noticeable changes in significant characteristics of an individually significant historic structure, or would involve a large group of contributing elements in a National Register district. Mitigation measures for historic structures and cultural landscape resources include measures to avoid impacts, such as rehabilitation and adaptive reuse, designing new development to be compatible with surrounding historic resources, and screening new development from surrounding historic resources. In situations where a historic structure was proposed for removal, the National Park Service would first consider options for relocating the structure to another location in the park for adaptive reuse. Standard mitigation measures, as defined in the Programmatic Agreement, include documentation according to standards of the Historic American Buildings Survey/Historic American Engineering Record, as defined in the Re-Engineering Proposal (October 1, 1997). The level of this documentation, which includes photography and a narrative history, would depend on the significance of a resource (national, state, or local) and the nature of the resource (an individually significant structure, contributing elements in a cultural landscape or historic district, etc.). When a historic structure is slated for demolition, architectural elements and objects may be salvaged for reuse in rehabilitating similar structures, or they may be added to the park’s museum collection. In addition, the historical alteration of the human environment and reasons for that alteration would be interpreted to park visitors. Social ResourcesTransportationThis impact assessment focuses on the effect of temporary changes to the roadway system and parking spaces on traffic volumes and associated traffic flow, access and circulation, and safety conditions. It is assumed that vehicle access past the dam area would be maintained during and after dam removal. The analysis of effects is based on professional transportation engineering judgment. Relative to the No Action Alternative (Alternative 1), the action alternatives (Alternatives 2 and 3), which include complete or partial dam removal, would affect traffic flow, access and circulation, and safety during project work. Transportation impacts are evaluated in terms of their context, duration, and intensity, and whether the impacts are considered to be beneficial or adverse. Traffic Flow ConditionsThis section assesses potential changes in traffic volumes
associated with the dam removal (e.g., workers and vehicular activities).
Changes in traffic volumes are judged as to whether they would substantially
change the levels of congestion on the roadway system serving Traffic Access and CirculationThis section assesses potential changes to the road
network in the area affected by dam removal (e.g., realignment of Traffic Safety/ConflictsThis section assesses potential changes in parking facilities associated with dam removal (i.e., displacement of parking spaces to maintain traffic flow past the staging area). Changes to parking facilities are judged as to whether decreased parking would substantially affect the potential for traffic conflicts. Scenic ResourcesThe overriding management purpose of any national park, as
defined by the National Park Service 1916 Organic Act, is to conserve the
scenery and natural and historic objects. Following this direction, the
National Park Service determined impacts on scenic resources by examining the
potential effects of the Cascades Diversion Dam Removal Project on both the
physical component (any change to the landscape character and/or features) and
with respect to how that change is experienced (any change in visibility, vie Impacts of the project on scenic resources are examined and determined by: § Comparing the existing visual character of the landscape in terms of the color, textural scale, and formal attributes of landscape components and features, and the degree to which project actions would affect (i.e., contrast or conform with) that character § Analyzing changes in experiential factors, such as whether a given action would result in a visible change, the duration of any change in the visual character, the distance and viewing conditions under which the change would be visible, and the number of viewers that would be affected Scenic resources impacts consist of substantial changes
that would alter (1) existing landscape character, whether foreground,
intermediate ground, or background, and be visible from vie RecreationThis analysis evaluates the quality of recreation opportunities in terms of how they might be altered as a result of the alternatives. Developing a quantitative analysis of potential effects on recreation is not feasible. Analysis of effects is therefore qualitative, and professional judgment was applied to reach reasonable conclusions as to the context, intensity, and duration of potential impacts. Assumptions that frame the analysis included the following: § Visitor demand will increase over existing levels and will be the same among all of the alternatives § There will be no change to visitor access by private vehicle to the park Analysis was based on whether there was a complete loss of
a recreation opportunity, a change in access to or availability of a recreation
opportunity, or a change in the aggregate of recreation opportunities for the
visitor. This analysis evaluates how the alternatives would interact with all
independent and group opportunities available between Orientation and InterpretationThis analysis evaluates the quality of orientation and interpretation opportunities in terms of how they might be altered as a result of the alternatives. Developing a quantitative analysis of potential effects on orientation and interpretation is not feasible. Analysis of effects is therefore qualitative, and professional judgment was applied to reach reasonable conclusions as to the context, duration, and intensity of potential impacts. Assumptions that frame the analysis included the following: § Visitor demand will increase over existing levels and will be the same among all of the alternatives § There will be no change to visitor access by private vehicle to the park Analysis was based on whether there was a complete loss of a orientation and interpretation opportunity, a change in access to or availability of an orientation and interpretation opportunity, or a change in the aggregate of orientation and interpretation opportunities for the visitor. SocioeconomicsThe socioeconomic impact analysis qualitatively evaluates the effects of project alternatives on the regional economy. Due to the structure of the local economic relationships and the nature of the dam removal activities, these impacts are addressed in terms of the three-county region as a whole, and not at the individual county level. Professional judgment was applied to reach reasonable conclusions as to the context, duration, and intensity of potential impacts. The analysis considered both direct and secondary project-related spending effects. Direct effects represent the immediate spending within the sector of the economy where the initial stimulus occurs. Secondary effects include indirect effects and induced effects. Indirect effects represent the impact of the initial stimulus on the economy as a result of changes in business spending. Induced effects are the impacts of the initial stimulus on the economy from changes in personal consumption (as a result of changes in employee income). Generally, secondary spending effects increase direct spending effects by about 45 percent (NPS 2000a). Total project-related spending is the combination of both direct and secondary spending effects. The estimated cost of dam removal for the action alternatives is provided in 2003 dollars. Project-related economic output for the affected region is provided in 2000 dollars. The estimated project costs for the alternatives were deflated to 2000 dollars using the Consumer Price Index for All Urban Consumers (1982-84=100) to adjust for inflation (U.S. Bureau of Labor Statistics 2002). Park Operations and FacilitiesFor purposes of this analysis, an alternative is assumed to have an impact (negative or beneficial) on park operations and facilities if it: § Results in direct changes to park operation, facilities, or staffing requirements or policies associated with park operations § Causes indirect effects on park operations staffing, such as effects on utility and roadway infrastructure, flooding, and impacts on provision of utilities, especially potable water and sewer services Alternative 1 – No ActionThe No Action Alternative maintains the status quo at Cascades Diversion Dam, as described in Chapter III, Affected Environment. It provides a baseline from which to compare the action alternatives, to evaluate the magnitude of proposed changes, and to measure the environmental effects of those changes. Natural ResourcesGeology, Geologic Hazards, and SoilsAnalysisUnder Alternative 1, dam failure could be sudden or could occur
over a course of years or decades. This failure could occur in either a single
event during high river flows or could occur gradually, as the dam and
attendant structures degrade from continued scour. Dam failure represents a
public safety hazard because debris, including timbers and concrete, would be
released into the river and carried downstream, potentially causing injury. Uncontrolled
dam failure could also cause unpredictable riverflows that destabilize and erode
banks, resulting in soil loss, an undermining of trails and roads, endangerment
of the utility lines beneath Under Alternative 1, geologic hazards could cause further structural damage to Cascades Dam Diversion and contribute to greater structural degradation that could accelerate the eventual failure of the dam structure. Cascades Diversion Dam would continue to be subjected to structural damage due to earthquakes. The dam is located in an area of moderate seismicity, and earthquakes from several remote sources that could trigger groundshaking sufficient to cause observable ground movement at the dam site. A significant earthquake would likely damage the historic structural elements (i.e., crack concrete dam components and dislodge the timber crib) rather than cause their immediate failure. Earthquake-induced damage could accelerate degradation of the structural supports and contribute to the eventual failure of the dam. Groundshaking could also consolidate the sand and gravel deposits underlying the dam, leading to further structural damage. This would be a local, long-term, moderate, adverse impact. It is important to note that the dam has withstood numerous small and some relatively large earthquakes over the past 83 years without significant damage or failure. As in the past, Cascades Diversion Dam and the attendant
structures would be subject to damage from debris generated during rockfalls
along the shear cliffs of the Retrieval of dam materials scattered downstream due to continued degradation or dam failure would require multiple ingress and egress points for equipment and personnel, potentially destabilizing the riverbank in locations between Cascades Diversion Dam and Cascades Picnic Area. Under extreme high flows, dam debris could be transported as far downstream as El Portal. Debris retrieval activities would result in short-term impacts to soil resources and could include excessive erosion, soil compaction, and loss of topsoil caused by diverted floodwaters following dam failure. Dam debris retrieval activities would result in local, short-term, moderate, adverse impacts to soil resources. Continued dam degradation and dam failure and the subsequent deposition of sediment would benefit floodplain soils by providing a sediment source that would eventually settle out along the river and provide a substrate for development of a soils horizon. However, the amount of sediment that would be released from behind the dam would be relatively insignificant compared to the overall area of expected deposition (the braided river reach at Cascades Picnic Area) and the amount of sediment that is typically transported downstream on a continual basis. The contribution of sediment available for soil development in the area of the dam and reservoir is considered negligible. Soil resources throughout the remainder of the Summary of Alternative 1 Impacts. Continued degradation and eventual failure of the dam and retrieval of dam debris would cause bank destabilization, erosion, and soil loss, resulting in local, short- and long-term, moderate, adverse impacts to soil resources. Under Alternative 1, rockfall events would result in a local, long-term, minor, adverse effect on public health and safety. Cumulative ImpactsCumulative impacts to geological resources discussed
herein are based on analysis of past, present, and reasonably foreseeable
future actions in the Development projects intended to serve park visitors in Rockfall hazards are being reduced through projects that
relocate people and facilities away from rockfall hazard zones, such as the
removal of units at Housekeeping Camp and Certain projects could result in increased degradation of soil resources, such as construction projects related to campgrounds, lodging, employee housing, and other facilities, as identified in the Yosemite Valley Plan, Briceburg Bridge Reconstruction, and Yosemite Motels Expansion. However, other projects related to habitat restoration (e.g., Yosemite Valley Plan, Cook’s Meadow Ecological Restoration, and Eagle Creek Merced River Ecological Restoration) would have long-term, beneficial effects on soils. Although these types of projects may have slight site-specific, short-term, adverse effects (e.g., potential short-term construction erosion and soil loss), an objective of these projects is to restore and manage natural resources and reduce soil degradation. For example, full implementation of the Yosemite Valley Plan would restore approximately 177 acres of soil, of which approximately 136 acres would be high-value resource soils. The cumulative projects would result in a local, long-term, minor, beneficial, cumulative impact on soil resources. Alternative 1 and the cumulative projects would result in a local, long-term, minor, beneficial impact to public safety in the project region, due to the overall reduction in the density of facilities in the talus slope and rockfall shadow zones. The local, long-term, minor, beneficial impact to soil resources under the cumulative projects would be somewhat diminished by the potential soil erosion and bank destabilization under Alternative 1, resulting in a net local, long-term, negligible, beneficial impact to soil resources. ImpairmentThe No Action Alternative would result in a local, short-
and long-term, moderate, adverse impact to soil resources in the immediate
vicinity of Cascades Diversion Dam due to bank destabilization, erosion, and
soil loss. Although the Hydrology, Floodplains, and Water QualityAnalysisUnder Alternative 1, the existing condition and placement of Cascades Diversion Dam would continue to adversely influence river hydrology and present a potential flood hazard. A dam, like any fixed structure in a river, can alter flow dynamics and result in localized morphologic changes to the bed and banks of the river. Cascades Diversion Dam constricts the floodplain of the Merced River in the immediate area of the dam, alters hydrologic flows, widens the river channel (to approximately twice its natural width), and impedes free-flow and fluvial processes (e.g., sediment transport). Alternative 1 would have a local, long-term, moderate, adverse impact on hydrologic processes that influence river morphology. However, when the dam fails of its own accord under Alternative 1, river hydrology would be restored to more natural conditions, resulting in a local, long-term, moderate, beneficial impact on hydrologic processes. Under Alternative 1, Cascades Diversion Dam would remain
in its existing condition, without maintenance or repair. Serious damage to the
timber overflow crest of the dam was sustained during the 1997 flood. Over the
long term, Cascades Diversion Dam would continue to degrade and eventually fail,
and dam materials and impounded sediments would be released and eventually
collect downstream to the Cascades Picnic Area. Under extreme high flows, dam
materials and impounded sediments could be transported as far downstream as El
Portal. At the dam site, the channel would scour to a more natural condition,
becoming narrower and deeper. As this occurs, the localized water table is
expected to drop. Lateral movement of the channel or increased bank shear
stress could increase erosion. Over time, the channel of the Under Alternative 1, continued degradation of the dam and eventual failure would result in temporarily substantial water quality impacts associated with bank erosion. Upon dam failure, the majority of impounded sediment would settle at Cascades Picnic Area. While it is estimated that the impoundment area contains approximately 15,000 to 20,000 cubic yards of sediment (including rocks/boulders), some of this material was present in the riverbed prior to dam construction, and it is likely that some of the impounded material would remain following dam failure, including the island upstream from the dam. Sediment deposition in the reach between the dam and the Cascades Picnic Area could build sandbars and increase floodplain characteristics (e.g., riparian vegetation). Because less than 1% of the impounded material is finer than 0.07 millimeter in diameter, any turbidity impact to the river from release of this material would likely be small. The largest increase of turbidity above background values (see Chapter III, Affected Environment, Alluvial Processes) would likely occur the first time the river flows through the sediments formerly impounded by the dam. This rise in turbidity would probably last on the order of a few hours. After the initial rise in turbidity, there could be smaller spikes in turbidity as riverflows pass through the impoundment region and gradually erode more sediment and expose the fine material. However, each successive turbidity spike should be smaller than the previous one, until the turbidity levels return to normal (USBR 2001). Downstream bank erosion resulting from dam deterioration and eventual failure would release additional sediment into the river and result in a greater turbidity impact than that associated with release of impounded sediments. Fine-grained materials would be transported downstream some distance and would likely settle out and deposit in areas of low energy, such as pools and downstream reservoirs (NPS 2001a). Solid structural materials from the dam would constitute less of a water quality impact. However, retrieval of dam debris would require multiple ingress and egress points for equipment and personnel, and the use of equipment along the river between Cascades Diversion Dam and Cascades Picnic Area. Debris retrieval activities could dislodge sediment from the riverbed and banks, resulting in water quality impacts. Sediment and debris delivery to the river would continue if the dam remained and eventually failed; therefore, Alternative 1 would represent a local, short-term, moderate, adverse impact to water quality. Summary of Alternative 1 Impacts. Alternative 1 would have a local, short-term, moderate, adverse impact on hydrologic processes and water quality, due to continued deterioration and eventual failure of the dam and subsequent debris retrieval activities. Over the long term, the failed dam would be removed and more natural river hydrology would be restored in this area, which would have a local, long-term, moderate, beneficial impact on hydrologic processes. Cumulative ImpactsCumulative effects to hydrologic processes are based on
analysis of past, present, and reasonably foreseeable future actions occurring
in the The Removal of the The Merced River Plan protects river-related natural resources through the application of management elements, including the River Protection Overlay, management zoning, protection and enhancement of Outstandingly Remarkable Values, Section 7 determination process, and implementation of a Visitor Experience Resource Protection framework. Other future projects include the Happy Isles Gauging
Station Replacement Project, Eagle Creek Merced River Ecological Restoration,
and Replacement/Rehabilitation of Yosemite Valley Main Sewer Line.
