South Fork Merced River Bridge Replacement Project

This chapter describes the probable consequences (or impacts) that could result under the alternatives described in this environmental assessment. The chapter is divided into three parts. The Introduction describes the methodologies and assumptions that are common to all resource topic areas. The Methodologies and Assumptions section presents the methods used to assess impacts for each specific resource topic. The next section describes the impacts anticipated under each alternative, organized by resource topic. Environmental impacts are summarized in Table II-1: Summary of Environmental Consequences, located at the end of Chapter II, Alternatives, of this document.

Impact Analysis

Each alternative contains an impact analysis for each individual resource topic. Impacts are evaluated based on context, duration, intensity and whether they are direct, indirect, or cumulative. In addition, impairment to park resources and values is considered.

The following guidelines were used to identify the context, duration, intensity (or magnitude) and type of impact.

§ Context. The context considers whether the impact would be local or regional. For the purposes of this analysis, local impacts would be those that occur within the immediate vicinity of the South Fork Merced River Bridge Replacement Project, unless otherwise noted.

§ Duration. The duration of an impact is noted as either short term or long term and defined in a range of years.

§ Intensity. Indicators of the intensity of an impact, whether it is negligible, minor, moderate, or major, are included in the impact analysis and specifically defined by topic area in the methodology section that follows.

§ Type. The type of impact refers to whether the effect is considered beneficial or adverse. Beneficial impacts would improve resource conditions. Adverse impacts would deplete or negatively alter resources. Mitigating actions listed in Chapter II would be taken during implementation of the action alternatives. With the exception of the cultural resource analysis, all impacts have been assessed under the assumption that mitigating measures have already been implemented.

Alternative 1 (the No Action Alternative) describes the status quo. This alternative provides a baseline from which to compare other action alternatives, to evaluate the magnitude of proposed changes, and to measure the environmental affects of these changes.

Cumulative Impacts

The Council on Environmental Quality describes a cumulative impact as follows (Regulation 1508.7):

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 for this environmental assessment, projects within the South Fork Merced River and Wawona area were identified. The cumulative projects identified included past actions, and planning and development activities currently under implementation or planned for implementation in the reasonably foreseeable future. Appendix D contains the list of cumulative projects included in the cumulative impacts analysis. These actions are evaluated in the impact analysis in conjunction with the impacts of an alternative to determine if they have any additive effects on a particular natural, cultural, or social resource. When a cumulative project was in the planning stage, the evaluation of cumulative impacts was based on a general description of the project.

Projects and plans that were considered in the cumulative analysis were: (1) the Merced River Plan, which protects and enhances the Outstandingly Remarkable Values and free-flowing condition of the river; (2) South Entrance/Mariposa Grove Site Planning, which considers alternatives for restoring giant sequoia habitat; (3) Wilderness Boundary Protection Land Exchange, Seventh Day Adventist Camp, Wawona, which involves a land exchange to protect wilderness; (4) Wawona Campground Improvement, which would rehabilitate the existing campground and construct an additional campground; (5) South Fork and Merced Wild and Scenic River Implementation Plan, which provides long-term protection of natural and cultural resources on adjacent U.S. Forest Service and Bureau of Land Management lands; (6)Yosemite Valley Plan, which implements the goals of the 1980 General Management Plan in Yosemite Valley, is designed to meet resource preservation and visitor experience goals in Yosemite Valley, including natural and cultural resource management and restoration, visitor services and recreational opportunities, transportation, and employee housing; (7) Mariposa County General Plan Update, which provides guidance for land use, zoning, and development throughout Mariposa County; and (8) Yosemite Area Regional Transportation System (YARTS), which evaluates the feasibility of a regional transportation system and identifies the best options for initial implementation and upkeep of such a system.

Impairment

Impairment is an impact that, in the professional judgment 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. The need to analyze and disclose impairment impacts originates from the National Park Service Organic Act (1916). The Organic Act established the National Park Service with a mandate “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.”

