Introduction

This section presents topics included in the analysis of the Merced River Plan/FEIS and a rationale for their inclusion. These topics were selected based on federal law, regulations, and executive orders, National Park Service management policies, and concerns expressed by the public, park staff, or other agencies during scoping and comment periods. This section also provides a discussion of topics that were dismissed from further analysis.

A short rationale for each impact topic considered in this chapter is given below. A description of the existing conditions for each selected topic is provided later in this chapter. Existing conditions are represented by the most recent natural and cultural resources analyses completed for each topic area (for example, a portion of the flood analyses for Yosemite Valley and El Portal were completed in 1997 through 1999). The affected environment described in this chapter covers the geographical area included with all of the alternatives, and also includes areas adjacent to the Merced River corridor. The potential impacts of each alternative within each topic area are presented in Chapter IV, Environmental Consequences.

The federal and state Endangered Species Acts (and associated legislation), Clean Water Act, Clean Air Act, and National Environmental Policy Act require that the effects of any federal undertaking examine natural resources. In addition, the National Park Service management policies and natural resource management guidelines call for the consideration of natural resources in planning proposals. Significant natural resources, such as rare, threatened, and endangered species, exist within the park and could be affected by implementation of the alternatives.

Originating in Yosemite National Park, the Merced River traverses a region of abundant natural resources. It is therefore necessary to characterize both these natural resources and the environmental consequences to these resources that would result from implementation of Merced River Plan/FEIS alternatives.

Analysis was performed for the following natural resource topics:

• Geology, Geohazards, and Soils

• Hydrology, Floodplains, and Water Quality

• Wetlands

• Vegetation

• Wildlife

• Rare, Threatened, and Endangered Species

• Air Quality

• Noise

The National Historic Preservation Act, the Archeological Resources Protection Act, Native American Graves Protection and Repatriation Act, and the National Environmental Policy Act require that the effects of any federal undertaking on cultural resources be examined. In addition, National Park Service management policies and cultural resource management guidelines call for the consideration of cultural resources in planning proposals. Significant historic and archeological sites, museum collections, historic buildings and structures, cultural landscape resources, and traditional cultural properties exist within the park and could be affected by the alternatives.

Analysis was performed for the following cultural resource topics:

• Archeological Resources

• Ethnographic Resources

• Cultural Landscape Resources, including Historic Sites and Structures

Stewardship of Yosemite National Park requires the consideration of two integrated purposes: to preserve Yosemite's unique natural and cultural resources and scenic beauty, and to make these resources available to visitors for study, enjoyment, and recreation. Many management options (i.e., zone designations and prescriptions) considered in the Merced River Plan/FEIS would affect patterns of visitor use and the type and quality of visitor experiences.

Analysis was performed for the following visitor experience topics:

• Recreation

• Orientation and Interpretation

• Visitor Services

• Wilderness Experience

Analysis of social resources examines the effects of Yosemite visitation on the social environment within the park and in the surrounding region, and how visitors experience social resources within the park. Transportation is analyzed because management zoning designations and prescriptions considered in the Merced River Plan/FEIS could affect how visitors access the park. The park's scenic resources are a major component of the park visitor's experience. Conserving the scenery is a crucial component to the National Park Service 1916 Organic Act and the park's enabling legislation. The National Environmental Policy Act requires that socioeconomic impacts of the Merced River Plan/FEIS be addressed. The Merced River Plan could affect socioeconomic activity within the park and in the surrounding region.

Analysis was performed for the following social resource topics:

• Land Use

• Transportation

• Scenic Resources

• Socioeconomics

• Park Operations and Facilities

Environmental justice analyses determine whether a proposed action would have "disproportionately high and adverse human health or environmental effects...on minority populations and low-income populations." The National Park Service and other federal agencies have determined that a disproportionately high and adverse effect on minority and low-income populations means an adverse effect that:

Potential adverse effects identified in an environmental justice analysis include air, noise, and water pollution; soil contamination; destruction or diminution of aesthetic values; destruction or disruption of community cohesion and economic vitality; displacement of public and private facilities and services; increased traffic congestion; and exclusion or separation of minority or low-income populations from the broader community. Of particular concern is the effect on property acquisition and displacement of people.

No aspect of any alternative in the Merced River Plan/FEIS would result in disproportionately high and adverse human health or environmental effects on minority populations or low-income populations. Any restriction on travel, lodging accommodations, or access to any area of the park that might result from the Merced River Plan would be equally applied to all visitors, regardless of race or socioeconomic standing. As well, no alternative of the Merced River Plan/FEIS would change current management direction concerning housing policy in El Portal or other areas adjacent to the park. Policies concerning the future availability of housing in these areas are already in place and would not change as a result of the Merced River Plan. Therefore, the Merced River Plan would not result in destruction or disruption of community cohesion and economic vitality; displacement of public and private facilities and services; increased traffic congestion; and/or exclusion or separation of minority or low-income populations from the broader community.

Impacts on low-income populations are addressed in the Socioeconomics section in Chapter IV.

There are no known agricultural lands directly involved in this plan, and thus no further discussion of this topic is necessary. Also, this plan would not have any indirect effects to downstream agricultural lands.

Public health and safety is not presented as a separate topic in this plan since many sections (water quality, recreation, park operations, and others) evaluate park-related public health and safety issues.

The Museum Collection is not presented as a separate topic. However, actions associated with implementation of elements of the Merced River Plan/FEIS alternatives could indirectly affect the museum collections. These changes would involve additions to the collections generated from archeological data recovery conducted as mitigation for direct site impacts. These changes would be minimal and would require additional museum storage space and ongoing collections maintenance and management.

Regional Setting

Yosemite National Park lies on the western slope of the Sierra Nevada, a massive mountain range dividing central and northern California from more arid lands to the east. Elevations in the park range from approximately 2,000 to 13,114 feet. The total area within the park's authorized boundary is 761,266 acres. The El Portal Administrative Site is approximately 1,398 acres. U.S. Forest Service land surrounds the park and is divided into four national forests: Stanislaus, Toiyabe, Inyo, and Sierra. The four counties immediately surrounding the park consist of Mariposa, Tuolumne, Madera, and Mono (see figure III-1).

Yosemite National Park is located about 200 miles east and four hours by car from San Francisco, and about 320 miles northeast and six hours from Los Angeles. There are five entrances to the park. Four are on the west side of the Sierra Nevada: the Big Oak Flat Entrance along the Big Oak Flat Road; the Arch Rock Entrance Station on the El Portal Road; the Hetch Hetchy Entrance; and the South Entrance on the Wawona Road. The Tioga Pass Entrance on the Tioga Road offers seasonal access from the east side of the Sierra Nevada.

According to the bioregional characterizations developed as part of California's Agreement on Biological Diversity (a multiagency memorandum signed in 1993), the area is within the Sierra Nevada Bioregion. The region (which extends through the foothill zone on the west side and the base of the escarpment on the east side), is about 450 miles long and 100 miles wide (approximately 20,663,930 acres).