Cumulatively, these projects are anticipated to have a beneficial impact on
hydrologic processes and water quality of the While some of the past, present, and future projects in
the The past, present, and future projects in the Merced River watershed, considered cumulatively with Alternative 1, would have a local, long-term, minor, beneficial impact to hydrologic processes and water quality. The long-term beneficial effects associated with dam failure under Alternative 1 would contribute to the beneficial cumulative effects, and largely offset the short-term adverse effects associated with the continued deterioration and eventual failure of the dam. ImpairmentAlternative 1 would have a local, short-term, moderate,
adverse impact on hydrologic processes and water quality due to continued
deterioration and eventual failure of the dam, but a local, long-term, moderate,
beneficial impact on hydrologic processes associated with the ultimate removal
of the dam. Although the Merced River system and its associated hydrologic resources
are key natural resource components within the Merced River gorge, the adverse
effects of this alternative on river hydrology would be primarily localized
between Cascades Diversion Dam and Cascades Picnic Area (and potentially to El
Portal under extreme high flows), temporary in duration, and largely offset by
the long-term beneficial effects of ultimate dam removal. The short-term
adverse effects of this alternative would not be considered severe. Therefore,
Alternative 1 would not impair hydrologic resources within the WetlandsAnalysisUnder Alternative 1, the existing condition and placement
of Cascades Diversion Dam would continue to adversely influence the size,
connectivity, and integrity of river-related wetlands in the vicinity of the
dam, particularly palustrine forest, palustrine scrub shrub, and riverine
habitats. A dam, like any fixed structure in a river, can alter flow dynamics
and result in localized morphologic changes to the bed and banks and wetland
habitats of the river. Cascades Diversion Dam constricts the floodplain of the Cascades Diversion Dam restricts free flow of the Cascades Diversion Dam would continue to degrade and eventually fail. Dam materials and sediments would be released downstream to the Cascades Picnic Area (or as far as El Portal under extreme high flows) and could affect riparian and aquatic resources, either during transport (e.g., large dam debris could remove riparian trees) or upon deposition. While it is estimated that the impoundment area contains approximately 15,000 to 20,000 cubic yards of sediment (including rocks/boulders), some of this material was present in the riverbed prior to dam installation, and it is likely that some of the impounded material would remain following dam failure, including the island upstream from the dam. Sediment deposition could have adverse or beneficial effects to downstream wetland resources. For instance, suspended sediments would temporarily reduce dissolved oxygen levels, and sediment and debris deposition could bury riparian species, resulting in individual death. This latter effect would also be temporary, because natural revegetation would occur. Conversely, sediment deposition could result in larger sandbars capable of supporting riparian vegetation – a potential beneficial effect. Depending on the flows at the time of dam failure, large pieces of the dam structure could restrict and divert flows, leading to bank erosion or riverbank scour between the dam and Cascades Picnic Area. Until flow reduced sufficiently to allow the pieces to be removed from the river after dam failure, bank erosion would continue and possibly threaten to expose and undermine wetland resources. Flows diverted by debris could cause the river to leave the channel and result in localized flooding on either side of the river. Bank erosion and retrieval of dam debris could result in local, short-term, negligible to moderate, adverse impacts for wetland resources. At the dam site, the channel would be expected to scour to a more natural condition, becoming narrower and deeper. As this occurs, the localized water table is expected to drop, and existing riparian vegetation could transition to an upland community. Lateral movement of the channel or increased bank shear stress could increase erosion, which could also result in riparian loss. Although natural stabilization of the riparian and aquatic community would occur over time, restoration would not be complete for 10 or more years; therefore, this impact is considered a local, short- and long-term, minor to moderate, adverse effect on wetland and aquatic habitats. Summary of Alternative 1 Impacts. In the near term, Cascades Diversion Dam would remain and would continue to adversely affect the size, connectivity, and integrity of wetlands in the immediate vicinity of the dam, particularly palustrine forest and riverine habitats. Cascades Diversion Dam would degrade and eventually fail. Dam materials and impounded sediments would be released downstream and could affect riparian and aquatic resources during transport (e.g., large dam debris could remove riparian trees), upon deposition, or during debris retrieval activities, resulting in a local, short-term, negligible to minor, adverse impact to wetland resources. Although natural stabilization of the riparian and aquatic community would occur over time, restoration would not be complete for 10 or more years; therefore, this impact is considered a local, long-term, minor to moderate, adverse effect on wetland and aquatic habitats. Cumulative ImpactsCumulative effects to wetland and aquatic resources
discussed herein are based on analysis of past, present, and reasonably foreseeable
future actions in the Wetland and riparian systems of the ImpairmentAlternative 1 would result in a local, short-term,
negligible, adverse impact and a local, long-term, minor to moderate, adverse
impact to wetland and aquatic resources. Although the VegetationAnalysisThe size, connectivity, and integrity of vegetation in the
project area, particularly riparian vegetation, has been directly compromised
by Cascades Diversion Dam, which constricts the floodplain of the Cascades Diversion Dam restricts free flow of the Cascades Diversion Dam would continue to degrade and eventually fail. Dam materials and sediments would be released downstream to the Cascades Picnic Area (or as far as El Portal under extreme high flows) and could affect riparian habitats, either during transport (e.g., large dam debris could remove riparian trees) or upon deposition. While it is estimated that the impoundment area contains approximately 15,000 to 20,000 cubic yards of sediment (including rocks/boulders), some of this material was present in the riverbed prior to dam installation, and it is likely that some of the impounded material would remain following dam failure, including the island upstream from the dam. Sediment deposition could have adverse or beneficial effects to downstream vegetation. For instance, suspended sediments would temporarily reduce dissolved oxygen levels, and sediment deposition could bury riparian species, resulting in individual death. This latter effect would also be temporary, because natural revegetation would occur. Conversely, sediment and dam debris deposition could result in larger sandbars capable of supporting riparian vegetation – a potential beneficial effect. Depending on the flows at the time of dam failure, large pieces of the dam structure could restrict and divert flows, leading to bank erosion or riverbank scour between the dam and Cascades Picnic Area. Until flow reduced sufficiently to allow the pieces to be removed from the river after dam failure, bank erosion would continue and possibly threaten to expose and undermine riparian vegetation. Flows diverted by debris could cause the river to leave the channel and result in localized flooding on either side of the river, which could further affect local vegetation patterns. Bank erosion and retrieval of dam debris could result in local, short-term, negligible to moderate, adverse impacts on vegetation. At the dam site, the channel would be expected to scour to a more natural condition, becoming narrower and deeper. As this occurs, the localized water table is expected to drop, and existing riparian vegetation could transition to an upland community. Lateral movement of the channel or increased bank shear stress could increase erosion, which could also result in riparian loss. Although natural stabilization of the riparian and aquatic community would occur over time, restoration would not be complete for 10 or more years; therefore, this impact is considered a local, long-term, minor, adverse effect on vegetation. Summary of Alternative 1 Impacts. In the near term, Cascades Diversion Dam would remain and would continue to adversely affect the size, connectivity, and integrity of vegetation in the immediate vicinity of the dam, particularly palustrine forest and riverine habitats. Cascades Diversion Dam would degrade and eventually fail. Dam materials and impounded sediments would be released downstream and could affect downstream vegetation, especially riparian vegetation, during transport (e.g., large dam debris could remove riparian trees), upon deposition, or during debris retrieval activities, resulting in a local, short-term, negligible to moderate, adverse impact to vegetation. Although natural stabilization of river-associated vegetation would occur over time, restoration would not be complete for 10 or more years; therefore, this impact is considered a local, long-term, minor, adverse effect on vegetation. Cumulative ImpactsCumulative effects to vegetation resources discussed
herein are based on analysis of past, present, and reasonably foreseeable
future actions in the Vegetation within the ImpairmentAlternative 1 would result in a local, long-term,
negligible to moderate, adverse impact and a local, long-term, minor, adverse
impact to vegetation. Although the WildlifeAnalysisThe size, connectivity, and integrity of native fish and wildlife
habitat in the immediate vicinity of Cascades Diversion Dam, particularly the
riparian corridor and aquatic environment of the Merced River, has been
directly altered by the dam, which constricts the floodplain of the Merced
River in the immediate area of the dam and alters hydrologic flows. Within the
impoundment, the Cascades Diversion Dam would continue to degrade and eventually fail. Dam materials and impounded sediments would be released downstream to the Cascade Picnic Area (or as far as El Portal under extreme high flows) and could temporarily affect aquatic resources, either during transport (e.g., large dam debris could directly impact individual species) or upon deposition. While it is estimated that the impoundment area contains approximately 15,000 to 20,000 cubic yards of sediment (including rocks/boulders), some of this material was present in the riverbed prior to dam installation, and it is likely that some of the impounded material would remain following dam failure, including the island upstream from the dam. Downstream sediment deposition could have adverse or beneficial effects on downstream fish and wildlife habitats. For instance, suspended sediments would temporarily reduce dissolved oxygen levels, which could lead to suffocation of aquatic invertebrates. Conversely, sediment deposition could result in larger sandbars capable of supporting riparian vegetation, increasing wildlife habitat – a potential beneficial effect. Depending on the flows at the time of dam failure, large pieces of the dam structure could restrict and divert flows, leading to bank erosion or riverbank scour between the dam and Cascades Picnic Area that could further affect local fish and wildlife and their habitat. Until flow reduced sufficiently to allow the pieces to be removed from the river after dam failure, bank erosion would continue and possibly threaten to expose and undermine riparian wildlife habitats adjacent to the river-right bank. Flows diverted by debris could cause the river to leave the channel and result in localized flooding on either side of the river, which could further affect local fish and wildlife patterns. Bank erosion and retrieval of dam debris could result in local, short-term, negligible to moderate, adverse impacts on wildlife. Dam failure would eliminate approximately 2.5 acres of
unnatural slack-water aquatic habitat upstream of the dam. At the dam site, the
channel would scour to a more natural condition, becoming narrower and deeper.
Water temperature through this reach would decrease, and oxygen levels would be
expected to increase (generally a beneficial effect for aquatic species).
Lateral movement of the channel or increased bank shear stress could increase erosion,
which could also result in loss of riparian wildlife habitat. Over time, the
channel of the Summary of Alternative 1 Impacts. In the near term, Cascades Diversion Dam would remain and would continue to adversely affect the size, connectivity, and integrity of wildlife and aquatic habitat in the immediate vicinity of the dam. Cascades Diversion Dam would degrade and eventually fail. Dam materials and impounded sediments would be released downstream and could have both beneficial and adverse affects on aquatic wildlife resources. Bank erosion and dam retrieval activities could result in local, short-term, negligible to moderate, adverse impacts on wildlife. Although natural stabilization of the riparian and aquatic community would occur over time, restoration would not be complete for 10 or more years; therefore, this impact is considered a long-term effect. Overall, Alternative 1 would result in a local, long-term, minor to moderate, adverse impact to aquatic wildlife habitat and associated native fish and wildlife. Cumulative ImpactsCumulative effects to native fish and wildlife discussed
herein are based on analysis of past, present, and reasonably foreseeable
future actions in the Native fish and wildlife communities have been manipulated almost since the inception of the park. Regional wildlife has been historically affected by logging, fire suppression, rangeland clearing, grazing, mining, draining, damming, diversions, and the introduction of non-native species. Fur-bearing mammals were trapped by park rangers until 1925; lions were considered dangerous predators and controlled through the 1920s; bears were artificially fed as a tourist attraction until 1940. Natural wildfires, with their generally beneficial effects on wildlife habitat, were routinely suppressed until 1972 (Wuerthner 1994). Past and ongoing activities include recreational use and installation of dams, diversion walls, bridges, roads, pipelines, riprap, buildings, campgrounds, and other recreational features. Implementation of the Yosemite
Valley Plan is an example of a project that could have adverse or
beneficial effects on wildlife. Full implementation of the Yosemite Valley Plan would restore 141 acres of
river-associated wildlife habitat in ImpairmentAlternative 1 would result in a local, short-term,
negligible to moderate, adverse impact and a local, long-term, minor to
moderate, adverse impact to native fish and wildlife resources. Although the Special-Status SpeciesAnalysisSpecial-status species known or likely to occur in the immediate vicinity of Cascades Diversion Dam include Wawona riffle beetle and nine species of bats (refer to Chapter III, Affected Environment, and Appendix D, Special-Status Species Evaluation, for additional information). The following subsections discuss impacts of Alternative 1 on these species or their habitat. Wawona Riffle Beetle and
Harlequin Duck. Cascades Diversion Dam adversely affects habitat
for Wawona riffle beetle and harlequin duck in the area of the impoundment by
altering hydrology flows and patterns. Within the impoundment, the Cascades Diversion Dam would continue to degrade and eventually fail. Dam materials and impounded sediments would be released downstream to the Cascades Picnic Area (or as far as El Portal under extreme high flows) and could temporarily affect the aquatic environment, either during transport or upon deposition. While it is estimated that the impoundment area contains approximately 15,000 to 20,000 cubic yards of sediment (including rocks/boulders), some of this material was present in the riverbed prior to dam installation, and it is likely that some of the impounded material would remain following dam failure, including the island upstream from the dam. Water quality would be adversely affected in the short term during sediment transport, which could adversely affect (e.g., suffocation, habitat displacement) individuals, populations, or habitat for Wawona riffle beetle or harlequin duck. Downstream deposition could also affect existing riffles and pools that currently provide habitat for Wawona riffle beetle. Depending on the flows at the time of dam failure, large pieces of the dam structure could restrict and divert flows, leading to bank erosion or riverbank scour between the dam and Cascades Picnic Area, further affecting habitat for Wawona riffle beetle and harlequin duck. Until flow reduced sufficiently to allow the pieces to be removed from the river after dam failure, bank erosion would continue and possibly threaten to expose and undermine riparian vegetation adjacent to the river-right bank. Flows diverted by debris could cause the river to leave the channel and result in localized flooding on either side of the river, which could further affect local habitat for Wawona riffle beetle and harlequin duck. Retrieval of dam debris could result in local, short-term, negligible to moderate, adverse impacts on Wawona riffle beetle and harlequin duck habitat. Overall, dam failure is expected to have local, short-term, moderate, adverse impacts on individuals and/or habitat for Wawona riffle beetle that occur downstream of dam. Dam failure would also eliminate approximately 2.5 acres of unnatural slack-water aquatic habitat upstream of the dam. At the dam site, the channel would scour to a more natural condition, becoming narrower and deeper. Water temperature through this reach would decrease, and oxygen levels would be expected to increase. The impoundment portion of the river would be returned to a more natural condition, which would include deeper rifles and pools as well as full connectivity to downstream portions of the river during periods of low flow, creating an overall local, long-term, minor to moderate, beneficial effect on Wawona riffle beetle by increasing habitat for the species. Special-Status Species of
Bats and Summary of Alternative 1
Impacts. In the near term, Cascades Diversion Dam would remain
and would continue to adversely affect the size, connectivity, and integrity of
habitat for Wawona riffle beetle and harlequin duck in the immediate vicinity
of the dam, resulting in a continued minor to moderate, adverse impact on
beetles and their habitat. Cascades Diversion Dam would degrade and eventually
fail, creating a local, short-term, moderate, adverse impact on individuals or
habitat for Wawona riffle beetle and harlequin duck that occur downstream of
the dam. In the impoundment area, eventual dam failure would return this area
to a more natural condition, creating an overall local, long-term, minor to
moderate, beneficial effect on Wawona riffle beetle and harlequin duck by
increasing habitat for the species. Alternative 1 is unlikely to significantly
affect special-status species of bats or the Cumulative ImpactsCumulative effects to special-status species discussed
herein are based on analysis of past, present, and reasonably foreseeable
future actions in the Natural habitats of special-status species have been manipulated almost since the inception of the park. Their habitats have been historically affected by logging, fire suppression, rangeland clearing, grazing, mining, draining, damming, diversions, the introduction of non-native species, and recreational features and use. An example of a project that could have adverse or
beneficial effects on special-status species includes the implementation of the
Yosemite Valley Plan. Full
implementation of the Yosemite Valley
Plan would restore 141 acres of river-associated habitat in ImpairmentThe No Action Alternative would result in both beneficial
and adverse effects on Wawona riffle beetle and harlequin duck in the vicinity
of Cascades Diversion Dam. Although the Air QualityAnalysisUnder Alternative 1, air quality would be adversely affected by both “fugitive” sources (i.e., emissions released by means other than through a stack or tailpipe) and by tailpipe emissions. Although no action would be taken under Alternative 1, the
National Park Service would continue to make minor repairs to attendant
structures such as the screenhouse, concrete platform, and safety railing on
the dam intake structure. Maintenance-related activities conducted prior to
eventual dam failure would result in temporary increases in traffic and
associated tailpipe emissions. It is likely that maintenance activities would
be conducted using similar air quality best management practices as those
described in Chapter II, Alternatives. Emissions related to ongoing maintenance
activities could adversely affect regional air quality, but impacts would be
both temporary and negligible relative to emissions generated by other vehicle
traffic along Upon dam failure, the National Park Service would remove
large debris from the river and banks, which would result in air quality effects.