An impact would be less likely to constitute an impairment if it is an unavoidable result, which cannot reasonably be further mitigated, of an action necessary to preserve or restore the integrity of park resources or values (NPS 2000a). An impact would be more likely to constitute impairment to the extent that it affects a resource or value whose conservation is:

§ Necessary to fulfill specific purposes identified in the establishing legislation or proclamation of the park

§ Key to the natural or cultural integrity of the park or to opportunities for enjoyment of the park

§ Identified as a goal in the park’s General Management Plan or other relevant National Park Service planning documents

The evaluation of impairment of park resources was based on the type and intensity of impacts and the types of resources affected. Overall, beneficial impacts would not constitute impairment. With respect to the intensity of impacts, negligible and minor adverse impacts are not of sufficient magnitude to constitute impairment. Moderate and major adverse impacts may constitute impairment, but do not automatically do so. Rather, these impacts must be analyzed with respect to the three bulleted criteria above. Impairment is generally considered for geologic, hydrological, biological, cultural, and scenic resources and recreation Impairment is addressed in the conclusion section of each impact topic under each alternative.

Methodologies and Assumptions

This section presents the methodologies and assumptions used to conduct the environmental impact analyses for each resource topic.

Geology, Geologic Hazards, and Soils

This impact assessment focuses on effects that geologic processes in Yosemite National Park could have on visitors, personnel, and facilities under each alternative of the South Fork Merced River Bridge Replacement Project. Geologic processes negatively affect visitors, personnel, and facilities when events such as earthquakes, and severe soil instability result in injury, death, or damage to facilities. The assessment also focuses on the effect of project alternatives on geologic processes, namely the formation and conservation of soil resources. Project-related actions could affect soil resources through accelerated erosion, soil loss, or soil removal.

Several assumptions regarding facility placement, geologic design parameters, and public safety were integrated into this assessment, as summarized below.

§ Geologic risks to public safety are rarely predictable, and the extent of potential harm to people and property cannot be quantified. While the Wawona area is not prone to earthquakes or rockfalls, it is not possible to avoid risks due to geologic hazards, 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.

§ Geotechnical studies to determine soil stability conditions would be performed prior to placing, designing, or relocating a facility within the park, and facility design within Yosemite National Park would conform to accepted building costs, particularly regarding seismic design parameters.

§ 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.

Duration of Impact

Short-term impacts are considered temporary or transitional in nature. Short-term impacts would be associated with South Fork Bridge removal, South Fork Bridge construction, and temporary Bailey bridge removal, and the subsequent period of time for site restoration. Long-term impacts are typically those that are evident for periods longer than 10 years following the project, and may be permanent. Geologic impacts related to seismic events would likely be long-term and permanent.

Intensity of Impact

The intensity of an impact was based on its location within the park and what the types of activities and facilities are proposed in that location. The intensity of the impact would be negligible if facilities of any kind are located outside geologic hazard zones, or in rock areas with no soil resources.

There will always be a potential for adverse impacts to life and property due to seismic hazards, especially in developed areas. Therefore, management actions to avoid placement of facilities in areas susceptible to seismic hazards may decrease the risks but would not necessarily reduce the intensity of the impact.

For soils, impact intensity was characterized as negligible, minor, moderate, or major. Negligible impacts would be imperceptible or not detectable. Minor impacts would be slightly perceptible and localized. Moderate impacts would be apparent and have the potential to become larger. Major impacts would be substantial, highly noticeable, and may be permanent.

Type of Impact

All seismic events are potentially hazardous. The type of impact is related to risk, and it is difficult to estimate risk involving natural events. In general, reducing risk would be considered a beneficial impact. Generally, maintaining facilities within or moving facilities into a zone of higher risk or exposing people to greater levels of risk would be considered adverse.

Beneficial impacts to soils protect or restore natural soil conditions including abiotic and biotic components, soil structure, and moisture. Adverse impacts would result in degradation of chemical, physical, abiotic, or biotic soil components.