The Sierra Nevada range contains the headwaters of 24 major river basins, two of which are in the park: the Merced River and the Tuolumne River. In 1984, Congress established portions of the main stem of the Tuolumne River, and the Dana and Lyell Forks, as part of the National Wild and Scenic Rivers System. In 1987, the Wild and Scenic Rivers Act was amended to include 114 miles of both the main stem and the South Fork of the Merced River as Wild and Scenic.

The Merced River flows from the headwaters in the high elevations of the Sierra Nevada, through Yosemite Valley, and down to the San Joaquin Valley, where it contributes to the San Joaquin River. The main stem of the Merced River drains approximately 250,000 acres from the headwaters within the park to the Foresta Bridge in the El Portal Administrative Site. The main stem of the Merced River flows a total of 140 miles from its headwaters to the confluence with the San Joaquin River. Principal tributaries of the Merced River within the park boundaries include Lyell Fork, Triple Peak Fork, Red Peak Fork, Merced Peak Fork, Sunrise Creek, Illilouette Creek, Tenaya Creek, Yosemite Creek, Sentinel Creek, Ribbon Creek, Bridalveil Creek, Cascade Creek, Grouse Creek, Avalanche Creek, and Indian Creek. The principal tributaries of the South Fork of the Merced River within park boundaries include Chilnualna Creek, Big Creek, and Alder Creek. The South Fork drains the southern portion of the park, an area of approximately 76,000 acres. The Tuolumne River drains the northern portion of the park, an area of approximately 435,000 acres.

The major vegetation zones of the Sierra Nevada ecosystem form readily apparent large-scale north-south elevational bands along the axis of the Sierra. Major east-west watersheds that dissect the Sierra into steep canyons form a secondary pattern of vegetation. On the west side, forest types change from ponderosa pine to mixed conifer to firs with increasing elevation. A subalpine and alpine vegetation zone is located on the crest of the Sierra Nevada range. Fire suppression and changing land use practices have dramatically affected natural fire regimes, altering ecological structures and functions in Sierra Nevada plant communities (SNEP 1996).

The Sierra Nevada is rich in plant diversity. As a group, Sierra Nevada plants are most at risk where habitat has been reduced or altered. However, rare local geologic formations and their derived unique soils have led to the evolution of ensembles of plant species restricted to these habitats. This is true in the El Portal area, which supports a number of California state-listed rare species that are sustained in a unique contact zone of metamorphic and granitic rock.

About 300 terrestrial vertebrate species (including mammals, birds, reptiles, and amphibians) use the Sierra Nevada as a significant part of their range. Three modern vertebrate species once well distributed in the range are now extinct from the Sierra Nevada: Bell's vireo, California condor, and grizzly bear. Sixty-nine species of terrestrial vertebrates (17% of Sierra Nevada fauna) are considered at risk by state or federal agencies. These species include bighorn sheep, Yosemite toad, foothill yellow-legged frog, mountain yellow-legged frog, and the western pond turtle. The most important identified cause of the decline of Sierran vertebrates has been loss of habitat, especially foothill and riparian habitats and late successional forests.

Aquatic and riparian systems are the most altered and impaired habitats of the Sierra Nevada. Dams and diversions throughout most of the Sierra Nevada have profoundly altered stream-flow patterns and water temperatures. Foothill areas below about 3,300 feet appear to have the greatest loss of riparian vegetation of any region in the Sierra Nevada (SNEP 1996).

Natural Resources

Yosemite National Park occupies approximately 1,170 square miles within the central portion of the Sierra Nevada (Spanish for "snowy mountain range"). The Sierra Nevada is the highest and most continuous mountain range in California, extending approximately 450 miles north to south and averaging approximately 100 miles wide. The range is generally asymmetrical, with a gentle west slope and a steep east escarpment. Elevations approach sea level on the western side and reach about 14,000 feet at the crest.

The Sierra Nevada is essentially an uplifted block of the Earth's crust that was tilted westward by normal faults on the eastern boundary. Granitic bedrock is widespread in Yosemite National Park and dominates a significant portion of the Sierra Nevada. The granitic rock formed deep within the Earth as a pluton of melted rock. About 200 million years ago (mya), as the granitic rocks were formed, heated, and melted, they slowly migrated toward the Earth's surface and began to cool, forming subsurface bodies of solidified granitic rock called the batholith.

Between 100 mya and 65 mya, magma formation slowed and a long period of erosion began in the Sierra Nevada. Erosion removed the overlying rocks and exposed the underlying core of the granitic batholith. Eroded material was transported westward and filled the present-day Central Valley with deposits that were tens of thousands of feet thick. About 15 mya, the relief of the Sierra Nevada in the Yosemite region had gently rolling upland topography and a much lower elevation than the present-day range. The Merced River flowed westward at a gentle gradient through a broad river valley. Volcanic activity, prevalent in the northern Sierra Nevada from about 38 to 10 million years ago, deposited ash, filled valleys, buried streams, and altered river courses.

Mountain-building activity was reactivated about 25 to 15 mya, uplifting and tilting the Sierra Nevada to form its relatively gentle western slope and the more dramatic, steep eastern slopes. The uplift increased the gradients of the rivers and resulted in deeply incised river valleys.

Between 2 mya and 3 mya, snow and ice accumulated as glaciers at the higher alpine elevations and began to move westward down the mountain valleys. At least three major glacial periods occurred during the ice age in the Sierra Nevada and are known as the Pre-Tahoe (oldest), the Tahoe (intermediate), and the Tioga (youngest). The downslope movement of the ice masses cut and sculpted the valleys, cirques, and other glacially formed landforms throughout the Yosemite region and the Sierra Nevada. The depositional and erosional glacial features viewed today in Yosemite are primarily the result of the Tioga event, although the cumulative effects of the previous glaciations are responsible for the overall shape and character of the region.

The Tioga was the last glaciation event and began as late as 60,000 years ago, when the climate cooled sufficiently to allow small glaciers to form on erosional features sculpted by earlier glaciers. Throughout this period in the Yosemite area, the ice field grew and pushed fingers of ice into the major drainages on the west slopes, until it reached the maximum extent about 20,000 years ago. The Tioga glacier reached only as far as Bridalveil Meadow and left behind features such as erratics, glacial till, and moraines. The Tioga glacial event left the landscape scoured and small basins filled with silt and sediment (Huber 1989).

Granitic and metamorphic rocks dominate Yosemite National Park, with the granitic rocks being most abundant and metamorphic rocks constituting less than 5% of the area within the park (Huber 1989). The metamorphic rocks represent the older rock that the granitic plutons intruded. Granitic rocks form from the cooling and solidification of molten rock within the Earth's crust.