Such effects would be primarily related to use of equipment, vehicle trips to
and from the area, and dust. Cascades Diversion Dam is located in a region that
experiences exceedances of the state ozone and particulate matter (with
diameters of 10 microns or less) emission standards. Debris removal activities
would temporarily affect pollutant concentrations along the Debris removal would also result in tailpipe emissions
associated with use of mobile debris removal equipment, worker commute trips,
and truck trips to haul debris materials from the Summary of Alternative 1 Impacts. Emissions associated with maintenance activities prior to dam failure and debris removal after dam failure under Alternative 1 would result in a local, short-term, negligible to moderate, adverse impact to air quality. There would be no long-term impact on air quality under Alternative 1. Cumulative ImpactsCumulative effects to air quality discussed herein are
based on analysis of past, present, and reasonably foreseeable future actions
in the Since 1950, the population of The Yosemite Valley
Plan proposes to enhance the quality of the visitor experience in Short-term adverse impacts on air quality could result
from many of the reasonably foreseeable projects planned or approved within the
Although cumulative growth in the region will tend to
adversely affect air quality, implementation of ongoing state and federal
mobile-source control programs will ameliorate this effect to some degree. With
respect to particulate matter, conditions in the Alternative 1 and the cumulative projects would have a local, long-term, minor, beneficial impact on air quality. Alternative 1 would reduce the intensity of this beneficial impact to negligible in the short term, due to emissions that would be generated during maintenance activities and debris removal after dam failure. ImpairmentImpairment is not addressed in the air quality analysis because this resource topic is peripheral to the protection of the park for future generations. NoiseAnalysisUnder Alternative 1, ambient noise levels would be affected by noise associated with maintenance-related activities prior to dam failure and debris removal activities after eventual dam failure. Although no action would be taken under Alternative 1, the National Park Service would continue to make minor repairs to attendant structures such as the screenhouse, concrete platform, and safety railing on the dam intake structure. Maintenance-related activities conducted prior to eventual dam failure would result in temporary increases in traffic and associated roadside noise levels. It is likely that maintenance activities would be conducted using similar noise best management practices as those described in Chapter II, Alternatives. Noise generated by ongoing maintenance activities could adversely affect the noise environment, but impacts would be both temporary and negligible relative to noise generated by other vehicle traffic. Because no management action would be taken to repair or
remove the dam under Alternative 1, eventual uncontrolled failure of the
overflow portion of the dam structure would be expected. Dam failure could be
sudden or could occur over a course of years or decades. An uncontrolled
failure of the overflow portion of the structure or continued deterioration of
the dam over time would result in a release of concrete and timber debris and
grouted rockfill, which would litter the downstream channel of the The specific mix of equipment to be used in debris removal is unknown, but could include the use of cranes, excavators, backhoes, skid steer loaders, and trucks. Noise levels would decrease by about 6 dBA with each doubling of distance from the noise source (e.g., noise levels from crane use would be in the range of 83 to 88 dBA at 100 feet from the site, and about 77 to 82 dBA at 200 feet from the site). Noise associated with debris removal would constitute a local, short-term, moderate, adverse impact. Typical Noise Levels from Heavy-Duty Equipment
dBA = A-weighted decibels
* Adjusted to a distance of 50 feet; original data of 107 to 111 dBA corresponds to a distance of 75 feet. Measurement of rock blasting reflects use of nonglycerin dynamite.
SOURCES: Federal Transit Authority 1995, except for rock blasting; rock blasting data provided by the National Park Service (NPS 2000b)
Therefore, noise generated by routine maintenance activities and debris removal after dam failure under Alternative 1 would result in a local, short-term, negligible to moderate, adverse impact on the noise environment. Over the long term, the acoustical environment in the vicinity of Cascades Diversion Dam would be shaped largely by natural sources of sound (i.e., rushing water and wind) punctuated by human-caused sources of noise, such as motor vehicles and aircraft. Summary of Alternative 1 Impacts. Noise generated by routine maintenance and debris removal activities under Alternative 1 would result in a local, short-term, negligible to moderate, adverse impact to the ambient noise environment. There would be no long-term impact on the noise environment under Alternative 1. Cumulative ImpactsCumulative effects to the ambient noise environment
discussed herein are based on the analysis of past, present, and reasonably
foreseeable future actions in the The Yosemite Valley
Plan proposes to enhance the quality of the visitor experience in Short-term adverse impacts on ambient noise levels could
result from construction activities associated with some of the reasonably
foreseeable projects planned or approved within the Over the long term, the gradual increase in annual visitation to the park could potentially offset the beneficial effects of the cumulative projects discussed above, resulting in a net local, long-term, minor, adverse effect on the noise environment. Alternative 1 would contribute to this cumulative impact in the short term. ImpairmentImpairment is not addressed in the noise analysis because this resource topic is peripheral to the protection of the park for future generations. Cultural ResourcesArcheological ResourcesAnalysisThere are no known archeological resources located within
the immediate vicinity of the dam (NPS 1987b). Under Alternative 1, there would
be no change in management and treatment of archeological resources. The dam
would continue to degrade and eventually fail, and dam materials and impounded
sediments would be released downstream. Debris could dam the river, divert the
river from its channel, or substantially erode the otherwise stable riverbanks
in the area between the dam and Cascades Picnic Area (or potentially as far as
El Portal). These activities could unearth as-yet unknown sensitive
prehistoric or historic archeological resources in the vicinity of the dam downstream
to Cascades Picnic Area. Although the banks of the Summary of Alternative 1 Impacts. There would be no change in the treatment and management of archeological resources as a result of Alternative 1. Dam failure and subsequent bank erosion could have a long-term adverse effect on archeological resources in the vicinity of the dam downstream to Cascades Picnic Area. Any site-specific planning and compliance actions would be performed in accordance with stipulations in the park’s 1999 Programmatic Agreement. Overall, Alternative 1 would result in a local, long-term, minor to moderate, adverse impact to archeological resources. Cumulative ImpactsCumulative impacts to archeological resources discussed
herein are based on analysis of past, present, and reasonably foreseeable
future actions within the main stem of the In general, any archeological resources within the main
stem of the Reasonably foreseeable future actions proposed in the
region that could have an adverse cumulative effect on archeological resources
in the main stem of the The Merced River Planprovides a framework for decision-making on future management actions
within the The cumulative projects within and in the vicinity of the main stem of the Merced River would result in a local, long-term, negligible to minor, adverse impact on archeological resources, due to the potential disturbance of such resources. Alternative 1 and the cumulative projects within and in
the vicinity of the main stem of the ImpairmentAlthough archeological sites along the river are key
cultural resources within the Ethnographic ResourcesAnalysisThere are potential ethnographic resources within the Retrieval of dam debris would require multiple ingress and egress points for equipment and personnel, and the use of equipment along the river between Cascades Diversion Dam and Cascades Picnic Area, which could result in further disturbance of potential ethnographic resources. Any debris retrieval actions undertaken by the National Park Service would be performed in accordance with stipulations in the park’s 1999 Programmatic Agreement, and the park would continue to consult with culturally associated American Indian tribes under this Programmatic Agreement and the cooperative agreement for traditional uses. Overall, Alternative 1 would result in a local, long-term, minor to moderate, adverse impact to ethnographic resources. Summary of Alternative 1 Impacts. There would be no change in the treatment and management of ethnographic resources as a result of Alternative 1. Dam failure and subsequent bank erosion could have a long-term adverse effect on ethnographic resources in the vicinity of the dam downstream to Cascades Picnic Area. Any actions taken by the National Park Service would be performed in accordance with stipulations in the park’s 1999 Programmatic Agreement, and the park would continue to consult with culturally associated American Indian tribes under this Programmatic Agreement and the cooperative agreement for traditional uses. Overall, Alternative 1 would result in a local, long-term, minor to moderate, adverse impact to ethnographic resources. Cumulative ImpactsCumulative impacts to ethnographic resources discussed
herein are based on analysis of past, present, and reasonably foreseeable
future actions within the main stem of the Ethnographic resources and their traditional cultural
associations have been lost or damaged in the Merced River region through past
development, visitor use, natural events, and widespread disruption of cultural
traditions. Nevertheless, In general, the ethnographic resources within the main
stem of the Reasonably foreseeable future actions proposed in the region that could have an adverse cumulative effect on ethnographic resources in the region include development-related projects, such as implementing the Yosemite Valley Plan. Under the Yosemite Valley Plan, traditional gathering areas would be disturbed due to the expansion of modern development into historic village areas. Overall, implementation of the Yosemite Valley Plan would have a local, long-term, adverse effect on ethnographic resources. The Merced River Plan provides a framework for
decision-making on future management actions within the An example of a reasonably foreseeable project that could
beneficially affect ethnographic resources in The cumulative projects within and in the vicinity of the main stem of the Merced River would result in a local, long-term, minor, adverse impact on ethnographic resources due to the disturbance of such resources. Alternative 1 and the cumulative projects within and in
the vicinity of the main stem of the ImpairmentAlthough ethnographic resources along the river are key
cultural resources within the Cultural Landscape Resources, Including Historic Sites and StructuresAnalysisCascades Diversion Dam is expected to eventually fail, resulting in the loss of this contributing feature to the Yosemite Hydroelectric Power Plant (also known as the Cascades Powerhouse). Uncontrolled dam failure could also damage downstream elements of the Yosemite Hydroelectric Power Plant, the Merced Canyon Travel Corridor, and the Coulterville Stage Road, resulting in an adverse impact to the cultural landscape. As described in Chapter VI, Consultation and Coordination, Cascades Diversion Dam has been the subject of previous evaluation and mitigation actions. In 1986, the National Park Service evaluated the impacts associated with the demolition, relocation, and/or rehabilitation of all components of the Yosemite Hydroelectric Power Plant. This evaluation included the removal of the dam. With the completion of this evaluation, the National Park Service completed the consultation process associated with Section 106 of the National Historic Preservation Act of 1966, as amended. As part of this process, the National Park Service signed a Memorandum of Agreement with the California State Historic Preservation Officer and the Advisory Council on Historic Preservation, which included stipulations for dam removal.[5] The National Park Service has complied with all stipulations of the Memorandum of Agreement and has sent a letter to the State Historic Preservation Officer indicating that the Section 106 consultation process is complete. In addition, any actions undertaken by the National Park Service (i.e., debris removal) would be performed in accordance with stipulations in the park’s 1999 Programmatic Agreement. Therefore, the adverse impact would be reduced to moderate. Nationally significant historic resources throughout the remainder of the Merced River corridor, such as designed landscapes and developed areas, historic buildings, and circulation systems (trails, roads, and bridges), would be unaffected. Summary of Alternative 1 Impacts. Uncontrolled dam failure would result in the loss of the dam, a cultural resource, and could cause damage to downstream elements of the Yosemite Hydroelectric Power Plant, the Merced Canyon Travel Corridor, and the Coulterville Stage Road, resulting in an adverse impact to the cultural landscape. Because demolition, relocation, and/or rehabilitation of all components of the Yosemite Hydroelectric Power Plant have been evaluated and the National Park Service has complied with all stipulation of the 1986 Memorandum of Agreement, and because any actions undertaken by the National Park Service (i.e., debris removal) would be performed in accordance with stipulations in the park’s 1999 Programmatic Agreement, the adverse impact would be reduced to moderate. Cumulative ImpactsCumulative impacts to cultural landscape resources
discussed herein are based on analysis of past, present, and reasonably
foreseeable future actions within the main stem of the Cultural landscape resources have been lost or damaged in
the Reasonably foreseeable future actions proposed in the
region that could affect cultural landscape resources include campground
rehabilitation projects, such as those identified in the Yosemite Valley Plan. The Yosemite
Valley Plan would remove, relocate, or modify historic buildings and
structures, and introduce modern facilities and development within historic
districts and contributing portions of the cultural landscape. The Yosemite Valley Plan calls for removal
of the five Cascades residences and garages north of Protection of river-related cultural resources that were
not intended to impede the free-flow of the The cumulative projects would result in a local, long-term, minor, adverse impact on the cultural landscape due to the disturbance of cultural landscape resources. Alternative 1 and the cumulative projects would result in a local, long-term, minor, adverse impact on such resources. ImpairmentAlthough the dam is a contributing element of the Yosemite
Hydroelectric Power Plant, the effect of this alternative on cultural landscape
resources would be primarily localized and would not be considered severe. In
addition, Alternative 1 would not change the treatment of cultural landscape
resources. Cultural landscape resources throughout the remainder of the Social ResourcesTransportationAnalysisUnder Alternative 1, the dam would be retained and would not affect transportation patterns. The 12-space parking lot northeast of the El Portal Road/Big Oak Flat Road intersection is used as a place for groups to meet and carpool to other areas of the park, for access to nearby rockclimbing locations, and for access to the intake structure, which is used as an informal viewing platform. Carpools reduce traffic volumes and thereby improve traffic flow; however, because of the number of parking spaces available and the low number of parked cars observed at the parking lot, carpool formation at this lot is very low. In addition, the parking area would continue to provide a location for park operations staging, emergency use by disabled vehicles, and for orientation to park destinations (i.e., consulting maps). Under Alternative 1, the continued use of the parking area would constitute a local, long-term, minor, beneficial impact to traffic flow. Use of the intake structure as a public vie Although no action would be taken under Alternative 1, the National Park Service would continue to make minor repairs to attendant structures such as the screenhouse, concrete platform, and safety railing on the concrete platform of the intake structure. Maintenance-related activities before eventual failure of the dam would result in temporary increases in traffic. Due to the intermittent and short-term duration of these activities, associated effects would be negligible. Thus, maintenance-related activities prior to dam failure would result in a local, short-term, negligible, adverse impact to traffic flow. Because no management action would be taken to repair or
remove the dam under Alternative 1, eventual uncontrolled failure of the