Hydrology, Floodplains, and Water Quality

Impacts on hydrology, floodplain values, and water quality are discussed under this resource topic. Hydrology refers to hydrologic processes such as flooding, erosion and deposition, and channel movement. Particular attention was given to alterations or restoration of water flow (e.g., placement or removal of facilities in the South Fork Merced River channel). Floodplain values are attributes of flooding that contribute to ecosystem quality, such as recharge of riparian ground water. Particular attention was given to alterations or restoration of the floodplain (e.g., placement or restoration of facilities in a floodplain). Water quality refers to the suitability of surface water for recreational use and wildlife habitat, particularly the enhancement or degradation of water quality. The National Park Service Freshwater Resource Management Guidelines (found in Procedural Manual-77) requires the National Park Service to “maintain, rehabilitate, and perpetuate the inherent integrity of water resources and aquatic ecosystems.” The Clean Water Act requires the National Park Service to “comply with all Federal, State, interstate, and local requirements, administrative authority, and process and sanctions respecting the control and abatement of water pollution.

This assessment focuses on the physical and chemical processes of the Merced River, and how (relative to the No Action Alternative – Alternative 1) the action alternative would affect hydrologic processes, both during project activities and following project completion. The hydrology impact assessment herein evaluates how project activities would affect channel morphology, flooding, and water quality.

Channel Morphology

The analysis examines potential changes to channel morphology (channel depth, position, and streamflow) as a result of the 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.

Floodplains

National 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. This section qualitatively analyzes the impacts or benefits to the river’s floodplain for the two alternatives.

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 1993b]). 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.

Water Quality

This section identifies potential effects on water quality associated with project activities, such as the eventual collapse of the South Fork Bridge and associated rupturing of the sewerline attached to the bridge.

Duration of Impact

Short-term impacts are considered temporary or transitional. Short-term impacts would be associated with South Fork Bridge removal, South Fork Bridge construction, and temporary Bailey bridge removal, and the subsequent period of time for site restoration. Long-term impacts are typically those that are evident for periods longer than 10 years following the project, and may be permanent.

Intensity of Impact

Negligible impacts would be imperceptible or not detectable. Minor impacts would be slightly perceptible and localized, without the potential to expand if left alone. Moderate impacts would be apparent and have the potential to become larger. Major impacts would be substantial, highly noticeable, and may be permanent.

Type of Impact

Adverse impacts alter natural hydrologic conditions (e.g., impede flood flows, cause unnatural erosion or deposition, etc.) or degrade water quality (e.g., increase pollution or bacteria levels). Beneficial impacts would be those that restore natural hydrologic conditions (e.g., remove impediments to flood flows, stabilize riverbanks, etc.) or improve water quality (e.g., reduce potential for nonpoint source and point source pollution).

Wetlands

Wetlands and riparian areas are relatively rare in the context of the entire landscape. However, modification of even small wetland areas induces effects that are proportionally greater than elsewhere in an ecosystem (UC Davis 1996b).

The National Park Service is committed to minimizing wetland loss. The wetland protection statutes that guide 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 United States. Wetland impacts were estimated using wetland-specific data collected in the field during the fall of 2002. Wetland data were compared with each alternative to determine the area of potential effect. This analysis considers whether proposed actions could breach applicable federal laws, regulations, or executive orders.

Duration of Impact

Intensity of Impact

Three primary measures were used to evaluate the intensity of impacts on wetlands: the size and type of the wetland, the integrity of the wetland, and the connectivity of the wetland to adjacent habitats.

The intensity of impacts has been described as negligible, minor, moderate, or major. Negligible impacts would be imperceptible or not detectable. Minor impacts would be slightly detectable, localized within a small area, and would not affect the overall viability of wetlands in the park. Moderate impacts would be apparent and have the potential to become major impacts. Major impacts would be substantial, highly noticeable, and could become permanent.