The granitic batholith of Yosemite National Park is not monolithic, but rather was formed through a series of intrusive events over a period of 130 million years. The separate episodes of intrusion and solidification formed more than 100 discrete plutonic masses, making up several granitic rock types. The particular type of granitic rock is distinguishable by the varying mineral composition, texture, and percentages of primary minerals. Granitic rocks in Yosemite National Park include granite, granodiorite, and tonalite.

The upper reaches of the main stem of the Merced River are dominated by the interaction of a wild river flowing through granitic landscapes. This glaciated valley is narrow with steep gradients in some areas, and wider in other areas where the river flows at a gradual slope and forms a floodplain. The width of the river valley can range from 960 feet in the narrower, steeper sections to 2,600 feet in the wider areas. The Bunnell Cascades is an example of steep gradient flow within a relatively steep canyon; the Merced River through Little Yosemite Valley exemplifies a river flowing on a wider floodplain.

Yosemite Valley is primarily granitic in composition and glacially carved, with its floor ranging from 3,800 to 4,200 feet above mean sea level (msl). The Valley is oriented in an east-west direction, and its sides rise 1,500 feet to 4,000 feet above the essentially flat Valley floor. Yosemite Valley. not including Tenaya Canyon or Little Yosemite Valley. is about 6.8 miles long and varies from a little under ½ mile wide to around ¾ mile wide. The east Valley branches into the Tenaya Canyon to the north and the Little Yosemite Valley to the south.

The downslope movement of the ice masses cut and sculpted the U-shaped valley that is present today. When glaciers melt, the rock debris they transport (till) is deposited in a ridge-shaped landform known as a moraine. Two prominent moraines were formed in Yosemite Valley after the last glacier (the Tioga) retreated about 15,000 years ago. A terminal moraine, marking the furthest extent of the glacier, lies just east of Bridalveil Meadow. The El Capitan moraine, lying further east, is a recessional moraine, formed after the leading edge of the glacier had retreated up the Valley from its furthest extent. After the last glacier melted, water flow was dammed by morainal material that formed what is now referred to as the prehistoric Lake Yosemite. Stream deposits then filled in Lake Yosemite, adding to the 1,000-foot-thick sediment that underlies the present-day floor of Yosemite Valley and covers the glacially disturbed granite rock below.. The moraines in the Valley, especially those below El Capitan Meadow and near Bridalveil Fall, along with other geological features, have been identified as Outstandingly Remarkable Values.

The Merced River gorge begins at the west end of Yosemite Valley where the gradient of the Merced River abruptly increases and the river enters the gorge. The gorge has remained an incised, V-shaped feature because most recent glacial events did not extend down the Merced River beyond Yosemite Valley. The transition from the U-shaped, glaciated Yosemite Valley to the steep-gradient, V-shaped, incised Merced River gorge, is identified as a geologic Outstandingly Remarkable Value.

The granitic rocks within the Merced River gorge consist primarily of tonalite; the Bass Lake tonalite is the dominant bedrock feature. Among some of the oldest rocks found in the Sierra Nevada are those just east of El Portal, in the walls of the Merced River gorge. These rocks are metamorphic and are remnants of ancient sedimentary and volcanic rocks that were deformed and metamorphosed, in part by granitic intrusions (Huber 1989). The transition from igneous to metasedimentary rocks is identified as a geologic Outstandingly Remarkable Value. This sedimentary rock (which includes banded chert) was once part of the ocean floor that covered the region about 200 mya (Huber 1989).

From its headwaters, the South Fork flows west at a relatively consistent gradient through a glaciated alpine environment and then enters a V-shaped, unglaciated river canyon below Wawona. Glaciation sculpted the upper reaches of the South Fork. Compared to the main stem, there is more variation of the bedrock regime along the South Fork of the Merced River. At the headwaters, the South Fork is in contact with metamorphic volcanic rocks, including ash flow deposits. As it flows eastward, the South Fork contacts granitic rocks, metamorphic rocks near Gravelly Ford, and granite, similar to that found in Yosemite Valley, eight miles east of Wawona.

Wawona Dome, visible from the river, is an exfoliating granite dome with an elevation of approximately 6,900 feet above sea level. Upon entering Wawona, the South Fork cuts through the granitic tonalite, a predominant granitic rock found along the southwest boundary of the park. The riverbed remains within tonalite, except for a short section underlain by metamorphic rocks near the park boundary. These rocks are among the oldest exposed along the South Fork.

The Merced River flows through geologically active areas, where geologic and hydrologic forces continue to shape the landform. Geohazards associated with these forces, such as earthquakes and rockfalls, present potentially harmful conditions to visitors, personnel, and facilities in Yosemite National Park.

The Sierra Nevada range of Yosemite National Park is not considered an area of particularly high seismic activity. No active or potentially active faults have been identified in the mountain region of the park (CDMG 1990). However, Yosemite can undergo seismic shaking associated with earthquakes on fault zones to the east and west margins of the Sierra Nevada range, as it has done in the past. These fault zones include the Foothills fault zone, the volcanically active area in the Mono Craters. Long Valley Caldera area, and along the various faults within the Owens Valley fault zone (CDMG 1996).

The Foothills fault zone extends in a north-south direction within the foothills of the Sierra Nevada, approximately 50 miles west of Yosemite Valley. This fault zone has not experienced movement in the last 2 million years and thus is not considered active or potentially active (CDMG 1990).

The Mono Lake fault is located approximately 35 miles northeast of Yosemite Valley within the Mono Craters. Long Valley Caldera region. Over the last 12 years, this area has been one of the most seismically active regions in California. Earthquakes have been attributed to movement on the Mono Lake fault (Sierra Nevada frontal fault) and movement associated with resurgent volcanic activity of the Long Valley Caldera. The Mono craters last erupted 600 years ago. Most recently, the Mono Lake fault experienced a 5.7 Richter movement in October 1990. This earthquake was felt as far west as Sacramento and the San Francisco Bay Area and caused landslides and rockfalls at Tioga Pass and on the Big Oak Flat Road (McNutt et al. 1991).

The Owens Valley fault, located approximately 100 miles southeast of Yosemite Valley, has experienced movement within the last 200 years, and the California Division of Mines and Geology considers this fault active (CDMG 1990). The most notable earthquake recorded in Yosemite National Park was the Owens Valley earthquake of March 26, 1872. The Owens Valley earthquake is estimated to have had a Richter magnitude of 7.6 and was one of the largest earthquakes in U.S. history (USGS 1991a). This earthquake reportedly caused damage in Sacramento and San Joaquin Valleys and caused significant rockfalls in Yosemite Valley.