overflow portion of the dam structure would be expected. Dam failure could be
sudden or could occur over a course of years or decades. The timing and manner
of dam failure cannot be accurately predicted but could be catastrophic,
requiring an immediate emergency response and generating traffic associated
with law enforcement, fire, and emergency medical units, and the evacuation of
visitors. Such activities could require traffic control and rerouting. An
uncontrolled failure of the overflow portion of the structure or continued deterioration
of the dam over time would result in a release of concrete and timber debris
and grouted rockfill, which would litter the downstream channel of the Summary of Alternative 1 Impacts. Under Alternative 1, the continued use of the parking area would constitute a local, long-term, minor, beneficial effect on traffic flow. Under Alternative 1, continued use of the parking area by visitors for access to the intake structure would constitute a local, long-term, minor, adverse impact to traffic safety. Maintenance-related activities prior to dam failure under Alternative 1 would result in a local, short-term, negligible, adverse impact to traffic flow. Emergency response and debris removal activities under Alternative 1 would result in a local, short-term, minor to moderate, adverse impact to traffic flow. Cumulative ImpactsCumulative effects to transportation discussed herein are
based on analysis of past, present, and reasonably foreseeable future actions
in the Past, present, and reasonably foreseeable projects that
could have a long-term beneficial effect on transportation include the Yosemite Valley Plan and the Yosemite
Area Regional Transportation System. Since
1950, the population of Construction of some of the reasonably foreseeable
projects planned or approved within the Collectively, the cumulative projects discussed above
would have a local, long-term, major, beneficial impact on transportation
conditions along the ImpairmentImpairment is not addressed in the transportation analysis because this resource topic is peripheral to the protection of the park for future generations. Scenic ResourcesAnalysisUnder Alternative 1, the diversion dam, abutments, screenhouse,
and intake structure would remain in their present condition, without
maintenance or repair (other than ongoing safety repair). Use of the intake
structure as an informal viewing platform would continue. The dam and attendant
structures would continue to visually intrude on the scenic character of this
area of the river corridor and on views from the Following dam failure, sections of the structure would likely
gouge the banks and scour the river bottom downstream to the Cascades Picnic
Area, leaving substantial and highly visible evidence of damage to the banks
and to stands of vegetation. Under Alternative 1, it is assumed that debris
deposited in the channel by the dam’s continued deterioration or failure would
be removed by the National Park Service as soon as feasible. However, depending
on the time of year and river conditions, completion of debris cleanup could be
delayed for a period of months. Debris removal activities and equipment
transport could extend downstream to Cascades Picnic Area (or as far as El
Portal) and would temporarily increase the visual intrusion effects. Once
debris was removed from the river, damage to the riverbanks and vegetation
could continue to be visible for a period of years. The continued deterioration
of the dam, deposition of debris in the river from deterioration and subsequent
failure, and operation of equipment to remove and transport dam debris would
result in a local, short-term, minor, adverse effect on scenic resources in the
In the long term, failure of the dam would remove a
structure that is a source of visual intrusion upon the scenic character of the
Summary of Alternative 1
Impacts. Alternative 1 would result in a local, short-term, minor,
adverse impact to scenic resources within the Cumulative ImpactsCumulative impacts to scenic resources are based on
analysis of past, present, and reasonably foreseeable future actions in the Scenic resources have been affected by numerous past
actions. Primary among these is the alteration of natural communities by Euro-American
settlers. For example, agricultural activities and the development of tourism
resulted in the drying out of the Valley by breaching the moraine and
controlling naturally occurring fires, which affected vegetation patterns along
the Actions that could have a net beneficial cumulative effect
on scenic resources include those that improve the general health of ecosystems
that can be seen from or within the Merced River corridor (e.g., Eagle Creek
Merced River Ecological Restoration, and implementation of the Yosemite Valley Plan), and those
projects that could reduce the number of vehicles entering the park and
therefore the frequency of intrusion of vehicles into the scenic landscape
(e.g., the Yosemite Area Regional Transportation System). The Merced River Plan
prescribes the restoration of degraded areas of the Reasonably foreseeable projects that could have an adverse
effect on scenic resources include development-related projects (e.g., construction
of lodging at Yosemite Lodge and The cumulative projects within and in the vicinity of the Merced River corridor would result in a local, long-term, major, beneficial cumulative impact on scenic resources along the Merced River corridor because of the overall emphasis of these projects on restoring disturbed or developed land to natural conditions and improving the health of ecosystems. Alternative 1 and the cumulative projects within the ImpairmentAlternative 1 would result in a short-term adverse impact,
but a long-term beneficial impact to scenic resources within the RecreationAnalysisIn the near term, the dam would be retained and would not
affect recreation patterns. Sightseers use the intake structure of Cascades
Diversion Dam to view the river and rock formations of the Continued deterioration and eventual failure of the dam would deposit debris and sediment within the gorge and near Cascades Picnic Area (and potentially downstream to El Portal). Failure of the dam under Alternative 1 would affect river-dependent recreational uses downstream from the dam, including swimming, wading, and fishing. The National Park Service would employ early warning evacuation procedures for downstream areas in the event of an uncontrolled dam failure. However, depending on how and when dam debris is released, people recreating in the river could be exposed to debris washing downstream following failure, and localized flooding, potentially resulting in serious injuries or fatalities. The potential for injury and/or fatality in the event of dam failure would constitute a local, short-term, moderate, adverse impact to active recreational activities in the immediate vicinity of the dam as well as downstream to Cascades Picnic Area. In addition, debris deposited in the river channel and increased sedimentation following dam failure, and subsequent bank erosion, could temporarily degrade water quality and alter water flows, adversely affecting river conditions that currently support active recreational pursuits (e.g., swimming and fishing) and passive recreational activities (e.g., sightseeing and photography) downstream to the Cascades Picnic Area, and potentially to El Portal. Thus, due to these effects on water quality and flows, dam failure would result in a local, short-term, moderate, adverse impact to active recreational activities downstream from the dam to the Cascades Picnic Area. Following failure of the dam under Alternative 1, nearby parking areas and trails from the dam area to Cascades Picnic Area would likely be obstructed or closed temporarily, resulting in delays and restrictions on use. Cleanup tasks under Alternative 1 likely would extend from the dam area to the Cascades Picnic Area (and potentially to El Portal) due to the uncontrolled transport of dam debris in the river, and could halt or curtail recreational activities and access for an extended period of time in this reach. Accordingly, Alternative 1 would result in a local, short-term, minor, adverse effect on recreational access to areas between the dam and the Cascades Picnic Area. Summary of Alternative 1
Impacts. The potential for injury and/or fatality due to
pedestrian hazards on Cumulative ImpactsCumulative impacts on recreation are based on analysis of
past, present, and reasonably foreseeable future actions in the Reasonably foreseeable projects that could have a beneficial
cumulative effect on recreational opportunities include projects proposed under
the Yosemite Valley Plan, such as
development of a new visitor center in The Merced River Planwould have a beneficial cumulative effect on recreation in the The cumulative projects would have a local, long-term,
moderate, beneficial effect on recreation due to expanded recreational
opportunities in the Alternative 1 and the cumulative projects in the ImpairmentAlternative 1 would result in local, short-term, minor to moderate,
adverse impacts on river-related recreational activities due to ongoing safety
hazards at the dam, and continued deterioration and eventual dam failure.
Although the Interpretation and OrientationAnalysisIn the near term, the dam would be retained and would not
affect interpretation and orientation. Dam failure would release dam debris and
could result in downstream bank erosion, which could undermine Summary of Alternative 1 Impacts. Dam failure under Alternative 1 could affect orientation and interpretation opportunities, resulting in a local, short-term, negligible to minor, adverse impact to orientation (i.e., roadside signs) and interpretation opportunities downstream from the dam to the Cascades Picnic Area. Cumulative ImpactsCumulative impacts on orientation and interpretation are
based on analysis of past, present, and reasonably foreseeable future actions
in the Reasonably foreseeable projects that could have a
beneficial cumulative effect on orientation and interpretation opportunities
include projects proposed under the Yosemite
Valley Plan, such as development of a new visitor center in The Merced River Planwould have a beneficial cumulative effect on orientation and interpretation
in the The cumulative projects would have a local, long-term,
minor, beneficial effect due to expanded orientation and interpretation
opportunities in the Alternative 1 and the cumulative projects in the ImpairmentDam failure under Alternative 1 would result in local,
short-term, negligible to minor, adverse impacts on orientation and
interpretation opportunities. Although orientation and interpretation
opportunities within the SocioeconomicsAnalysisBecause no active management action would be taken to repair or remove the dam under Alternative 1, eventual uncontrolled failure of the overflow portion of the dam structure would be expected. Debris removal would necessitate additional spending on labor and equipment and would therefore temporarily increase employment, income, and taxable retail sales. The amount of spending is unknown and would depend on the amount of labor and types of equipment needed; however, this spending would likely be greater than spending for planned, controlled dam removal. Due to the uncertainty regarding the magnitude and timing of project-related equipment spending associated with potential dam failure, economic impacts cannot be definitively projected. Given currently available information, however, it is expected that Alternative 1 would have a regional, short-term, negligible, beneficial impact on the socioeconomy due to the temporary nature of the dam debris removal activity and the small magnitude of spending for debris removal compared with the size of the construction industry in the affected region. Uncontrolled dam failure would also likely result in damage to natural resources or to local utilities infrastructure downstream; however, these property damages are not readily quantifiable. The effects on natural resources and park operations are addressed under those resource topics. Summary of Alternative 1 Impacts. Due to the uncertainty regarding the magnitude and timing of project-related equipment spending associated with potential dam failure, economic impacts cannot be definitively projected. Given currently available information, however, it is expected that Alternative 1 would have a regional, short-term, negligible, beneficial impact on the socioeconomy due to the temporary nature of the dam debris removal activity and the small magnitude of spending for debris removal compared with the size of the construction industry in the affected region. Cumulative ImpactsCumulative socioeconomic impacts discussed herein are
based on analysis of reasonably foreseeable future actions in the Reasonably foreseeable future projects by the National Park Service that could have a beneficial cumulative effect on the regional economy are related to construction activity, including the Yosemite Valley Plan; Cook’s Meadow Ecological Restoration; Curry Village Employee Housing; Eagle Creek Merced River Ecological Restoration; Ecological Restoration of Flood-Damaged Campgrounds; the El Portal Road Improvement Project (from Cascades Diversion Dam to Pohono Bridge); Happy Isles Dam Removal; Happy Isles Fen Habitat Reclamation Project; Happy Isles Gauging Station Bridge Removal; Happy Isles to Vernal Falls Trail Reconstruction; Improvements to Curry Village and East Yosemite Valley Campgrounds; Lower Yosemite Fall Project; Replacement/Rehabilitation of Yosemite Valley Main Sewer Line; Utilities Master Plan; Yosemite Lodge Area Redevelopment; and Yosemite Valley Shuttle Bus Stop Improvements. Private development projects such as the Yosemite Motels Expansion just outside of the park entrance at El Portal and other facilities constructed under the Mariposa County General Plan Update would also result in short-term spending in the construction sector. The cumulative planning, transportation, facility improvement, and development-related projects identified would generate construction-related output, employment, and income in the regional economy. Construction spending associated with the cumulative projects would also generate secondary output impacts as a result of local spending on materials and wages. Some of this construction-related spending would be expected to occur outside of the affected region. The total construction-related spending in the affected region associated with the cumulative projects would have a short-term, major, beneficial impact on the regional economy resulting from the substantial and highly noticeable increase in construction output, income, and employment in the three-county region. The increase in construction-related output would be highly noticeable when compared with the total output of the construction industry in the affected region. Some projects would increase the amount of visitor
overnight accommodations in or near the park, including the Improvements to The cumulative projects within and in the vicinity of Yosemite National Park would result in a local, long-term, negligible, beneficial impact to the regional economy, and a local, short-term, major, beneficial impact during construction. Alternative 1 would contribute to this effect. ImpairmentImpairment is not addressed in the socioeconomics analysis because this resource topic is peripheral to the protection of the park for future generations. Park OperationsAnalysisUnder Alternative 1, Cascades Diversion Dam would remain in place without maintenance or repair; however, the National Park Service would continue to make minor repairs to attendant structures such as the screenhouse, concrete platform, and safety railing on the river-right dam intake structure. The dam would continue to deteriorate and eventually fail, likely during high-flow conditions. The failed dam could divert riverflows, resulting in substantial erosion on both riverbanks as well as other adverse impacts to the river between the dam and Cascades Picnic Area (and potentially to El Portal). The timing and manner of dam failure cannot be predicted but could be catastrophic, requiring immediate emergency response from law enforcement, fire, and emergency medical units and the evacuation of park visitors. Park operations staff would be required to remove the dam debris as soon as feasible, under emergency conditions, and repair facilities damaged by dam debris. Excess erosion on the river-right bank would threaten a
wastewater line (which extends between El Portal and Yosemite Valley) and
electrical conductors (which extend from a substation at the Cascades
Powerhouse to a substation in Yosemite Valley), both of which are located under
El Portal Road. Damage to these utilities would interrupt wastewater and
electrical service in Dam failure could result in a short-term (immediate) and
dramatic increase in demand for the full range of park operations and emergency
response staff to remove dam debris and repair damaged facilities downstream
from the dam to the Cascades Picnic Area (and potentially to El Portal), a
local, short-term, moderate to major, adverse impact. In addition, uncontrolled
failure of the dam could damage the wastewater line and electrical conductors
for Under Alternative 1, the parking area north of the dam would continue to serve as a park operations staging area, and the public telephone at the parking area would continue to be available to visitors for access to the park’s emergency response system. Under Alternative 1, continued use of the parking area and public telephone would constitute a local, long-term, minor, beneficial impact to park operations. Summary of Alternative 1