Type of Impact

Adverse impacts would degrade the size, integrity, or connectivity of wetland. Conversely, beneficial impacts would enlarge the size or enhance the integrity and connectivity of wetlands.

Vegetation

Impacts on vegetation communities were assessed in terms of duration, type, and intensity in site-specific, parkwide, and regional contexts.

Duration of Impact

Intensity of Impact

Impacts 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.

Non-native species are discussed in terms of presence on previously disturbed sites and as invasive species within existing plant communities. Mitigation measures were applied, as applicable, to prevent impacts related to the introduction and spread of non-native plant species; however, they would continue to be managed by park staff in conjunction with National Park Service programs responsible for protection and long-term management of vegetation resources.

Human use impacts such as recreational use and foot traffic can extend beyond developed areas and affect plant community size and continuity. Human use can disturb or compact soils, create conditions favorable for non-native species or introduce non-native species, and trample native vegetation cover. Human use impacts that extend beyond development boundaries were considered as factor in determining the intensity of impacts on vegetation.

New development within an otherwise intact and undisturbed area may fragment or disassociate plant communities. Small areas of restoration surrounded by existing or new development may constitute a lesser beneficial impact on plant communities than restoration of a small area adjacent to a larger intact community. In general, reducing and limiting fragmentation, and maintaining connections within and among plant communities can minimize adverse effects on plant communities.

The evaluation of the integrity of plant communities was based on:

§ Biodiversity

§ Opportunities for natural processes to occur such as fire and flooding

§ Exotic species introduction and spread

§ Resilience of the plant community

In this document, biodiversity refers to the diversity of communities within an ecosystem, the diversity of species within a community, and genetic variation among individual species. Measures of biodiversity may include plant community structure and composition, connectivity of ecosystems, variation in age, structure (density and arrangement), individual species composition and abundance, and the presence or absence of natural structural layers.

Natural processes such as fire and flooding sustain many plant communities. This impact analysis considered whether changes would occur to opportunities for natural processes (or management options such as prescribed burning) to take place. For example, new development may prohibit opportunities for prescribed natural fire.

Non-native species can alter soil chemical and physical properties, hamper native species establishment, and ultimately alter native plant community structure and function. This impact analysis considered whether proposed actions would favor the establishment of non-native species, and the ability to contain and reverse non-native plant infestation.

Negligible impacts would have no measurable or perceptible changes in plant community size, continuity, or integrity. Minor impacts would be measurable or perceptible and localized within a relatively small area and the overall viability of the plant community would not be affected. Moderate impacts would cause a change in the plant community (e.g., size, continuity, and integrity); however, the impact would remain localized. The change would be measurable and perceptible, but could be reversed. Major impacts would be substantial, highly noticeable, and could be permanent in their effect on plant community size, diversity, continuity, or integrity.

Type of Impact

Impacts were classified as adverse if they would reduce the size, continuity, or integrity of a plant community. Conversely, impacts were classified as beneficial if they would increase the size, continuity, or integrity of a plant community.

Wildlife

This section addresses the effects of alternatives on wildlife and their habitat, as represented by general vegetation types and riverine conditions present. The correlation of vegetation impacts and effects on wildlife is described within this section. Adverse effects to wildlife without modifications to wildlife habitat, are also considered.

In general, adverse effects on wildlife can be minimized by reducing and limiting habitat fragmentation; that is, by preserving and restoring large areas of habitat, patches of habitat, and maintaining connections within and among habitat types. Larger patches of habitat tend to support higher numbers and diversity of wildlife species than smaller ones, and connections between habitat patches enable the movement of wildlife between areas, enhancing reproduction and survival. Small patches of habitat can serve as stepping-stones for wildlife moving between larger blocks.

Ultimately, the value of a restored area or the impact of a developed area to wildlife is determined by the characteristics of the species affected. Home range size, tolerance of human disturbance, and life-history characteristics determine whether a species reoccupies a restored area or abandons a disturbed area.

Impacts on wildlife have been 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.