Although earthquakes that are felt by people in Yosemite National Park are relatively infrequent, they have occurred in the past and will likely occur in the future. Groundshaking is typically expressed in peak acceleration due to gravity as a percent of 1 g (g is acceleration due to gravity, or 980 centimeters. 32 feet. per second squared). The peak accelerations estimated in the Yosemite National Park region of the Sierra Nevada are between 0.1 and 0.2 g (CDMG 1999). Most people would likely feel this range of groundshaking, but structural damage would be negligible to slight in buildings constructed according to modern building standards.

Rockfall is used as a generic term to refer to all slope movement processes, including rockfall, rockslide, debris slide, debris flow, debris slump, and earth slump. Rocks have become dislodged and fallen off the sheer granite cliffs throughout the geologic history of Yosemite. Rockfalls can displace large volumes of rock and can occur due to such processes as the climate-related expansion and contraction of rock, seismic shaking, or exfoliation. Exfoliation is caused by differential stresses that form within the rock mass as the stress of the overburden is released. This process causes concentric granitic plates, ranging in size from inches to several feet, to become dislodged from the granite face.

Expansion and contraction caused by alternating freezing and thawing of water in the cracks of Yosemite's cliffs weaken its structure and result in periodic rockfalls. Rockfalls have created steep talus slopes along each side of the Yosemite Valley that provide better drained soils and warmer microhabitats than are found on the adjacent Valley floor, as well as crevices and caves that are home to many animal species.

Most rockfalls are associated with triggering events such as earthquakes, rainstorms, or periods of warming that produce a rapid melting of snow. The magnitude and proximity of the earthquake, intensity and duration of the rainfall, and the thickness of the snow-pack/pattern of warming all influence the triggering of rockfalls. However, some rockfalls occur without a direct correlation to an obvious event and are probably associated with gradual stress release and exfoliation of the granitic rocks (USGS 1998b).

More than 400 rockfalls have been recorded within Yosemite National Park and some have resulted in injury and on occasion, death. Rockfalls can also result in the damage or destruction of roads, trails, and buildings.

A prehistoric rockfall dammed Tenaya Creek and formed Mirror Lake. Famed writer and naturalist John Muir was in Yosemite Valley when the 1872 Owens Valley earthquake occurred and described the earthquake-triggered rockfall he observed:

Two types of areas of potential rockfall impact have been identified. The first is the area closest to the Valley or canyon walls and is called the Talus Zone. The second area, referred to as the Rockfall Shadow Zone, extends out from the Talus Zone and is the area in which rocks may travel out from the talus. The frequency and magnitude of rockfall events vary considerably. Many small rockfalls may occur every year and go unnoticed, while larger rockfalls occur much less frequently (USGS 1998b). The National Park Service, in cooperation with the U.S Geological Survey, is currently identifying potential geologic hazards in developed areas, including areas most susceptible to rockfalls (Wieczorek et al. 1999). The National Park Service is revising its management policies regarding geologic hazards, with the intent to better protect park visitors and staff by avoiding placement of structures in areas with a high potential for rockfall impact.

Yosemite Valley is in the upper or middle portion of the canyon of the Merced River, which was deepened by several episodes of glacial erosion. The most recent glaciation (Tioga) extended eastward of Bridalveil Meadow, where the Merced River now meanders across the relatively flat Valley. Except for large rock avalanches, the talus from rockfall and rockslide deposits seldom reaches the center of the Valley. However, debris flows (which are very fluid in nature) can carry boulder debris far into the Valley, even on moderately gentle slopes. The Yosemite Valley narrows to the west of Bridalveil Meadow, and talus from rockfalls and rockslides extends from the cliffs down to the banks of the Merced River.

Accumulating talus (angular rock fragments), ranging in size from small rocks to large boulders, forms slopes at the base of the sheer rock cliffs at the Valley edge. The rockfalls and associated talus slopes contribute to the natural topography and to the formation of soils on the Valley floor. Rockfalls from the sheer Valley walls have, over time, created talus cones of debris spreading away from the edges of the cliffs. Some of the rockfalls are sizable and have contributed to altering the course of the Merced River.

Rockfalls have left abundant deposits of talus around the base of almost all the walls of Yosemite Valley. The extent of talus around the edge of the Valley is estimated at some places to be greater than 300 feet thick (Wieczorek and Jaeger 1996). At some locations, such as below El Capitan, where large prehistoric rock avalanches have occurred, these deposits extend from the base of the wall about 1,400 feet across the Valley floor.

Rockfalls in Yosemite National Park range in size from small individual blocks of less than 1 cubic meter to rock avalanches of several million cubic meters. All such events pose a potential hazard; even a rapidly moving small boulder can cause serious injury to people, vehicles, or buildings. The frequency of different-size rockfalls has been determined from an analysis of historical events (Wieczorek et al. 1995).

Significant incision of the river has created the present-day relief of the gorge and a change of gradient of over 2,000 feet in just over seven miles between Pohono Bridge to the park boundary. The gorge area has had more rockfall incidences than any place in the park. Several of these have occurred along El Portal Road. The high incidence of rockfalls is partly due to the steep, narrow configuration of the gorge, river bank undercutting, and historic human activity such as the construction of El Portal Road. These events have been well documented (USGS 1992a) and provide information regarding historic rockslide hazards along the Merced River gorge and in areas where unstable rock slopes are known to pose a risk of future rockfall events. Rockfall hazards are similar in El Portal to those in the Merced River gorge. Areas with steep cliffs surrounding El Portal are susceptible to rockfall events, especially on cliffs composed of highly fractured granitic and metamorphic rocks. Hazards associated with seismic groundshaking would affect El Portal as they would the Merced River gorge and elsewhere in Yosemite National Park.

All soils form as a result of the combined effect of several factors, including geologic parent material, climate, biologic activity, topographic position/relief, and time. Within the park, topography is the most important factor contributing to soil differentiation. Topography influences surface runoff, groundwater, the distribution of stony soils, and the separation of various-aged alluvial soils (NPS 1980a). More than 50 soil types are found within the park; general or local variations depend upon glacial history, microclimatic differences, and the ongoing influences of weathering and stream erosion/deposition (NPS 1978).

Soils of the Yosemite National Park region are primarily derived from underlying granitic bedrock and are of similar chemical and mineralogical composition. Except for meadow soils, most high country soils developed in glacial material (glacial soils) or developed in place from bedrock (residual soils). Extensive areas above 6,000 feet are covered by glacial moraine material, a mixture of fine sand, glacial flour, and various-sized pebbles and boulders. Alluvial soils developed along streams through erosion and deposition and tend to have sorted horizons of sandy material. Various areas of Yosemite National Park have meadow soils consisting of accumulated clays, silts, and organic debris that are subjected to occasional flooding. The El Capitan fine-sandy loam, found in and around El Capitan Meadow, is an example of a meadow soil. Colluvial soils have developed along the edges of cliffs where landslides and rockslides have occurred and are composed of various-sized rocks that have high rates of infiltration and permeability. Weathering processes break down talus to smaller-sized particles that are then transported by water and eventually become deposited in alluvial fans or in stream channels. The surface soil in Yosemite Valley, for instance, consists primarily of granitic sands in various stages of decomposition (USGS 1996a).