Impacts. Dam failure could result in a short-term (immediate) and
dramatic increase in demand for the full range of park operations and emergency
response staff to respond to evacuation and medical emergencies, remove dam
debris, and repair damaged facilities downstream from the dam to the Cascades
Picnic Area, a local, short-term, moderate to major, adverse impact. In
addition, uncontrolled failure of the dam could damage the wastewater line and
electrical conductors for Cumulative ImpactsCumulative effects on park operations and facilities are
based on analysis of past, present, and reasonably foreseeable future actions
in the Park operations and facilities have been affected by numerous National Park Service management decisions and projects since the inception of the park, including the implementation of Merced River Plan and Yosemite Valley Plan. Overall, there is no net adverse or beneficial effect of these past actions on park operations and facilities. Actions that would have a beneficial cumulative effect on park operations and facilities are those that could reduce the number of visitors entering the park, reduce the number or amount of facilities within the park, or reduce long-term maintenance activities. An example is the Replacement/Rehabilitation of the Yosemite Valley Main Sewer Line. Although this project would have a short-term, adverse effect associated with planning, construction, replacement, and rehabilitation, its overall effect would be to reduce long-term maintenance. Therefore, this project would have a long-term, beneficial cumulative impact on park operations and facilities. Examples of projects that would have an adverse effect on
park operations and facilities are the Merced River Plan and the Yosemite Valley Plan. Implementation of
these plans would substantially increase demand on park operations and
facilities in the short term during planning, repair, rehabilitation,
construction/demolition, development of the Visitor Experience and Resource
Protection framework, and replacement of facilities (e.g., construction of new
campsites and restoration of large areas of Overall, the past, present, and reasonably foreseeable future actions would have a local, moderate, adverse cumulative impact because of the increased demand on park operations, services, and facilities, over both the short and long term. These cumulative effects, in combination with Alternative 1, would result in a local, short- and long-term, moderate to major, adverse impact on park operations and facilities, depending upon the nature and extent of damage to facilities. ImpairmentPark operations are not subject to the impairment standard. The National Park Service has a management responsibility to conserve the scenery and natural and historic objects and the wildlife therein; park operations are not included within this management responsibility. Alternative 2 – Complete Dam RemovalAlternative 2 includes complete removal of the dam, the dam abutments, the intake structure, and the screenhouse, and restoration of the related river channel located beneath the dam site (see figure II-3). Approximately 4,400 to 5,400 cubic yards of sediments (including rocks and boulders) in the area upstream of the dam would be excavated and repositioned to stabilize the river-right bank and decrease the potential for sediment erosion. Figure II-4 indicates the river profile at Cascades Diversion Dam before and after removal of the dam structure and sediments. Natural river processes would continue to transport remaining sediments (up to a maximum range of 9,600 to 15,600 cubic yards of sediment) from the impoundment area over time, allowing for a gradual re-establishment of the natural river channel and related riparian habitat. It is expected that the river would fully recover incrementally over time as sediments are transported from the impoundment area. However, the rate of natural channel recovery and restoration would be monitored to determine if additional restoration actions were necessary. Following removal of the river-right abutment, intake structure, and screenhouse, the river-right bank would be stabilized using a bioengineered bank stabilization system to prevent erosion of the river-right bank. The objective of this alternative would be to restore the natural river character with a mixture and distribution of boulders, cobbles, gravels, sand, silt, soil, and vegetation similar to those found in adjacent riverbank segments. Natural ResourcesGeology, Geologic Hazards, and SoilsAnalysisDam removal would have a short-term, adverse, removal-related
impact to soils along the riverbank adjacent to the dam, at the parking lot
north of In the event of a rockfall under Alternative 2, boulders and talus could be deposited into the river reach in the former dam location, but would not damage structures. Rockfall material could redirect riverflows, but given that the channel would down-cut and water velocity would increase through a narrower, rock-lined channel with river-right bank stabilization, the potential for bank scour adjacent to the road would decrease, and Alternative 2 would have a local, long-term, minor, beneficial effect on public health and safety. Under Alternative 2, the deposition of sediment following dam removal would benefit floodplain soils by providing a sediment source that would eventually settle out along the river and provide a substrate for development of a soils horizon. However, the amount of sediment that would be released from behind the dam would be relatively insignificant compared to the overall area of expected deposition (the braided river reach at Cascades Picnic Area) and the amount of sediment that is typically transported downstream on a continual basis. The contribution of sediment available for soil development in the area of the dam and reservoir would be considered negligible. Soil resources throughout the remainder of the Summary of Alternative 2
Impacts. Dam removal would result in a short-term impact to soils
related to ground disturbance activities. However, dam removal activities would
occur in a controlled manner, with the application of best management
practices. Since Alternative 2 would avoid the more extensive adverse effects described
under Alternative 1 (i.e., bank destabilization, erosion, and soil compaction
and loss due to uncontrolled dam failure and debris retrieval activities),
Alternative 2 would have a local, short-term, minor, beneficial effect on soil
resources compared to Alternative 1. Site restoration and stabilization would
reduce the potential for erosion and sedimentation, help stabilize channel
shape and slopes, repair banks, and increase the protection of riverbanks, the
adjacent roadway, and utility lines under Cumulative ImpactsThe cumulative impact analysis for geology under Alternative 2 is the same as described under the No Action Alternative. See the discussion of cumulative effects under Alternative 1. Past, present, and reasonably foreseeable future actions would result in a long-term, minor, beneficial cumulative impact to soil resources and to public health and safety with respect to geologic hazards. Alternative 2 and the cumulative projects would result in a local, long-term, minor, beneficial impact to soil resources and public safety with respect to geologic hazards. Alternative 2 would avoid the more extensive adverse effects of soil erosion and bank destabilization compared to Alternative 1. Overall, the cumulative projects would restore soils in the project region, reduce soil degradation, and decrease the density of people and facilities in the talus slope zone. ImpairmentAlternative 2, with the application of best management practices, would result in beneficial effects on soil resources. Alternative 2 would not impair geologic resources. Hydrology, Floodplains, and Water QualityAnalysisUnder Alternative 2, removal of Cascades Diversion Dam would
improve riverflow dynamics and hydrologic processes by restoring the
free-flowing condition of the Removal of Cascades Diversion Dam under Alternative 2 would cause minor amounts of fine-grained sediment originating from behind the dam to be released into the river. Sediment sources include riverbed materials dislodged during excavation of sediments, installation of the diversion channel, and diversion of flows between the river-right and river-left; concrete dust generated and friable concrete dislodged during dam removal activities; and timber fragments. Fine-grained materials (less than 0.07 millimeter in diameter) make up less than 1% of impounded materials and would not cause excessive turbidity downstream. The sediment dislodged during removal activities is expected to impact water quality only temporarily within a localized area, and the sediment would settle out downstream in areas of low energy, particularly considering that dam removal activities would take place during periods of low flow. Sediment excavation and dam removal would occur in a controlled manner (e.g., within a delineated work area, during low-flow conditions, and with the application of best management practices [see Chapter II, Alternatives]). Measures to control sediment sources using barriers would serve to capture the majority of sediment released during dam removal. Following dam removal, some portion of the remaining impounded sediments (approximately 9,600 to 15,000 cubic yards, including rocks/boulders) would wash downstream. As described under Alternative 1, some portion of impounded sediment and materials would likely remain following dam removal. Downstream sediment transport and evolution of natural channel dynamics under Alternative 2 is the same as described under Alternative 1; however, the total amount of sediment available for downstream transport would be reduced. Further, dam-related debris would not be washed downstream; and Alternative 2 would avoid the water quality and turbidity impacts associated with downstream erosion and with riverbed and bank disturbance during dam debris retrieval that would occur under Alternative 1. Therefore, Alternative 2 would have a local, short-term, moderate, beneficial effect on water quality compared to Alternative 1. Water quality could be compromised if petroleum components were discharged from heavy equipment. The proposed best management practices implemented under this alternative, as described in Chapter II, Alternatives, would ensure that petroleum releases from heavy equipment were minimized within the dam removal area. Although there are potential sources of pollutants (i.e., petroleum products) associated with the dam removal phase of this project, removal of the dam would avoid the more extensive adverse effects of erosion. As a result, Alternative 2 would have a local, short-term, minor, beneficial effect on water quality compared to Alternative 1. Summary of Alternative 2 Impacts. Dam removal would have a short-term water quality impact related to the discharge of petroleum components. However, dam removal activities would occur in a controlled manner, with the application of best management practices. Compared to Alternative 1, Alternative 2 would have local, short- and long-term, minor to moderate, beneficial impact on hydrologic processes and water quality by avoiding bank erosion and localized flooding associated with continued deterioration and eventual dam failure, reducing sedimentation, and controlling removal of the dam. Cumulative ImpactsThe cumulative impacts assessment for hydrologic resources under Alternative 2 is the same as under Alternative 1. The cumulative projects would result in an overall local, long-term, minor, beneficial impact to hydrologic processes and water quality. The past, present, and reasonably foreseeable future projects considered cumulatively with Alternative 2 would result in a local, long-term, minor, beneficial impact on hydrologic processes. The beneficial impacts associated with Alternative 2 would nominally contribute to overall beneficial cumulative impacts on hydrologic processes and water quality. ImpairmentAlternative 2, with the application of best management practices, would have a local, short- and long-term, minor to moderate, beneficial impact on hydrologic processes and water quality. Alternative 2 would not impair hydrologic resources. WetlandsAnalysisDam removal would have short-term, adverse, removal-related effects on approximately four acres of wetland and aquatic habitat. Effects would be related to heavy equipment and dam removal activities and could include soil compaction, dust, vegetation removal, root damage, erosion, and introduction and spread of non-native species. The addition of silt, the resuspension of sediment, or the introduction of pollutants (i.e., fuels, lubricants) related to dam removal operations could temporarily degrade the quality of native wetland and aquatic habitats in the immediate vicinity of the dam. The application of mitigation measures described in Chapter II, Alternatives (e.g., best management practices) would reduce the potential adverse impacts to wetland and aquatic habitats to a negligible intensity. Because dam removal would occur in a controlled manner, with the application of mitigation measures, Alternative 2 would have local, short-term, negligible, beneficial effect on wetland and aquatic habitat compared to Alternative 1. Removal of Cascades Diversion Dam would restore the free-flowing
condition of the Summary of Alternative 2
Impacts. Dam removal activities would have a short-term impact to
wetland and aquatic habitat resources associated with ground disturbance and
the potential introduction of pollutants. However, dam removal activities would
occur in a controlled manner, with the application of mitigation, reducing the
adverse effect to a negligible intensity. Therefore, Alternative 2 would have a
local, short-term, negligible, beneficial effect on wetland and aquatic habitat
compared to Alternative 1. Removal of Cascades Diversion Dam would restore the
free-flowing condition of the Cumulative ImpactsThe cumulative impact analysis for wetland resources under
Alternative 2 is the same as described under the No Action Alternative. See the
discussion of cumulative effects under Alternative 1. Past, present, and
reasonably foreseeable future projects in combination with Alternative 2 would
have a net long-term, major, beneficial effect on wetland patterns within the ImpairmentGiven the incorporation of mitigation into the design of this alternative, Alternative 2 would result in a local, short- and long-term, negligible to moderate, beneficial impact to wetland and aquatic resources. Alternative 2 would not impair wetland resources or values. VegetationAnalysisDam removal would have short-term, adverse, removal-related effects to riparian and aquatic habitat. Effects would be related to heavy equipment and dam removal activities and could include soil compaction, dust, vegetation removal, root damage, erosion, and introduction and spread of non-native species. The addition of silt, the resuspension of sediment, or the introduction of pollutants (i.e., fuels, lubricants) related to dam removal operations could degrade the quality of native vegetation. The application of mitigation measures described in Chapter II, Alternatives (e.g., best management practices) would reduce these potential short-term adverse dam removal impacts to vegetation to a negligible intensity. Because dam removal would occur in a controlled manner, with the application of mitigation measures, Alternative 2 would result in a local, short-term, negligible, beneficial impact to vegetation compared to Alternative 1. Removal of Cascades Diversion Dam would restore the
free-flowing condition of the Summary of Alternative 2
Impacts. Dam removal activities would have a short-term impact to
vegetation associated with ground disturbance and the potential introduction of
pollutants. However, dam removal activities would occur in a controlled manner,
with the application of mitigation, reducing the adverse effect to a negligible
intensity. Therefore, Alternative 2 would have a local, short-term, negligible,
beneficial effect on vegetation compared to Alternative 1. Removal of Cascades
Diversion Dam would restore the free-flowing condition of the Cumulative ImpactsThe cumulative impact analysis for vegetation under
Alternative 2 is the same as described under the No Action Alternative. See the
discussion of cumulative effects under Alternative 1. Past, present, and
reasonably foreseeable future projects in combination with Alternative 2 would
have a net long-term, major, beneficial effect on vegetation patterns within
the ImpairmentGiven the incorporation of mitigation into the design of this alternative, Alternative 2 would result in a local, short- and long-term, negligible to moderate, beneficial impact to vegetation. Alternative 2 would not impair vegetation resources or values. WildlifeAnalysisLocalized, minor, short-term, temporary effects on native fish and wildlife could occur during dam removal. Effects would be related to heavy equipment and human intrusion and could include increased dust, vegetation removal, noise, or decreased oxygen levels. Dam removal would have short-term, adverse, removal-related effects on wildlife habitat. These actions could result in direct losses of nests or burrows, and indirect effects through the disturbance of nesting birds or roosting bats. The addition of silt, the resuspension of sediment, or the introduction of pollutants (i.e., fuels, lubricants) could degrade the quality of the aquatic environment and the wildlife habitat it provides. The application of mitigation measures described in Chapter II, Alternatives (e.g., best management practices) would reduce the potential adverse impacts to native fish and wildlife to a negligible intensity. Because dam removal would occur in a controlled manner, with the application of mitigation measures, Alternative 2 would result in a short-term, negligible, beneficial impact to wildlife compared to Alternative 1. Removal of Cascades Diversion Dam would restore the