Duration of Impact

Short-term impacts are considered temporary or transitional. Short-term impacts would be associated with South Fork Bridge removal, South Fork Bridge construction, and temporary Bailey bridge removal, and the subsequent period of time for site restoration. These impacts would end with cessation of construction activity, or soon thereafter, and include:

§ Noise, dust, and light emanating from construction sites could affect the use of surrounding habitats by wildlife.

§ Vegetation removed, trampled, or run-over during temporary use of some habitat as areas for staging of machinery or materials would affect wildlife until such areas could be restored after the project.

§ Diversion of water flows during construction would result in unnatural drying or wetting of habitats adjacent to sites.

§ Wildlife could be killed by traffic or machinery associated with construction.

§ Pits and trenches could entrap wildlife, resulting in their death.

§ Spills of fuel, oil, hydraulic fluid, antifreeze, and other toxic chemicals could affect wildlife, especially those in aquatic environments.

§ Construction personnel, at in-park residences or at work sites, could provide a source of human food to wildlife, resulting in conditioning of wildlife and in human/wildlife conflicts.

Long-term impacts are typically those that are evident for periods longer than 10 years following the project, and may be permanent.

Intensity of Impact

Negligible impacts are impacts that would not be measurable or perceptible. Minor impacts would be measurable or perceptible and would be localized within a relatively small area; however, the overall viability of the resource would not be affected. Without further impacts, negative effects would be reversed, and the resource would recover. Moderate impacts would be sufficient to cause a change in the resource (e.g., abundance, distribution, quantity, or quality); however, the impact would remain localized. The change would be measurable and perceptible, but negative effects could be reversed. Major impacts would be substantial, highly noticeable, and could be permanent without active management.

Type of Impact

Impacts were classified as adverse if they would negatively affect the size, continuity, or integrity of wildlife habitat, or result in unnatural changes in the abundance, diversity, or distribution of wildlife species. Conversely, impacts were classified as beneficial if they would positively affect the size, continuity, or integrity of wildlife habitat.

Special-Status Species

Wildlife

This analysis includes species listed under the Endangered Species Act as threatened or endangered; species that are Candidates for listing under the Endangered Species Act; species given Species of Concern status by the United States Fish and Wildlife Service; species listed by the State of California as threatened, endangered, or species of concern; and locally rare species of special importance to the park. The impact evaluation for special-status wildlife species was based on the following: (1) the known or likely occurrence of a species or its preferred habitat in the vicinity of the project area; (2) the direct physical loss or adverse modification of habitat; (3) the effective loss of habitat (through avoidance or abandonment) due to construction activity or noise, or the species’ sensitivity to human disturbance.

Habitat fragmentation is also a critical factor for special-status species. Restored blocks of habitat should be large enough to support viable populations, and intact habitat should not be reduced or affected to the point that it would no longer support viable populations. A more detailed discussion of impact duration, intensity, and type is included in the preceding Wildlife section.

Plants

This analysis includes species given Species of Concern status by the United States Fish and Wildlife Service; species listed by the State of California as threatened, endangered, rare, or species of concern; and locally rare species of special importance to the park. The impact evaluation for special-status plant species was based on the following: (1) the known or likely occurrence of a species or its preferred habitat in the vicinity of the project area; (2) the direct physical loss of habitat; (3) the effective loss of habitat through loss of habitat features such as surface water flows.

Duration of Impact

Intensity of Impact

Type of Impact

Impacts were classified as adverse if they would negatively affect population size, or habitat size, continuity, or integrity of a special-status species. Conversely, impacts were classified as beneficial if they would positively affect population size, or the size, continuity, or integrity of habitat.

Air Quality

The creation of pollutants resulting from the implementation of an alternative can contribute to an impact on air quality; however, air quality is a regional issue that is influenced by factors outside the immediate area. For example, the California Environmental Protection Agency concluded that the ozone exceedances in 1995 in the southern portion of the Mountain Counties Air Basin (i.e., Tuolumne and Mariposa Counties) were caused by transport of ozone and ozone precursors from the San Joaquin Valley Air Basin.