Organic content within the upper soil profile varies with local influences of moisture and drainage. Thick sedges and grasses have significantly contributed to the organic content of soils near ponds, lakes, and streams. Coniferous forest soils have a high organic content and are relatively acidic. Soils lacking organic accumulations are frequently a result of granitic weathering, consist largely of sand, and support only scattered plants tolerant to drought conditions (NPS 1980b).

Soils specific to the upper Merced River have not been mapped but are similar in chemical and mineralogical composition to those in the Yosemite Valley region. Glacial history, weathering, fluvial process, and erosion contribute to the local variations in soil compositions. High country soils (excluding meadow soils) are typically glacial or residual, and alluvial soils can be found near streams. Glacial moraines and deposits cover areas above 6,000 feet.

Most of Yosemite Valley is an active floodplain of the Merced River. During Merced River flood events, alluvial soils are formed and removed as floodwaters deposit and erode material over the floodplain. The active flooding builds river terraces of fine- to coarse-textured sands. Old riverbeds of boulders and gravel may be buried under the terrace soils. Residual soils are scattered throughout Yosemite Valley where bedrock weathering has occurred. Glacial soils are associated principally with moraines. Colluvial soils have developed on the talus slopes along the edges of the Valley floor. Valley soil textures vary from fine sand to fine gravel. Most soils have a relatively undeveloped profile, indicating their relatively recent origin and young geologic age.

The Natural Resource Conservation Service identified 21 soil series/types in Yosemite Valley. Each soil type has specific characteristics that influence plant growth, water movement, land use capabilities, etc. Land use limitations are commonly associated with frequent flooding, seasonally high water table, poor drainage, steep slopes, high rock concentration, and a poor soil structure. The El Capitan fine sandy loam, found in and around El Capitan Meadow, is an example of a Yosemite Valley soil with physical constraints that limit land use due to occasional flooding.

The soils in relatively flat topographic positions in the Merced River gorge and El Portal form from glacial and alluvial sediment deposition processes originating in Yosemite Valley, or by alluvial and colluvial deposition occurring locally within the gorge or near El Portal. Soils that formed in old river channels consist of alluvial boulders, cobbles, river wash, and loamy sands. These soils have, for the most part, moderate to severe development limitations and thus require the implementation of engineering and mitigation measures.

Soils in the upper reaches of the South Fork are similar in chemical and mineralogical composition to those in the upper Merced River. Parent rock type, glacial history, weathering, fluvial process, and erosion contribute to the local variations in soil compositions. High country soils (excluding meadow soils) are typically glacial or residual, and alluvial soils typically form near streams.

Soils of the Wawona area are primarily residual on slopes and alluvial in the Valley. Soil depth varies from 2 to 4 feet above bedrock; these soils are moderately to strongly acidic. Most soils are subject to erosion after disturbance or loss of vegetative cover. The major soil types are distinguished by their mixtures of loam, sand, and silt, and by the amount and type of rock fragments.

Yosemite National Park has a variety of surface water features, some of which are major attractions for visitors, such as Yosemite, Bridalveil, Nevada, and Vernal Falls. Hydrologic processes. including glaciation, lake to meadow succession, and fluvial geomorphic response. have been fundamental in creating surface water features and landforms in the park. Flowing water (including glacial flow) has helped to create the existing landscape and will continue to modify the landscape through erosion and alluvial processes. The park includes the headwaters and significant stream reaches of the Tuolumne and Merced Rivers and contains approximately 1,591 lakes and 1,700 miles of streams within its boundary.

The Merced River basin encompasses the main stem of the Merced River and its watershed area, and the South Fork of the Merced River basin encompasses the South Fork and its watershed area. Within Yosemite National Park, these areas contain separate and unique watersheds, sustain separate hydrologic and aquatic resources, and support differing levels of development. Therefore, these watersheds are addressed separately in this discussion.

The park is drained by two major watersheds: the Tuolumne and the Merced River, both of which are tributaries of the overall San Joaquin River basin. The Merced River basin includes the main stem and the South Fork. The Tuolumne and Merced River systems originate along the crest of the Sierra Nevada mountains, eventually carving river canyons 3,000 to 4,000 feet deep on their paths to the Central Valley. The Tuolumne River drains the entire northern portion of the park, an area of approximately 435,000 acres (681 square miles). The Merced River basin begins in the southern peaks of the park, primarily the southern aspects of the Cathedral Range and the Clark Range, and drains the southern one-third, or 326,000 acres (511 square miles), of the park.

The main stem of the Merced River flows from the headwaters in the high elevations of the Sierra Nevada, through Yosemite Valley, and down to the Central Valley where it contributes to the San Joaquin River. This river basin drains 250,000 acres (391 square miles) within the boundaries of the park. The main stem of the Merced River flows a total of 140 miles from its headwaters to the confluence with the San Joaquin River (USGS 1992). Principal tributaries of the Merced River within the park boundaries and the El Portal Administrate Site include the Merced Peak, Lyell, Triple Peak, and Red Peak Forks, as well as Sunrise, Illilouette, Tenaya, Yosemite, Sentinel, Ribbon, Bridalveil, Cascade, Grouse, Avalanche, Indian, and Crane Creeks.

For the purpose of discussion within this section, the Merced River basin is divided into three hydrologic segments: the upper Merced River, Yosemite Valley, and the Merced River gorge (which includes the El Portal Administrative Site). This division is based upon the unique watershed characteristics of the three river areas. Discharge flows within the different areas reflect the contribution of the overall watershed upstream of the noted streamflow gauging location.

Upper Main Stem Watershed. The upper Merced River watershed is located on the western slope of the Sierra Nevada mountains in Yosemite National Park.^[1] The watershed encompasses 114,843 acres (181.9 square miles), with elevations ranging from 4,000 feet at Happy Isles Bridge to over 13,000 feet at Mt. Lyell. Located within the watershed are the sub-basins of the upper Merced River and Illilouette Creek as well as over 100 lakes and ponds (Williamson et al. 1996a). The watershed consists of mountainous valleys with steep walls, large areas of exposed granite, and forested areas common along the valley floors. The upper Merced River watershed topography is characterized by jagged peaks, precipitous cliffs, steep canyons, broad inter-stream areas of glacially smoothed granite, small lakes and meadows, and thin, granitic soils. Above 9,600 feet are alpine and subalpine zones with little vegetation and low soil permeability. From 8,000 to 9,600 feet is a lodgepole pine zone with limited ability to hold soil moisture. Much of the area from 6,000 to 8,000 feet is red fir forest, which intercepts a high percentage of the rainfall, which is held in alluvial soils. Mixed coniferous forests grow on thin to moderate depth soils from 4,000 to 7,000 feet.