free-flowing condition of the Summary of Alternative 2
Impacts. Dam removal activities would have a short-term impact to
wildlife associated with ground disturbance and the potential introduction of
pollutants. However, dam removal activities would occur in a controlled manner,
with the application of mitigation, reducing the adverse effect to a negligible
intensity. Therefore, Alternative 2 would have a local, short-term, negligible,
beneficial effect on wildlife compared to Alternative 1. Removal of Cascades
Diversion Dam would restore the free-flowing condition of the Cumulative ImpactsThe cumulative impact analysis for fish and wildlife under
Alternative 2 is the same as described under the No Action Alternative. See the
discussion of cumulative effects under Alternative 1. Past, present, and
reasonably foreseeable future projects in combination with Alternative 2 would
have a net long-term, minor to moderate, beneficial effect on fish and wildlife
patterns within the ImpairmentGiven the incorporation of mitigation into the design of this alternative, Alternative 2 would result in a local, short- and long-term, negligible to moderate, beneficial impact to native fish and wildlife. Alternative 2 would not impair fish and wildlife resources or values. Special-Status SpeciesAnalysisSpecial-status species known or likely to occur in the
immediate vicinity of the project area include Wawona riffle beetle, harlequin
duck, Wawona Riffle Beetle and Harlequin Duck. Dam removal would have localized, short-term, minor, adverse effects on Wawona riffle beetle and harlequin duck. Effects would be related to heavy equipment and human intrusion and could include vegetation removal, decreased oxygen levels, the addition of silt, resuspension of sediment, or the introduction of pollutants (i.e., fuels, lubricants). These actions could result in direct losses of individuals or habitat for Wawona rifle beetle and harlequin duck downstream of the dam in the gorge and El Portal river segments. The application of mitigation measures described in Chapter II, Alternatives (e.g., best management practices) would reduce the potential adverse impacts to Wawona riffle beetle and harlequin duck to a negligible intensity. Because dam removal would occur in a controlled manner, with the application of mitigation measures, Alternative 2 would result in a local, short-term, minor, beneficial impact to Wawona riffle beetle and harlequin duck compared to Alternative 1. Removal of Cascades Diversion Dam would restore the
free-flowing condition of the Special-Status Species of
Bats and Summary of Alternative 2
Impacts. Dam removal activities would have a short-term impact to
special-status species associated with ground disturbance and the potential
introduction of pollutants. However, dam removal activities would occur in a
controlled manner, with the application of mitigation, reducing the adverse
effect to a negligible intensity. Therefore, Alternative 2 would have a local,
short-term, negligible, beneficial effect on special-status species compared to
Alternative 1. Removal of Cascades Diversion Dam and revegetation would
restore the free-flowing condition of the Merced River and return this portion
of the river to a more natural state, thereby enhancing the biological
integrity of this segment for Wawona rifle beetle and harlequin duck, resulting
in a local, long-term, minor, beneficial effect on habitat for Wawona riffle
beetle and harlequin duck at this location. The bioengineered bank
stabilization and revegetation would have a local, long-term, negligible to
minor, beneficial effect on habitat for special-status bats and Cumulative ImpactsThe cumulative impact analysis for special-status species
under Alternative 2 is the same as described under the No Action Alternative. See
the discussion of cumulative effects under Alternative 1. Past, present, and
reasonably foreseeable future projects in combination with Alternative 2 would
have a net long-term, moderate, beneficial effect on habitat for special-status
species within the ImpairmentGiven the incorporation of mitigation into the design of this alternative, Alternative 2 would result in a local, short- and long-term, negligible to minor, beneficial impact to special-status species. Alternative 2 would not impair special-status species. Air QualityAnalysisUnder Alternative 2, air quality effects would relate primarily
to temporary use of equipment, dust, stockpiling, and vehicle travel, as well
as tailpipe emissions from equipment. Alternative 2 would avoid the more
extensive adverse effects of debris retrieval activities on air quality
described under Alternative 1, because dam removal and sediment excavation
would occur under controlled conditions and in a limited area. Dam removal and
sediment excavation under the action alternatives would occur over a five-month
period and would increase traffic volumes and associated tailpipe emissions on
El Portal Road during transport of heavy-duty equipment, worker commute trips,
and truck trips to haul debris materials. Emissions would be generated by truck
trips required to remove excavated rock, concrete, and timber over the course
of the two to three months of in-channel removal. Moreover, the staging area on
the paved travel lanes of Summary of Alternative 2 Impacts. Dam removal activities would result in a short-term air quality impact associated with vehicle emissions. Because dam removal and sediment excavation would occur in a controlled manner and would include the application of best management practices, Alternative 2 would avoid the more extensive adverse effects of debris retrieval activities on air quality described under Alternative 1. Therefore, Alternative 2 would have a local, short-term, negligible, beneficial effect on air quality compared to Alternative 1. Alternative 2 would not result in a long-term impact to air quality. Cumulative ImpactsThe cumulative impact analysis for air quality under Alternative 2 is the same as described under Alternative 1. See the discussion of cumulative effects under Alternative 1. Cumulative actions would have a local, long-term, minor, beneficial impact on air quality. Alternative 2 and the cumulative projects would result in a local, long-term, minor, beneficial impact on air quality. Dam removal under Alternative 2, as compared to Alternative 1, would contribute to beneficial impacts on air quality in the short term. ImpairmentImpairment is not addressed in the air quality analysis because this resource topic is peripheral to the protection of the park for future generations. NoiseAnalysisDam removal under Alternative 2 would occur over a five-month period. Earthmoving activities (in-channel removal) would generate the highest noise levels and would occur over a two- to three-month period. Material haul trips would also raise ambient noise levels along haul routes. Operation of heavy-duty equipment at the site during dam removal and sediment excavation would generate high noise levels. Table IV-1 provides typical noise levels generated by various heavy-duty equipment. Equipment operation could generate substantial amounts of noise and would occur within close proximity to river-related recreational uses. Other sensitive land uses (e.g., Cascades Picnic Area), located farther from the site, would be affected to a lesser extent. Noise effects in the project area would vary depending upon a number of factors, such as the number and types of equipment in operation on a given day, usage rates, the level of background noise in the area, and the distance between sensitive uses and dam removal activities. Alternative 2, however, would avoid the more extensive adverse effects of debris retrieval activities on the ambient noise environment described under Alternative 1 by working within a delineated area, under controlled conditions, and according to a planned timeline. In addition, best management practices described in Chapter II, Alternatives (e.g., limiting hours of dam removal operations, and shielding or muffling equipment) would reduce noise impacts from equipment associated with removal activities. Therefore, Alternative 2 would have a local, short-term, negligible, beneficial effect on the ambient noise environment compared to Alternative 1. Summary of Alternative 2 Impacts. Dam removal activities would result in short-term noise impacts associated with equipment operation. Because dam removal would occur in a controlled manner and would include the application of best management practices, Alternative 2 would avoid the more extensive adverse effects of noise generated by debris retrieval activities on the ambient noise environment described under Alternative 1. Therefore, Alternative 2 would have a local, short-term, negligible, beneficial effect on the ambient noise environment compared to Alternative 1. Cumulative ImpactsThe cumulative impact analysis for noise under Alternative 2 is the same as described under the No Action Alternative. See the discussion of cumulative impacts under Alternative 1. The cumulative actions would result in a local, long-term, minor, adverse effect on the noise environment. The local, short-term, negligible, beneficial effect under Alternative 2 would not improve this cumulative effect and, overall, Alternative 2 and the cumulative projects would result in a local, long-term, minor, adverse effect on the noise environment. ImpairmentImpairment is not addressed in the noise analysis because this resource topic is peripheral to the protection of the park for future generations. Cultural ResourcesArcheological ResourcesAnalysisThere are no known archeological resources located within
the project area or in the vicinity of the dam. Further, the project area has
been disturbed by past actions associated with dam, roadway, and utilities
construction, from ongoing maintenance of those facilities, and from ongoing
use of Pohono Quarry for staging and materials storage for park projects. Under
Alternative 2, dam removal activities would require staging and operation along
the banks of the Summary of Alternative 2 Impacts. Ground-disturbing activities under Alternative 2 could have a local, long-term, minor, adverse impact to as-yet unknown archeological resources. Any actions would be performed in accordance with stipulations in the park’s 1999 Programmatic Agreement. Cumulative ImpactsBecause it is not anticipated that this alternative would
lead to disturbance of archeological resources, the cumulative impact analysis
for archeological resources in Alternative 2 is the same as described under Alternative
1. See the discussion of cumulative impacts under Alternative 1. Alternative 2
and the cumulative projects within and in the vicinity of the main stem of the ImpairmentAlthough archeological sites along the river are key
cultural resources within the Ethnographic ResourcesAnalysisThere are potential ethnographic resources within the
project area, consisting of plant material traditionally gathered for basketry,
food, ceremonies, insect repellant, etc. Under Alternative 2, dam removal
activities would require staging and operations along the banks of the Summary of Alternative 2 Impacts. Ground-disturbing activities under Alternative 2 could have an adverse impact to ethnographic resources. Any actions would be performed in accordance with stipulations in the park’s 1999 Programmatic Agreement, such as disturbance avoidance or culturally sensitive design measures. In addition, the park would continue to consult with culturally associated American Indian tribes under this Programmatic Agreement and the cooperative agreement for traditional uses. Therefore, Alternative 2 would result in a local, long-term, negligible, adverse impact to ethnographic resources. Cumulative ImpactsThe cumulative impact analysis for ethnographic resources under Alternative 2 is the same as described under Alternative 1. See the discussion of cumulative effects under Alternative 1. The cumulative projects within and in the vicinity of the
main stem of the Merced River would result in a local, long-term, minor,
adverse impact on ethnographic resources due to the disturbance of such
resources. Alternative 2 and the cumulative projects within and in the vicinity
of the main stem of the ImpairmentAlthough ethnographic resources along the river are key
cultural resources within the Cultural Landscape Resources, Including Historic Sites and StructuresAnalysisUnder Alternative 2, the potential for degradation of cultural landscape resources would be similar to that described under Alternative 1, since the Cascades Diversion Dam would no longer exist under either alternative. The primary difference is that the dam would be removed in a controlled manner under Alternative 2, compared to uncontrolled failure under Alternative 1. The controlled removal of the dam under Alternative 2, performed in accordance with stipulations in the park’s 1999 Programmatic Agreement (see best management practices described in Chapter II, Alternatives), would avoid downstream impacts to other elements of the Yosemite Hydroelectric Power Plant, the Merced Canyon Travel Corridor, and the Coulterville Stage Road. Cascades Diversion Dam is a historic resource that
contributes to the cultural landscape as an element of the Yosemite Hydroelectric
Power Plant. Removal of the dam would constitute a local, long-term, major,
adverse impact on a historic property. However, at the present time, the dam is
in a significantly deteriorated condition, is an unnatural obstruction in the Nationally significant historic resources throughout the remainder of the Merced River corridor, such as designed landscapes and developed areas, historic buildings, and circulation systems (trails, roads, and bridges), would be unaffected. Summary of Alternative 2 Impacts. Controlled removal of Cascades Diversion Dam would occur in compliance with stipulations in the park’s 1999 Programmatic Agreement, and compliance with the 1986 Memorandum of Agreement is already complete; therefore, Alternative 2 would result in a local, long-term, moderate, adverse impact to the cultural landscape. Cumulative ImpactsThe cumulative impact analysis for cultural landscape resources under Alternative 2 is the same as described under Alternative 1. See the discussion of cumulative effects under Alternative 1. The cumulative projects would result in a local, long-term, minor, adverse impact on the cultural landscape due to the disturbance of cultural landscape resources. Alternative 2 and the cumulative projects would result in a local, long-term, minor, adverse impact on such resources. ImpairmentAlthough the dam is a contributing element of the Yosemite
Hydroelectric Power Plant, the effect of this alternative on cultural landscape
resources would be primarily localized and would not be considered severe. In
addition, Alternative 2 would not change the treatment of cultural landscape
resources. Cultural landscape resources throughout the remainder of the Social ResourcesTransportationAnalysisAlternative 2 would remove Cascades Diversion Dam and
attendant structures, including the intake structure, currently used as an
informal river-viewing platform, but the paved parking area and public
telephone across Alternative 2 would avoid the more extensive adverse
effects of debris retrieval activities on traffic flow described under
Alternative 1 because dam removal would occur under controlled conditions and in
a limited area. Dam removal activities under Alternative 2 would occur over a five-month
period and would increase traffic volumes on El Portal Road from transport of
heavy-duty equipment, worker commute trips, and truck trips to haul debris
materials. Truck trips would be required to remove excavated rock, concrete,
and timber over the course of the two to three months of in‑channel removal.[7] There would be an estimated 260 truck loads of
debris transported over the course of the project. In addition, project
activities would generate additional truck trips between the dam and the
secondary staging area at Pohono Quarry for the transport of equipment used
infrequently during project activities and for transport of material removed
from the dam. The number of truck trips on area roadways at any one time would
vary, as the trips would be spread over the days of the construction period,
and over the hours of the work days; a maximum of about 35 truck loads per day
is expected. Moreover, by making use of the paved parking area north of the dam,