The air quality impact assessment involved the identification and qualitative description of the types of activities associated with each of the alternatives that could affect air quality, corresponding emissions sources and pollutants, and relative source strengths. Based on the relative source strengths, a 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 were 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 (NPS 2003a). Without such recommendations, the typical approach is to qualitatively evaluate the significance of temporary demolition-related 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.

Air quality impacts were evaluated in terms of intensity and duration and whether the impacts were considered beneficial or adverse. Cumulative effects on air quality were also considered based on past, present, and reasonably foreseeable future actions occurring in Yosemite National Park, in combination with the potential air quality effects of each alternative.

Duration of Impact

Short-term impacts are considered temporary, transitional, or bridge-removal related impacts associated with the project activities. Short-term impacts would be associated with South Fork Bridge removal, South Fork Bridge construction, and temporary Bailey bridge removal, and the subsequent period of time for site restoration. Long-term impacts are typically those that are evident for periods longer than 10 years following the project, and may be permanent.

Intensity of Impact

Type of Impact

Impacts were considered beneficial or adverse to air quality. Beneficial air quality impacts would reduce emissions or lower pollutant concentrations, while adverse impacts would increase emissions or raise pollutant concentrations.

Noise

The 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 an increase in ambient noise levels. 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.

Duration of Impact

Intensity of Impact

Type of Impact

Impact type was evaluated using the following definitions: beneficial impacts would be created through a reduction in decibels, and adverse impacts would be created through an increase in decibels.

Cultural Resources

This impact analysis methodology applies to three types of cultural resources: archeological sites, ethnographic resources, and cultural landscape resources (including individually significant historic structures and historic districts).

Section 106 of the National Historic Preservation Act, as amended, requires a federal agency to take into account the effects of its undertakings on properties included in, eligible for inclusion in, or potentially eligible for inclusion in the National Register of Historic Places, and provide the Advisory Council on Historic Preservation the reasonable opportunity to comment. A Programmatic Agreement (1999) was developed among the National Park Service at Yosemite, the California State Historic Preservation Officer, and the Advisory Council on Historic Preservation, in consultation with American Indian tribes and the public, to take into account the effects of park planning and operations on historic properties.

The methodology for assessing impacts to historic resources is based on stipulations V through VIII of the Programmatic Agreement (ACHP 1999). This 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 the National Environmental Policy Act, Section 106 of the National Historic Preservation Act, and the 1999 Programmatic Agreement regarding the Planning, Design, Construction, Operations and Maintenance of Yosemite National Park.

Duration of Impact

Impacts to historic properties (cultural resources) could be of short term, long term, or permanent duration. Analysis of the duration of impacts is required under National Environmental Policy Act, but is not required and is not usually considered in assessing effects in terms of National Historic Preservation Act.

Type of Impact

Impacts are considered either adverse or beneficial to historic properties (cultural resources) when analyzed under the National Environmental Policy Act. However, impact type is not viewed this way when conducting analysis under Section 106 of the National Historic Preservation Act. For the purposes of assessing effects to historic properties under the National Historic Preservation Act, effects are either adverse or not adverse. Effects under both the National Environmental Policy Act and the National Historic Preservation Act are considered adverse when they diminish the significant characteristics of a historic property.

Impacts can be either direct or indirect. Direct impacts result from specific actions, such as demolition of historic structures. Indirect impacts generally occur after project completion, and are a result of changes in visitor-use patterns or management of resources fostered by implementation of an action.