The upper Merced River descends from its headwaters through a glacially carved canyon at a gradient of about 8,000 feet over 24 miles, or an average gradient of approximately 330 feet per mile (USGS 1992). Generally, the streambank and floodplains are vegetated with mature fir, pine, and cedar trees and abundant understory species.

Human infrastructure in the watershed includes hiking trails, bridges, a diversion wall, small utility systems, and wilderness campsites. Bridges in this upper watershed consist of footbridges made of wood and stone that can be obstructions to the free flow of the river during high flows. Before the turn of the century, a diversion wall was constructed at Nevada Fall to divert flow away from what is now the Mist Trail in order to protect the trail that once led to the former La Casa Nevada Hotel just below Nevada Fall.

The average daily discharge rate of the upper Merced River watershed (measured at the Happy Isles gauging station) is approximately 355 cubic feet per second (cfs), and the average annual total discharge is approximately 257,400 acre-feet (USGS 1998).

Yosemite Valley Watershed. The Yosemite Valley watershed includes Yosemite Valley and its tributary areas. The main tributaries to the Merced River in Yosemite Valley are Tenaya Creek, Illilouette Creek, Yosemite Creek, and Bridalveil Creek. At Pohono Bridge, the overall Merced River basin encompasses 205,000 acres (321 square miles) (USGS 1999). Historic discharge in the river, measured at the Pohono Bridge gauging station, has ranged from a high of about 25,000 cfs to a low of less than 10 cfs. The mean daily discharge rate is about 600 cfs, with an average annual total discharge of approximately 435,400 acre-feet (NPS 1978).

During the most recent period of glaciation in Yosemite Valley, a glacier extended to approximately the location Pohono Bridge. Following glacial retreat, a large lake (Lake Yosemite) developed and eventually filled with sediment from the El Capitan moraine to upstream of Happy Isles (Huber 1989). The resulting valley floor has a very mild slope and is responsible for the meandering pattern of the present-day river. The Merced River has a relatively mild slope, with an average of 0.1% through Yosemite Valley (USGS 1992). The Merced River is an alluvial river within Yosemite Valley, and the bed and banks of the channel are composed of smaller sediments and cobbles and soil layers. This condition makes for a dynamic river that alters its course periodically by eroding and depositing bed and bank material. In most locations, the river flows through a shallow channel approximately 100 to 300 feet wide. In the middle of Yosemite Valley, the Merced River has the capacity to convey an amount between the two- and five-year flow within the existing channel banks (NPS 1998d). Eleven bridges cross the Merced River between Happy Isles and the Pohono Bridge. Many of these bridges influence the width, location, and velocity of the Merced River (NPS 1991a). In a natural river channel, the stream banks slope at an angle away from the stream, resulting in a wider channel as flows increase. However, arched bridges such as Stoneman and Pohono (see figure III-2) confine flows in the Merced River and result in a narrowing of the channel as flows increase. The velocity of the river through a bridge can be accelerated, causing increased channel scouring directly downstream of the bridge. Substantial scour around bridge abutments is evident at several bridges in Yosemite Valley.

If flow cannot be conveyed through a bridge during periods of high discharge, water backs up behind the bridge. This backwatering can inundate low-lying areas or overflow channels. The Merced River within Yosemite Valley is constricted at all bridge sites between Happy Isles and Pohono Bridge (Milestone 1978).

Merced River Gorge and El Portal Watershed. The Merced River gorge watershed includes the watershed area from Pohono Bridge through the El Portal Administrative Site. Within this area, the Merced River has a much steeper gradient than in Yosemite Valley, and consists mostly of continuous rapids. As the river exits Yosemite Valley, it cascades at an average gradient of approximately 70 feet per mile through the narrow, steep-sided Merced River gorge. The riverbed and banks are largely composed of boulders and cobbles, ranging in size from a few inches to several yards in diameter.

The steeper river gradient in this area prevents the river from meandering as extensively as in Yosemite Valley. Additionally, riverbank areas in many locations have been developed and hardened for road and facility protection. Because of the steep gradient and development, the shifting of the river channel in El Portal usually occurs only during periods of large floods.

Flow volumes through the gorge are not available (there are no gauges in the immediate area), but should be only slightly larger than the volumes recorded at the Pohono Bridge gauge station. Tributaries within the gorge are relatively minor, although Cascade Creek confluences with the Merced River as the river enters the steepest part of the gorge.

Cascades Diversion Dam is located near the far western end of Yosemite Valley as the river transitions from the Valley floodplain into the steep river gorge. This dam was originally constructed to divert water from the Merced River into a hydropower plant that is no longer in use. The dam is relatively small and easily spills over during moderate flows. Suspended sediments and bedload discharging from Yosemite Valley collect behind Cascades Diversion Dam. Recent estimates indicate that approximately 15,000 cubic yards of sediment is held behind the dam (Central Federal Lands Highway Division 1997).

The South Fork of the Merced River is the Merced River's major tributary in the park vicinity. The watershed area of the South Fork at Wawona is approximately 63,000 acres (98 square miles) and expands to 154,000 acres (76,000 acres within the park boundary) by the time the South Fork confluences with the main stem outside of the park boundary. The headwaters of the South Fork originate near Triple Divide Peak at an elevation of approximately 10,500 feet. The South Fork flows westward over granitic bedrock to Wawona and then flows northwest over an area underlain by sedimentary rocks at a 3,500-foot elevation (USGS 1995a). Upstream from Wawona, tributaries enter the steep-walled canyon (glacial gorge) of the South Fork from the north and south. In the Wawona area, the river meanders through a large floodplain meadow (part of a deep alluvial valley) with substantial gravel bars within the channel.

The total length of the South Fork is 43 miles from its headwaters to the confluence with the Merced River several miles downstream from the western park boundary (USGS 1992). The average annual flow at its confluence with the Merced River is 356 cfs, with a maximum recorded flow of 46,500 cfs and a minimum recorded flow of 2.2 cfs (USFS 1989).^[2] At Wawona, upstream of the Big Creek confluence, the average annual flow was 174 cfs between 1958 and 1968, with an estimated maximum flow of 15,000 cfs in December 1955.^[3] The 100-year flow volume of the river through the South Fork Bridge cross-section is estimated at 13,563 cfs. The average annual total discharge of the South Fork is approximately 250,000 acre-feet (NPS 1978).

Within the Wawona area, a small impoundment created to pool water at the intake of Wawona's surface water supply is located near the end of Forest Drive. This area is designed to maintain a sufficient water level for the intake. Over time, the pool has filled with small cobbles, sands, and other sediments but does not represent a major source of sediment or act as a significant barrier to river flow and dynamics.