two-way traffic on of the dam would have to find alternative locations. This would result in a negligible increase in conflicts, because very few visitors use this parking area during the fall, when dam removal would occur.) In addition, best management practices would be utilized to reduce transportation effects and would be made conditions of agreements with contractors. These practices are listed in Chapter II, Alternatives, and are common to all action alternatives. Generally, these practices include implementation of a traffic control plan, which would include measures (e.g., advance warning signs, flaggers to direct traffic, and advance notification of visitors about the location, timing, and duration of dam removal activity) to maintain safe and efficient traffic flow during the dam removal period. Controlled dam removal using best management practices under Alternative 2 would have a local, short-term, negligible, beneficial impact on traffic flow compared to Alternative 1. Summary of Alternative 2 Impacts. Under Alternative 2, avoidance of the traffic conflict to sightseers would result in a local, long-term, minor, beneficial impact compared to Alternative 1. Dam removal activities would result in a short-term increase in vehicle trips in the project area. Controlled dam removal using best management practices (see Chapter II, Alternatives) under Alternative 2 would have a local, short-term, negligible, beneficial impact on traffic flow compared to Alternative 1. Cumulative ImpactsThe cumulative impact analysis for transportation under Alternative 2 is the same as described under Alternative 1. See the discussion of cumulative effects under Alternative 1. The cumulative projects would have a local, long-term,
major, beneficial impact on transportation conditions along the Alternative 2 and the cumulative projects would result in
a local, long-term, major, beneficial impact on transportation conditions along
the ImpairmentImpairment is not addressed in the transportation analysis because this resource topic is peripheral to the protection of the park for future generations. Scenic ResourcesAnalysisUnder Alternative 2, Cascades Diversion Dam would be removed as soon as feasible. Removal of the dam would eliminate the adverse scenic resource impacts associated with leaving the structure in place under Alternative 1. Planned dam removal under Alternative 2 would prevent the deposition of dam debris in the river channel, the gouging of the riverbanks and channel, and the associated degradation of the scenic quality of these areas that would occur under Alternative 1. Accordingly, compared to Alternative 1, Alternative 2 would have a local, short-term, minor, beneficial impact. Like Alternative 1, Alternative 2 would require the use of
equipment to remove and transport dam materials from the existing site. The
presence and operation of the equipment would detract from the scenic resource
values of the As under Alternative 1, removal of the dam would have a long-term,
beneficial effect on the scenic resources of the natural landscape of the Damage to trees and streamside vegetation would be avoided, in contrast to the damage likely to occur along the riverbanks under Alternative 1 due to uncontrolled failure of the dam. Any area damaged during dam removal under Alternative 2 would be revegetated. Therefore, the riparian vegetation and natural landscape impact would be local, short-term, minor, and beneficial, as compared to Alternative 1. Site revegetation would increase riparian vegetation and natural landscape patterns in the area. The long-term effects of dam removal would be beneficial under both Alternative 1 and Alternative 2. However, due to the bank stabilization and restoration efforts included, Alternative 2 would result in a local, long-term, minor, beneficial impact to scenic resources compared to Alternative 1. Summary of Alternative 2 Impacts. In avoiding the effects associated with uncontrolled dam deterioration and eventual failure, which include deposition of debris in the river channel and visually prominent damage to the riverbanks and vegetation, Alternative 2 would have a local, short-term, minor, beneficial impact on scenic resources. The long-term effects of dam removal would be beneficial under both Alternative 1 and Alternative 2. However, due to the bank stabilization and restoration efforts included, Alternative 2 would result in a local, long-term, minor, beneficial impact to scenic resources compared to Alternative 1. Cumulative ImpactsThe cumulative impact analysis for scenic resources under Alternative 2 is the same as described under Alternative 1. See the discussion of cumulative effects under Alternative 1. The cumulative projects within and in the vicinity of the
Merced River corridor would result in a local, long-term, major, beneficial
impact on scenic resources along the Merced River corridor because of the
overall emphasis on restoring disturbed or developed land to natural conditions
and improving the health of ecosystems. Alternative 2 and the cumulative
projects within the ImpairmentAlternative 2 would have an overall beneficial impact on the visual landscape. Therefore, Alternative 2 would not impair scenic resources or values. RecreationAnalysisAlternative 2 would remove Cascades Diversion Dam and attendant
structures, including the intake structure, currently used as an informal river-viewing
platform. Removal of the dam would eliminate the potential for serious injury
and/or fatality to sightseers who cross Alternative 2 would avoid effects resulting from dam deterioration and eventual failure on river-dependent active recreational uses in the vicinity of the dam and downstream. Under Alternative 1, debris and increased sedimentation in the river following dam failure would temporarily prevent or disrupt swimming, wading, and fishing. Under Alternative 2, dam removal would be controlled to prevent deposition of debris in the river and increased sedimentation associated with bank erosion. Accordingly, compared to Alternative 1, Alternative 2 would result in a local, short-term, minor, beneficial impact on river-dependent active recreational uses. Removal of the dam under Alternative 2 would temporarily
interfere with access to recreational opportunities through the Summary of Alternative 2 Impacts. Compared to Alternative 1, elimination of the potential for injury and/or fatality to river-related recreation users under Alternative 2 would be a local, short-term, moderate, beneficial impact on recreation; a local, short-term, minor, beneficial impact on river-dependent recreation; and a local, short-term, negligible to minor, beneficial effect on recreational access. Cumulative ImpactsThe cumulative impact analysis for recreation under Alternative 2 is the same as described under the No Action Alternative. See the discussion of cumulative effects under Alternative 1. The cumulative projects would have a local, long-term,
moderate, beneficial effect on recreation due to expanded recreational
opportunities in the ImpairmentAlternative 2 would result in a local, short-term, negligible
to moderate, beneficial effect on river-related recreation in the Orientation and InterpretationAnalysisDam removal activities under Alternative 2 would not
affect directional signage located in the vicinity of the project area. During
dam removal activities, interpretive displays and information regarding the
Cascades Diversion Dam Removal Project would be made available at the Summary of Alternative 2 Impacts. Under Alternative 2, the inclusion of interpretation opportunities during dam removal activities and the avoidance of the potential for dam debris and erosion to affect downstream orientation and interpretation opportunities would be a local, short-term, minor, beneficial impact on orientation and interpretation compared to Alternative 1. Cumulative ImpactsThe cumulative impact analysis for orientation and interpretation under Alternative 2 is the same as described under the No Action Alternative. See the discussion of cumulative effects under Alternative 1. The cumulative projects would have a local, long-term,
minor, beneficial effect due to expanded orientation and interpretation
opportunities in the Alternative 2 and the cumulative projects in the ImpairmentAlternative 2 would result in a local, short-term, minor,
beneficial effect on orientation and interpretation opportunities in the SocioeconomicsAnalysisThe complete removal of Cascades Diversion Dam under Alternative 2 would result in direct spending on labor and equipment. The direct spending related to dam removal under Alternative 2 would be approximately $2 to $2.6 million (in 2003 dollars). Expressed in 2000 dollars, direct spending on complete dam removal would be between $1.9 and $2.5 million.[8] Indirect and induced spending would also result from dam removal and would be approximately 45% of direct spending (NPS 2000a); therefore, a total economic impact resulting from spending on dam removal (direct, indirect, and induced) would be between $2.8 and $3.6 million in 2000 dollars. Some portion of total construction spending would be expected to occur outside the affected region. The duration of controlled dam removal activity is estimated to be approximately five months. Employment that would result from this level of direct and total spending would be approximately 36 to 46 jobs and 57 to 73 jobs, respectively, over the course of the five-month construction period.[9] This increase in jobs represents a 2 to 2.7% employment increase in the number of construction jobs in the affected region. It is expected, however, that this employment increase would be somewhat less than 2%, because dam removal would occur during the fall (September through November), which is outside of the peak construction season for the affected region. Controlled dam removal activity under Alternative 2 would generate output, employment, and income in the construction and mining sector of the regional economy. Alternative 2 would have a short-term, negligible, beneficial impact on the regional economy, due to the temporary nature of the dam removal and the small magnitude of associated spending (less than 1 percent) compared with the size of the construction industry in the affected region. Summary of Alternative 2 Impacts. Alternative 2 would have a direct economic impact of $1.9 to $2.5 million and an indirect and induced impact of $0.9 to $1.1 million, which would result in a short-term, negligible, beneficial impact on the regional economy. Cumulative ImpactsThe cumulative impact analysis for socioeconomics under Alternative 2 is the same as described under the No Action Alternative. See the discussion of cumulative effects under Alternative 1. The cumulative projects within and in the vicinity of Yosemite National Park would result in a local, long-term, negligible, beneficial impact to the regional economy, and a local, short-term, major, beneficial impact during construction. Alternative 2 would contribute to this local, short-term, beneficial impact due to temporary spending on dam removal activities. ImpairmentImpairment is not addressed in the socioeconomics analysis because this resource topic is peripheral to the protection of the park for future generations. Park Operations and FacilitiesAnalysisBecause Alternative 2 would entail the use of heavy
equipment to remove the dam, implement the bioengineered bank stabilization
system, and revegetate the river-right bank, there is a risk of accidental
damage to existing park facilities, particularly the wastewater line and
electrical conduit under After the dam is removed, the river-right bank would be
stabilized and revegetated to match adjacent bank composition, including the use
of rocks, cobbles, and boulders. The large size of substrate materials along
this stretch of the river provides for bank stability, which would protect the
utility lines located under Alternative 2 would remove Cascades Diversion Dam and
attendant structures, including the intake structure that is currently used as
an informal river-viewing platform, but the paved parking area and public
telephone across Under Alternative 2, all aspects of the project would be overseen by park staff, including design and engineering, removal activities, utilities protection, and site restoration. Controlled removal of the dam under this alternative would place fewer demands on park operations staff than Alternative 1, which could require an emergency response to manage catastrophic dam failure and repair damaged facilities. As a result, Alternative 2 would have a local, short-term, minor, beneficial effect on park operations compared to Alternative 1. Summary of Alternative 2 Impacts. Alternative 2 would avoid potential catastrophic damage to park facilities, resulting in a local, long-term, minor, beneficial impact compared to Alternative 1. Dam removal activities could result in damage to park facilities. Controlled dam removal under this alternative, with the application of mitigation measures, would have a local, short-term, minor, beneficial impact on park operations, due to the reduced demands on park operations staff compared to Alternative 1, which would require an emergency response to dam failure and damaged facilities. Cumulative ImpactsThe cumulative impact analysis for park operations and facilities under Alternative 2 is the same as described under the No Action Alternative. See the discussion of cumulative impacts under Alternative 1. Overall, the past, present, and reasonably foreseeable future actions would have a local, moderate, adverse cumulative impact because of the increased demand on park operations, services, and facilities, over both the short and long term. These cumulative effects, in combination with Alternative 2, would result in a local, short- and long-term, moderate, adverse impact on park operations and facilities, due to the increased demand these projects would place on park operations, services, and facilities. The minor beneficial effects under Alternative 2 would not offset the adverse effects associated with the cumulative projects. ImpairmentPark operations are not subject to the impairment standard. The National Park Service has a management responsibility to conserve the scenery and natural and historic objects and the wildlife therein; park operations are not included within this management responsibility. Alternative 3 – Partial Dam RemovalAlternative 3 includes complete removal of the dam, the river-left dam abutment, and the screenhouse on the intake structure, and restoration of the related river channel located beneath the dam site (see figure II-3). Under this alternative, the river-right dam abutment and intake structure would be retained for use as a river-viewing platform. Approximately 4,400 to 5,400 cubic yards of sediments (including rocks and boulders) in the area upstream of the dam would be excavated and repositioned to stabilize the river-right bank and decrease the potential for sediment erosion. Figure II-4 indicates the river profile at Cascades Diversion Dam before and after removal of the dam structure and sediments. Natural river processes would continue to transport remaining sediments (up to a maximum range of approximately 9,600 to 15,600 cubic yards of sediment) from the impoundment area over time, allowing for a gradual re-establishment of the natural river channel and related riparian habitat. It is expected that the river would fully recover incrementally over time, as sediments are transported from the impoundment area. However, the rate of natural channel recovery and restoration would be monitored to determine if additional restoration actions were necessary. Following removal of the dam and screenhouse, the river-right bank would be stabilized upstream and downstream of the intake structure using a bioengineered bank stabilization system to prevent erosion of the river-right bank. The objective of this alternative would be to restore the natural river character with a mixture and distribution of boulders, cobbles, gravels, sand, silt, soil, and vegetation similar to those found in adjacent riverbank segments. Natural ResourcesGeology, Geologic Hazards, and SoilsAnalysisDam removal impacts on soils are the same as described under Alternative 2. See the discussion of these effects under Alternative 2. Potential damage to the river-right abutment and intake structure from potential future rockfalls would be a long-term public safety risk, as Alternative 3 would use the intake structure as a viewing platform following dam removal. Destabilization of the river-right abutment and intake structure following a rockfall could cause damage to or destruction of the viewing platform, which would result in a public safety risk for those in the immediate area and for those downstream of the released debris. In addition, a rockfall event would deposit boulders and talus into the river reach in the former dam location, but would not damage structures other than the viewing platform. Under Alternative 3, this geologic hazard would be permanent, whereas the geologic hazard under Alternative 1 would be removed once the dam failed. Rockfall material could redirect riverflows, but given that the channel would down-cut and water velocity would increase through a narrower, rock-lined channel following river-right bank stabilization, the potential for bank scour adjacent to the road would decrease. The river-right viewing platform would also be subject to future seismic events, and some level of damage could occur, although collapse of the viewing platform from groundshaking is unlikely. Overall, compared to Alternative 1, Alternative 3 would have a long-term, negligible, adverse impact to public health and safety, due to potential damage to the viewing platform from geologic hazards. The impact would be negligible because the risk of potential adverse effects to public health and safety is slight. Summary of Alternative 2 Impacts.