Intensity of Impact

The intensity of an impact on a cultural resource can be defined as negligible, minor, moderate, or major. Negligible impacts would be barely perceptible changes in significant characteristics of a historic property. Minor impacts would be perceptible and noticeable, but would remain localized and confined to a single element or significant characteristic of a historic property (such as a single archeological site containing low data potential within a larger archeological district, or a single contributing element of a larger historic district). Moderate impacts would be sufficient to cause a noticeable but not substantial change in significant characteristics of a historic property (such as an archeological site with moderate data potential or a small group of contributing elements within a larger historic district). Major impacts would result in substantial material alteration or destruction of the property or cause highly noticeable changes to any qualifying characteristics of a property that contribute to its historic significance (such as an archeological site with high data potential or a large group of contributing elements within a larger historic district).

The 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 intensity of impact from mitigation is an estimate of the effectiveness of this mitigation under the National Environmental Policy Act. It 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, the effects remain adverse.

Mitigation in this document is based on the Programmatic Agreement and includes the avoidance of adverse effects or the application of one or more standard mitigation measures 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.

Presented below are the specific discussions of duration, intensity, and type of impacts to cultural resources, and a description of typical mitigation measures.

Archeological Resources

Archeological resources are typically considered eligible for inclusion in the National Register of Historic Places because of the information they have or may be likely to yield (36 CFR 60.4).

Any change in the physical attributes of an archeological site is irreparable and considered adverse and of permanent duration. Adverse impacts to archeological resources most often occur as a result of earthmoving activities within an archeological site area, soil compaction or increased erosion, unauthorized surface collection, or vandalism. Beneficial impacts to archeological resources can occur when patterns of visitor use or management action are changed near archeological resources such that an ongoing impact, which would otherwise continue to degrade archeological resources, is reduced or arrested. 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 action near an archeological site, leading to things such as artifact collection, accelerated soil compaction, and 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 avoidance of sites through project design, or recovery of information that makes 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 considered not 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).^[1]

Ethnographic Resources

Ethnographic 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. They are considered long term if they involve a change in important vegetation or cultural feature, or addition of a new facility or visitor use that would change the physical character of or access to a resource for a noticeable period of time. 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 Structures

Impacts 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; removal of a significant structure or landscape resource; or addition of new, incompatible facilities in proximity to a historic site or structure. Indirect adverse impacts can also occur following project completion. These impacts 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 change in management of resources fostered by the completed plan.

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 twenty 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 (HABS/HAER). 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.

Socioeconomics

The 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 bridge replacement activities, these impacts are addressed in terms of Wawona and Mariposa Counties as a whole. 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). Total project-related spending is the combination of both direct and secondary spending effects.

Duration of Impact

Impact also included an assessment of duration. Distinguishing between short-term and long-term duration was necessary to understand the extent of the identified effects. In general, short-term impacts are temporary in duration and typically are transitional effects associated with implementation of an action (e.g., related to construction activities). In contrast, long-term impacts have a permanent effect on the social and economic environments (e.g., operational activities).

Intensity of Impact

The intensity of each impact was rated in terms of increasing severity, as negligible, minor, moderate, or major. Negligible impacts are effects considered not detectable and are expected to have no discernible effect on the socioeconomic environment. Minor impacts are slightly detectable and are not expected to have an overall effect on the character of the socioeconomic environment. Moderate impacts are detectable, without question, and could have an appreciable effect on the socioeconomic environment. Such impacts would have the potential to initiate an increasing influence on the socioeconomic environment (particularly if other factors have a contributing effect). Major impacts are considered to have a substantial, highly noticeable influence on the socio economic environments, and could be expected to alter those environments permanently.

Type of Impact

Impacts were recognized as beneficial if they would improve upon characteristics of the existing socioeconomic environment, as it relates to Wawona and Mariposa Counties as a whole. Conversely, impacts were considered adverse if they would degrade or otherwise negatively alter the characteristics of the existing environment.

Transportation

This 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. Vehicle access over the South Fork Merced River would be maintained during bridge replacement through the use of the temporary Bailey bridge.

The analysis of effects is based on professional transportation engineering judgment. Relative to the No Action Alternative (Alternative 1), the Preferred Alternative (Alternative 2), which calls for complete replacement of the South Fork Bridge, would affect traffic flows, 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.