The overall climate is temperate, with hot, dry summers and cold, wet winters. About 85% of the precipitation falls between November and April. December, January, and February have the highest average precipitation, with a monthly average of 6 inches in Yosemite Valley at 4,000 feet. Average annual precipitation in Yosemite Valley is 36.5 inches. Annual precipitation decreases to

25 inches in El Portal at 2,000 feet and increases to 70 inches in the red fir forest at 6,000 to 8,000 feet (Eagan 1998). Most precipitation in Yosemite Valley falls as rain; only 29 inches of snow falls during an average year. At elevations above 5,000 feet, 80% of the annual precipitation falls as snow. Snowmelt drives the peak stream flows that occur in May and June, and minimum river flow is observed in September and October.

In Wawona (4,000 feet), precipitation occurs either as rain or snow, which melts quickly and flows into streams. At higher altitudes of the South Fork basin, precipitation usually occurs as snow, which melts more slowly and sustains the flow of the river during the spring and early summer. Average annual precipitation at the South Entrance Station is approximately 40 inches. Precipitation averages 50 to 60 inches per year in the upstream reaches of the South Fork basin.

Yosemite National Park is composed of and underlain by various granite rock types, and thus weathering, erosion, and transport of sediment can be a very slow process. Unfractured granite is impermeable and weathers very slowly; however, granite weathers much more readily when the various granites are buried by soil and in contact with a chemically reactive mixture of water, atmospheric gases, and organic decay products. Joints and natural depressions further the weathering process.

Various areas of Yosemite National Park have significant soil layers where clays, silts, and organic debris have accumulated with the gravels and sands of the decomposed bedrock. These soils are subject to erosion and alluvial processes.

Sedimentation is a significant process within Yosemite Valley. The Merced River has a very low gradient within the Valley, approximately 0.1% or 6.25 feet per mile (Smillie et al. 1992). This low gradient allows for significant sediment deposition within Yosemite Valley and the formation of the meandering Merced River through this reach. This sediment deposition and subsequent formation of the floodplain allows the river to migrate laterally within the floodplain. River impoundments such as bridges tend to alter the sediment distribution and formative streamflows, thereby disrupting the natural alluvial processes.

Two of the most significant changes to sediment transport dynamics along the Merced River were the removal of a portion of the El Capitan moraine and the construction of the Cascades Diversion Dam. In 1879, El Capitan moraine was reduced in elevation by blasting to decrease flooding in Yosemite Valley. This moraine serves as a hydraulic control for the Merced River in Yosemite Valley and influences the rate and distribution of sediment deposition. The reduction of flooding may have allowed encroachment of the forest into meadow areas near the river due to a lowering of the water table and a lessening of meadow inundation by floodwaters.

The construction of Cascades Diversion Dam in 1917 and 1918 had the opposite effect of removing the El Capitan moraine. The dam provided a condition where sediment that normally moved through the river system would settle and become trapped. It is estimated that 4,450 cubic yards of sediment were deposited behind Cascades Diversion Dam in the ten years after its construction (NPS 1991a). Currently, there is an estimated 15,000 cubic yards of sediment retained behind the dam (Central Federal Lands Highway Division 1997).

Bank erosion provides soil for sediment transport and deposition and is a natural process along streams and particularly along meandering rivers. In an undisturbed state, a river will establish a dynamic equilibrium in which the eroded bank material is balanced by new deposition on point bars as the river migrates across its floodplain. Due to the heavy visitor use of the Merced River, bank erosion is a significant issue. Stabilizing vegetation and organic debris were previously removed or impaired along the streambanks as a result of past National Park Service management practices and overuse by park visitors. Within Yosemite Valley, multiple streambank restoration activities are currently ongoing in an effort to re-establish stable banks where human influences have degraded the channel.

Bank erosion is also apparent in the wilderness areas above Nevada Fall where hiking trails parallel the river through Little Yosemite Valley. Downstream of Yosemite Valley, bank stability and sediment transport are affected by the alteration of the channel and floodplain due to roads and development.

Alluvial processes along the South Fork have not been substantially affected compared to Yosemite Valley and the El Portal area, although this area still faces the same pressures from development. Development in the Wawona area has locally altered alluvial processes due to the placement of bridges and roads along streambanks.

This section describes floodplains and flood characteristics in the Merced River basin and addresses floodplain values, existing impacts associated with the occupation and modification of floodplains, and risks to life and property. A floodplain plays a necessary role in the overall adjustment of a river system. It exerts an influence on the hydrology of the basin and also serves as a temporary storage for sediment eroded from the watershed. Periodic flooding provides sediment and nutrients that are essential for the aquatic and vegetative health of the floodplain. Floodplains are features that are both the products of the river environment and important functional parts of the system. However, human-made structures such as bridges and buildings placed within a floodplain can impede natural flow and result in injury to visitors and damage to structures. Discussion of flooding and floodplains is most relevant to the potential loss of life and the influence on the river by development in the floodplain.

The 100-year floodplain^[4] is the area that would be inundated by the 100-year flood, or the peak flow that has a 1% chance of being equaled or exceeded in any given year. The 100-year floodplain is typically used to define the general floodplain boundary. Due to the variable and dynamic landscape of the park, the floodplain along the Merced River significantly changes from one location to the next and has not been defined in all areas, particularly in the upper reaches of the Merced River. Observed and recorded inundation areas during flood events provide the best delineation of floodplains.

Within the park, flood levels are dependent upon the amount of snowpack, water content of the snowpack, rate of snowmelt, and amount and timing of rainfall. Although most of the park's precipitation occurs between October and April, melting of the snowpack due to warming springtime temperatures usually signals the beginning of an increase in streamflow that persists into June (NPS 1991a). Flood events associated with this flow increase are often termed "spring floods." Under normal conditions most of the runoff occurs from mid-April through July, with peak flows in May and June. From 1916 through 1989, 124 of 140 recorded high flows on the Merced River in Yosemite Valley occurred in response to snowmelt (NPS 1991a). A second type of flood typical of the Merced River can occur between September and April and is commonly referred to as a "winter flood" or a "rain-on-snow event" (NPS 1991a). These floods occur when a storm is accompanied by warm air temperatures and rainfall and coincides with the presence of snow in the vicinity of the storm. Although these events account for only about 10% of the floods in the park, they are responsible for the highest floods recorded, as seen by the events of January 1997. The 1997 flood resulted from heavy warm rains and melting snow, with rain at elevations up to 10,000 feet (ERFO Report 1997). Rain alone occasionally causes peak discharge events that are usually local in nature but sometimes cover a large area.

In some areas, the floodplain is nonexistent due to narrowing of valley walls or incision of the channel into moraine deposits. The Merced River watershed has had six significant winter floods since 1937 that caused substantial damage to National Park Service property within the floodplain. All of these floods took place between November 1 and January 30 as a result of rain-on-snow events (Eagan 1998).

Upper Main Stem Watershed. The floodplains along the upper Merced River have not been defined in remote areas and support few human structures. Within Little Yosemite Valley, the floodplain likely encompasses most of the valley floor. Steep topography limits the floodplain in the upper canyon areas.