Dam removal would have a short-term impact to soils as a result of ground
disturbance activities. However, dam removal activities would occur in a
controlled manner, with the application of best management practices. Since
Alternative 3 would avoid the more extensive adverse effects of bank
destabilization, erosion, and soil compaction and loss due to uncontrolled dam
failure and debris retrieval activities described under Alternative 1,
Alternative 3 would have a local, short-term, minor, beneficial effect on soil
resources compared to Alternative 1. Site restoration and stabilization would
reduce the potential for erosion and sedimentation, help stabilize channel
shape and slopes, repair banks, and increase the protection of riverbanks, the
adjacent roadway, and utility lines under Cumulative ImpactsThe cumulative impact analysis for geology under Alternative 3 is the same as described under the No Action Alternative. See the discussion of cumulative effects under Alternative 1. Past, present, and reasonably foreseeable future actions would result in a long-term, minor, beneficial cumulative impact to soil resources and to public health and safety with respect to geologic hazards. Overall, the cumulative projects would restore soils in the project region, reduce soil degradation, and decrease the density of people and facilities in the talus slope zone. Alternative 3 and the cumulative projects would result in a local, long-term, minor, beneficial impact to soil resources and public safety with respect to geologic hazards. Alternative 3 would avoid the more extensive adverse effects of soil erosion and bank destabilization that would occur under Alternative 1. ImpairmentAlternative 3 would result in beneficial effects on soil
resources, but a local, long-term, negligible, adverse impact to public health
and safety due to potential damage to the viewing platform from geologic
hazards. Although the Hydrology, Floodplains, and Water QualityAnalysisDam removal effects on hydrologic processes and water quality under Alternative 3 are the same as described under Alternative 2. See the discussion of these effects under Alternative 2. Under Alternative 3, Cascades Diversion Dam would be
removed, thus restoring the free-flowing condition of the Summary of Alternative 3 Impacts. Dam removal would have a short-term water quality impact related to the discharge of petroleum components. However, dam removal activities would occur in a controlled manner, with the application of best management practices. Compared to Alternative 1, Alternative 3 would have a local, short- and long-term, minor to moderate, beneficial impact on hydrologic processes and water quality by avoiding bank erosion and localized flooding associated with continued deterioration and eventual dam failure, reducing sedimentation, and controlling removal of the dam. Cumulative ImpactsThe cumulative impacts assessment for hydrologic resources under Alternative 3 is the same as described under Alternative 1. See the discussion of cumulative effects under Alternative 1. The cumulative projects would result in an overall local, long-term, minor, beneficial impact to hydrologic processes and water quality. The past, present, and reasonably foreseeable future projects considered cumulatively with Alternative 3 would have a local, long-term, minor, beneficial impact on hydrologic processes. The beneficial impact associated with Alternative 3 would nominally contribute to the overall beneficial cumulative impact on hydrologic processes and water quality. ImpairmentAlternative 3 would have a local, short- and long-term, minor
to moderate, beneficial impact on hydrologic processes and water quality. Alternative
3 would not impair hydrologic resources within the WetlandsAnalysisThe effects of dam removal and revegetation on wetlands under Alternative 3 are the same as described under Alternative 2. See the discussion of these effects under Alternative 2. The intake structure would remain within the bed and banks
of the Summary of Alternative 3
Impacts. Dam removal activities would have a short-term impact to
wetland and aquatic habitat resources associated with ground disturbance and the
potential introduction of pollutants. However, dam removal activities would
occur in a controlled manner, with the application of mitigation, reducing the
adverse effect to a negligible intensity. Therefore, Alternative 3 would have a
local, short-term, negligible, beneficial effect on wetland and aquatic habitat
compared to Alternative 1. Removal of the overflow portion of Cascades
Diversion Dam and the river-left abutment would restore the free-flowing
condition of the Cumulative ImpactsThe cumulative impact analysis for Alternative 3 is the
same as described under the No Action Alternative. See the discussion of cumulative
effects under Alternative 1. Past, present, and reasonably foreseeable future
projects in combination with Alternative 3 would have a net long-term, major, beneficial
effect on wetland patterns within the ImpairmentGiven the incorporation of mitigation into the design of this alternative, Alternative 3 would result in a local, short- and long-term, negligible to moderate, beneficial impact to wetland and aquatic resources. Alternative 3 would not impair wetland resources or values. VegetationAnalysisThe effects of dam removal and revegetation on vegetation resources under Alternative 3 are the same as described under Alternative 2. See the discussion of these effects under Alternative 2. The intake structure would remain within the bed and banks
of the Summary of Alternative 3
Impacts. Dam removal activities would have a short-term impact to
vegetation associated with ground disturbance and potential the introduction of
pollutants. However, dam removal activities would occur in a controlled manner,
with the application of mitigation, reducing the adverse effect to a negligible
intensity. Therefore, Alternative 3 would have a local, short-term, negligible,
beneficial effect on vegetation compared to Alternative 1. Removal of the
overflow portion of Cascades Diversion Dam and the river-left abutment would
restore the free-flowing condition of the Cumulative ImpactsThe cumulative impact analysis for Alternative 3 is the
same as described under the No Action Alternative. See the discussion of
cumulative effects under Alternative 1. Past, present, and reasonably
foreseeable future projects in combination with Alternative 3 would have a net
long-term, major, beneficial effect on vegetation patterns within the ImpairmentGiven the incorporation of mitigation into the design of this alternative, Alternative 3 would result in a local, short- and long-term, negligible to moderate, beneficial impact to vegetation. Alternative 3 would not impair vegetation resources or values. WildlifeAnalysisThe effects of dam removal and revegetation on wildlife under Alternative 3 are the same as described under Alternative 2. See the discussion of these effects under Alternative 2. Special-Status SpeciesAnalysisThe effects of dam removal and revegetation on special-status species under Alternative 3 are the same as described under Alternative 2. See the discussion of these effects under Alternative 2. Air QualityAnalysisImpacts on air quality under Alternative 3 are the same as described under Alternative 2. See the discussion of these effects under Alternative 2. NoiseAnalysisImpacts on noise under Alternative 3 are the same as described under Alternative 2. See the discussion of these effects under Alternative 2. Cultural ResourcesArcheological ResourcesAnalysisImpacts on archeological resources under Alternative 3 are the same as described under Alternative 2. See the discussion of these effects under Alternative 2. Ethnographic ResourcesImpacts on ethnographic resources under Alternative 3 are the same as described under Alternative 2. See the discussion of these effects under Alternative 2. Cultural Landscape Resources, Including Historic Sites and StructuresImpacts on cultural landscape resources under Alternative 3 are the same as described under Alternative 2. See the discussion of these effects under Alternative 2. Social ResourcesTransportationAnalysisUnder Alternative 3, the paved parking area across El Portal Road from the dam site would be removed, the area revegetated, and the intake structure retained and redeveloped for use as a formal river-viewing platform. Development of the viewing platform would include a pedestrian walkway connecting to the vehicle turnout on the same side of the road. The parking area across the road from the viewing platform could no longer be used as a meeting place by visitors to form carpools, which currently reduce traffic volumes and thereby improve traffic flow. This effect is minor, however, because of the low number of parking spaces available, the low number of parked cars observed at the parking lot, and the frequency of use by park operations staff. Use of the area by visitors orienting themselves to park destinations (i.e., consulting maps), visitors with disabled vehicles, and putting on or removing tire chains would be restricted to the existing vehicle turnout west of the dam, to other nearby turnouts, and to parking areas downstream and upstream of the project area. Use of the existing parking area for equipment staging and snowplow turnaround would no longer be available. Such uses would be relocated to Cascades Picnic Area (approximately one mile downstream on El Portal Road), Foresta Overlook (approximately three miles away along Big Oak Flat Road), and the turnout on Southside in the Bridalveil Fall area and the Bridaveil parking lot (approximately two miles upstream along El Portal Road). Therefore, removal of the parking lot under Alternative 3 would have a local, long-term, minor, adverse impact on traffic flow. Removal of the parking area across Alternative 3 would have the same short-term dam-removal-related impacts as Alternative 2 because both action alternatives would include a similar number of truck trips, use of the same staging areas, and both would employ best management practices (see Chapter II, Alternatives), including implementation of a traffic control plan (see the discussion of dam-removal-related impacts for Alternative 2). Therefore, controlled partial dam removal using best management practices under Alternative 3 would have a local, short-term, negligible, beneficial impact on traffic flow compared to Alternative 1. Summary of Alternative 3 Impacts. Under Alternative 3, removal of the parking lot would have a local, long-term, minor, adverse impact on traffic flow. There would be a local, long-term, negligible, beneficial impact associated with traffic conflicts compared to Alternative 1. Dam removal activities would result in a short-term increase in vehicle trips in the project area. Controlled partial dam removal using best management practices (see Chapter II, Alternatives) under Alternative 3 would have a local, short-term, negligible, beneficial impact on traffic flow compared to Alternative 1. Cumulative ImpactsThe cumulative impact analysis for transportation under Alternative 3 is the same as described under Alternative 1. See the discussion of cumulative effects under Alternative 1. The cumulative projects would have a local, long-term,
major, beneficial impact on transportation conditions along the Alternative 3 and the cumulative projects would result in
a local, long-term, major, beneficial impact on transportation conditions along
the ImpairmentImpairment is not addressed in the transportation analysis because this resource topic is peripheral to the protection of the park for future generations. Scenic ResourcesAnalysisUnder Alternative 3, the short-term effects of dam removal activities on scenic resources would be the same as described for Alternative 2. See the discussion of these effects under Alternative 2. Under Alternative 3, the long-term effects of dam removal on
scenic resources would be the same as described for Alternative 2, with the
exception that the intake structure would be retained as a river-viewing
platform (the vehicle turnout west of the intake structure would also be
available for river viewing). This human-made structure would continue to
intrude upon the natural visual landscape at the dam site, particularly when
viewed as foreground or middle ground from vehicles traveling west on The long-term effects of dam removal would be beneficial under both Alternative 1 and Alternative 3. However, due to the bank stabilization and restoration efforts included, Alternative 3 would result in a local, long-term, minor, beneficial impact to scenic resources compared to Alternative 1. Summary of Alternative 3 Impacts. In avoiding the effects associated with uncontrolled dam deterioration and eventual failure, which include deposition of debris in the river channel and visually prominent damage to the riverbanks and vegetation, Alternative 3 would have a local, short-term, minor, beneficial impact on scenic resources. The long-term effects of dam removal would be beneficial under both Alternative 1 and Alternative 3. However, due to the bank stabilization and restoration efforts included, Alternative 3 would result in a local, long-term, minor, beneficial impact to scenic resources compared to Alternative 1. Cumulative ImpactsThe cumulative impact analysis for scenic resources under Alternative 3 is the same as described under the Alternative 1. See the discussion of cumulative effects under Alternative 1. The cumulative projects within and in the vicinity of the
Merced River corridor would result in a local, long-term, major, beneficial
impact on scenic resources along the Merced River corridor because of the
overall emphasis on restoring disturbed or developed land to natural conditions
and improving the health of ecosystems. Alternative 3 and the cumulative
projects within the ImpairmentAlternative 3 would have an overall beneficial impact on the visual landscape. Therefore, Alternative 3 would not impair scenic resources or values. RecreationAnalysisAlternative 3 would remove Cascades Diversion Dam and
attendant structures, but would retain the intake structure as a viewing
platform. Under Alternative 2, the parking lot north of Alternative 3 would avoid effects resulting from dam deterioration and eventual failure on river-dependent active recreational uses in the vicinity of the dam and downstream. Under Alternative 1, debris and increased sedimentation in the river following dam failure would temporarily prevent or disrupt swimming, wading, and fishing. Under Alternative 3, dam removal would be controlled to prevent deposition of debris in the river and increased sedimentation associated with bank erosion. Accordingly, compared to Alternative 1, Alternative 3 would result in a local, short-term, minor, beneficial impact on river-dependent active recreational uses. Removal of the dam under Alternative 3 would temporarily
interfere with access to recreational opportunities through the Alternative 3 would permanently eliminate the parking area
northeast of the Summary of Alternative 3 Impacts. Elimination of the potential for injury and/or fatality to river-related recreation users would be a local, short-term, moderate, beneficial impact on recreation; a local, short-term, minor, beneficial impact on river-dependent recreation; and a local, short-term, negligible to minor, beneficial effect on recreational access. Compared to Alternative 1, Alternative 3 would result in a local, long-term, negligible to minor, adverse effect on recreational access associated with the permanent removal of the parking area in the Cascades Diversion Dam vicinity. Cumulative ImpactsThe cumulative impact analysis for recreation under Alternative 3 is the same as described under the No Action Alternative. See the discussion of cumulative effects under Alternative 1. The cumulative projects would have a local, long-term,
moderate, beneficial effect on recreation due to expanded recreational
opportunities in the Alternative 3 and the cumulative projects in the ImpairmentAlternative 3 would result in a local, short-term, minor
to moderate, beneficial effect on river-related recreation in the Orientation and InterpretationAnalysisUnder Alternative 3, the short-term effects of dam removal activities on orientation and interpretation opportunities would be the same as described under Alternative 2. See the discussion of these effects under Alternative 2. Alternative 3 would remove Cascades Diversion Dam and
attendant structures, but would retain the intake structure for use as a formal
river-viewing platform. During dam removal activities, interpretive displays
and information regarding the Cascades Diversion Dam Removal Project would be
made available at the Summary of Alternative 3 Impacts. Under Alternative 3, the inclusion of interpretation opportunities during dam removal activities and avoidance of the potential for dam debris and erosion to affect downstream orientation and interpretation opportunities would be a local, short-term, minor, beneficial impact on orientation and interpretation compared to Alternative 1. Because Alternative 3 would include interpretive displays in the project area and Alternative 1 would have no long-term effect, Alternative 3 would result in a local, long-term, minor, beneficial impact to orientation and interpretation opportunities compared to Alternative 1. Cumulative ImpactsThe cumulative impact analysis for orientation and interpretation under Alternative 3 is the same as described under the No Action Alternative. See the discussion of cumulative effects under Alternative 1. The cumulative projects would have a local, long-term,
minor, beneficial effect due to expanded orientation and interpretation
opportunities in the Alternative 3 and the cumulative projects in the ImpairmentAlternative 3 would result in a local, short-term and
long-term, minor, beneficial effect on orientation and interpretation opportunities
in the SocioeconomicsAnalysisImpacts to socioeconomics under Alternative 3 are the same as described under Alternative 2 because the same amount of spending would occur under both action alternatives. See the discussion of these effects under Alternative 2. Park Operations and FacilitiesAnalysisThe short-term effects of dam removal activities and bank stabilization and revegetation under Alternative 3 on park operations and facilities would be the same as described for Alternative 2. See the discussion of these effects under Alternative 2. The long-term effects of dam removal under Alternative 3
on park operations would be the same as described for Alternative 2, with the
exception that the intake structure would be retained as a river-viewing
platform and the parking area north of the dam would be removed. The National
Park Service would make minor repairs (e.g., to the safety railing or concrete
platform) as necessary. While minor repairs would also occur under Alternative
1, repair activities would not continue once the dam failed, and the long-term
commitment of park operations for repairs would no longer be required under
Alternative 1. Park operations that currently utilize the parking lot (heavy
equipment staging and snowplow turnaround, equipment staging, and traffic
control for road closure activities) would be restricted to the existing turnout
west of the dam, to other nearby turnouts, and to parking areas downstream and
upstream of the project area, as described above under Transportation. Use of
the existing public telephone for access to the park’s emergency response
system would no longer be available. However, the frequency of mobile phone
use, the frequency of vehicle travel through the Summary of Alternative 3 Impacts. Alternative 3 would result in a local, long-term, minor, beneficial impacts on park facilities because of the avoidance of potential catastrophic damage to park facilities compared to Alternative 1. Dam removal activities could result in damage to park facilities. Controlled dam removal under this alternative, with the application of mitigation measures, would have a local, short-term, minor, beneficial impact on park operations due to the reduced demands on park operations staff compared to Alternative 1, which would require an emergency response to manage dam failure and repair potentially damaged facilities. Long-term minor repairs to the river-viewing platform under Alternative 3 would result in a local, long-term, minor, adverse impact to park operations compared to Alternative 1. Cumulative ImpactsThe cumulative impact analysis for park operations and facilities under Alternative 3 is the same as described under the No Action Alternative. See the discussion of cumulative impacts under Alternative 1. Overall, the past, present, and reasonably foreseeable future actions would have a local, moderate, adverse cumulative impact because of the increased demand on park operations, services, and facilities, over both the short and long term. These cumulative effects, in combination with Alternative 3, would result in a local, short- and long-term, moderate, adverse impact on park operations and facilities, due to the increased demand these projects would place on park operations, services, and facilities. The minor beneficial effects under Alternative 3 would not offset the adverse effects associated with the cumulative projects. ImpairmentPark operations are not subject to the impairment standard. The National Park Service has a management responsibility to conserve the scenery and natural and historic objects and the wildlife therein; park operations are not included within this management responsibility. [1] Throughout Chapter IV, Environmental Consequences, unless otherwise noted, “rockfalls” is used as a generic term to refer to rockfalls in the stricter sense but also to rockslides, debris avalanches, debris flow, and rock avalanches. [2] Under the Advisory Council on Historic
Preservation’s revised regulations of [3] Riparian vegetation through the gorge is naturally restricted by channel shape, slope, bed material, and flow. [4] Riparian vegetation through the gorge is naturally restricted by channel shape, slope, bed material, and flow. [5] Stipulations included the preparation of Historic
American Engineering Report documentation, and the submittal of archival
photographs and narrative to the State Historic Preservation Officer and the [6] Stipulations included the preparation of Historic
American Engineering Report documentation, and the submittal of archival
photographs and narrative to the State Historic Preservation Officer and the [7] Each truck load would generate two one-way trips (i.e., a trip by the loaded truck from the work site, and a return trip by the empty truck to the site). [8] Adjustment calculated using Consumer Price Index estimates provided by the U.S. Bureau of Economic Analysis (U.S. Bureau of Labor Statistics 2002). [9] Adjustment calculated using a multiplier from the Yosemite Valley Plan for estimating employment generated by construction spending (NPS 2000c).
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