Duration of Impact

Intensity of Impact

The intensities of impacts consider whether the impact would be negligible, minor, moderate, or major. Negligible impacts are effects considered not detectable and would have no discernible effect on traffic flow and/or traffic safety conditions. Minor impacts are effects on traffic flow and/or traffic safety conditions that would be slightly detectable, but not expected to have an overall effect on those conditions. Moderate impacts would be clearly detectable and could have an appreciable effect on traffic flow and/or traffic safety conditions. Major impacts would have a substantial, highly noticeable influence on traffic flow and/or traffic safety conditions and could permanently alter those conditions.

Type of Impact

Impacts are considered in the context of being either beneficial or adverse on traffic flow and/or traffic safety conditions. Beneficial impacts would improve traffic flow and traffic safety by reducing levels of congestion and occurrences of vehicle/vehicle, vehicle/bicycle, and vehicle/pedestrian conflicts. Adverse impacts would negatively alter traffic flow and traffic safety by increasing levels of congestion and occurrences of such conflicts.

Scenic Resources

The 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 South Fork Merced River Bridge Replacement Project on the landscape character and/or features and how any changes may be experienced (visibility, viewpoints, etc.).

Impacts of the South Fork Merced River Bridge Replacement Project on visual resources were examined and determined by:

§ Comparing the existing visual character of the landscape in terms of the color, contextual 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; and

§ 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 would be visible from viewpoints the National Park Service has established as important; (2) access to historically important viewpoints or sequence of viewpoints; or (3) the visibility of a viewpoint or sequence of viewpoints.

Duration of Impact

Intensity of Impact

The magnitude of impacts to the scenery within the view from specific vantage points and to specific scenic features is described as negligible, minor, moderate, or major as described below. Negligible impacts would be imperceptible or not detectable. Minor impacts would be slightly detectable or localized within a relatively small area. Moderate impacts would be those that are readily apparent. Major impacts would be substantial, highly noticeable, and/or result in changing the character of the landscape.

Type of Impact

Impacts were evaluated in terms of whether they would be beneficial or adverse to scenic resources. Beneficial impacts would enhance the existing landscape character, access to historically important viewpoints or sequence of viewpoints, or the visibility of a viewpoint or sequence of viewpoints. Adverse impacts would be effects that reduce the existing landscape character, access to historically important viewpoints or sequence of viewpoints, or the visibility of a viewpoint or sequence of viewpoints.

Recreation

This 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.

Yosemite National Park, including the South Fork Merced River and the Wawona area, offers a broad spectrum of recreation opportunities, including access to and availability of such activities as use of non-motorized watercraft (e.g., rafts, inner tubes, kayaks), swimming and wading, hiking, backpacking, camping, rock climbing, fishing, sightseeing, photography, nature study, and bicycling. In addition, every visitor to Yosemite National Park brings unique expectations and thus, each has a unique experience. As a result, the environmental assessment identifies, where possible, how the quality of the experience would change as a result of removing and replacing the South Fork Bridge and removing the temporary Bailey bridge.

An assumption that frames the analysis was that visitor demand will increase over existing levels and will be the same for both alternatives. 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.

Duration of Impact

Intensity of Impact

The intensity of impacts has been defined as negligible, minor, moderate, and major. Negligible impacts would result in little noticeable change in visitor experience. Minor impacts would result in changes but without appreciably limiting or enhancing opportunities for recreation. Moderate impacts would change the recreational opportunities. Major impacts would eliminate or greatly enhance recreational opportunities

Type of Impact

Impacts were evaluated in terms of whether they would be beneficial or adverse to recreational opportunities. Beneficial impacts would enhance visitor participation and the quality of visitor experience. Adverse impacts would be effects that reduce visitor participation and quality of visitor experience.

Park Operations and Facilities

For 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

Duration of Impact

Intensity of Impact

Type of Impact

Adverse impacts represent an increase in park operations staffing, from effects on utility and ro