Yosemite Valley Watershed. Yosemite Valley has a well-developed floodplain, with major roads and structures along or within both sides of the floodplain. The character of the floodplain varies in different locations because of local hydraulic controls. From Clark's Bridge to Housekeeping Camp in the east Valley, the Merced River floods areas outside the main river channel with shallow swift flows that cut across meander bends. Near Yosemite Lodge and downstream to the El Capitan moraine, floodwaters back up against the moraine and dense vegetation and tend to be deep and slow (Eagan 1998).

The broad floodplain that occurs in Yosemite Valley between the Housekeeping Camp area and the El Capitan moraine serves many hydrologic functions, including dissipation of floodwater energy as water spreads out over the flat, expansive plain. The meadows occur primarily in the floodplain and are maintained and rejuvenated by periodic floods. Development within the floodplains, such as roads across Stoneman, Ahwahnee, Cook's, Sentinel, and El Capitan Meadows, has varying degrees of influence upon the function of the floodplain. Loss of vegetation and soil compaction in highly visited areas, channel confinement due to riprap, and bridges can also influence functions of the floodplain. Development has also altered the hydrologic response of the Yosemite Valley watershed. The land uses and the subsequent infrastructure brought indirect changes to the watershed, such as loss of streamside vegetation, soil compaction, channel confinement, and loss of wetlands and riparian vegetation.

National Park Service field staff surveyed the extent of the 1997 flood inundation in Yosemite Valley and El Portal immediately after the event (see figure III-3 for the Valley's 1997 flood boundary). Flood flow rates during the 1997 flood were estimated by evaluating data collected at the U.S. Geological Survey gauging stations in Yosemite Valley. This data allowed hydrologists to verify previous flood extent maps and calibrate recent hydraulic models that were used to fully delineate the 1997 flood.

Significant flood events continue to alter the floodplain of Yosemite Valley and affect development within the park. The largest events occurred in 1937, 1950, 1955, and 1997 and were in the range of 22,000 to 25,000 cfs as measured at Pohono Bridge. These floods were the result of rain-on-snow events in which rain fell on winter snow pack, causing snowmelt in combination with rain-related runoff. At Pohono Bridge, the 100-year flood has been estimated by the U.S. Geological Survey to be in excess of 25,000 cfs (Eagan 1998).

The January 1997 flood was the largest recorded flood within the park. The flood inundated roads, picnic areas, park offices, and lodging units. The U.S. Geological Survey estimated that the flood had a peak discharge of 10,000 cfs at Happy Isles and 25,000 cfs at Pohono Bridge (Eagan 1998). The 1997 flood was estimated to have a recurrence interval of 90 years (NPS 1998d) and resulted in extensive damage to National Park Service facilities, including roads, bridges, buildings, and Yosemite Valley's electric, water, and sewer systems. The flood also altered natural features, causing downed trees, movement of landslide talus into streams, channel erosion, and significant changes in channel morphology (NPS 1997b).

Merced River Gorge and El Portal Watershed. From the Cascades Diversion Dam downstream through the El Portal Administrative Site, the river channel is extremely steep and confined in a narrow river gorge. In this area, the floodplain is quite narrow and the flow velocities are very high. The river channel in El Portal can shift during large floods, including movement of large boulders that define the channel. Within this area, El Portal Road has altered the floodplain by providing a barrier to channel migration. During extreme events, the Merced River has shown the capability to undermine or spill over and damage the roadway.

The South Fork has a limited floodplain (except in the Wawona area) due to the steep topography through which the river flows. The Wawona area has the only significant floodplain with an elongated alluvial valley. Development in the Wawona area has altered the floodplain. Diversion of surface water from the South Fork can affect the Wawona floodplain by reducing the water table in the floodplain during the drying phase, when no precipitation occurs and high runoff is not apparent. Water diversion is governed by the Wawona Water Conservation Plan, which includes provisions for reduction and/or cessation of withdrawals when streamflow drops to critical levels (NPS 1987d).

Waterfalls in the park occasionally produce a winter phenomenon called frazil ice at the base of the fall. Small ice crystals develop in turbulent super-cooled stream water, when air temperature suddenly drops to below freezing. The ice crystals join into slush and become pressed together as more crystals form. Frazil ice lacks the erosional force of regular stream ice, but it can cause streams to overflow their banks and change course. Frazil ice sometimes reaches a depth of more than 20 feet along Yosemite Creek at the Lower Yosemite Fall Bridge. A 1954 flow of frazil ice completely filled the streambed of the creek and covered the footbridge near Lower Yosemite Fall with many feet of ice (Hubbard and Brockman 1961). The Yosemite Fall footbridge was covered with frazil ice on February 27, 1996.

Although floods are significant to ecosystems because they can induce large changes in channel morphology and the floodplain landscape, low streamflow characteristics are also important. Low streamflow during the summer can affect the surrounding floodplain as riparian and wetland habitats undergo a drying phase. Diversion of river flows for human consumption can aggravate this normal balance and induce further reduction of riparian habitats and destabilization of streambanks. Prior to 1985, the National Park Service and the park concessioner in Yosemite Valley relied almost entirely on surface water diverted from the Merced River upstream of Happy Isles. It is estimated that up to 54% of the low streamflow discharge may have been diverted for park facilities (Medej 1991). This practice has been terminated in Yosemite Valley, and all potable water is now taken from groundwater wells. Water continues to be drawn from the South Fork for the Wawona area to augment groundwater supplies.

Water quality throughout Yosemite National Park is considered to be good and generally above state and federal standards. An inventory of water quality data performed by the National Park Service indicated excellent conditions in many parts of the park, but some water quality degradation in areas of high visitor use (NPS 1994h). The surface water quality of most park waters is considered beneficial for wildlife habitat, freshwater habitat, noncontact recreation, canoeing, rafting, and water contact recreation by the State of California, as indicated in the Central Valley Regional Water Quality Control Board's Water Quality Control Plan (Basin Plan).

Surface water draining granitic bedrock in the park exhibits considerable variability in chemical composition, despite the relative homogeneity of bedrock chemistry (Clow et al. 1996). Surface water in most of the Merced River basin is very diluted (lacking in dissolved solids), making the ecosystem sensitive to human disturbances and pollution (Clow et al. 1996). Studies have indicated a presence of Giardia lamblia and fecal coliform in various surface waters throughout the park, thereby limiting direct consumption of surface water by humans (Williamson et al. 1996b).

High water quality is critical for the survival and health of species associated with riparian and aquatic ecosystems. Water quality elements that affect aquatic ecosystems include water temperature, dissolved oxygen, suspended sediment, nutrients, and chemical pollutants. These elements interact in complex ways within aquatic systems to directly and indirectly influence patterns of growth, reproduction, and mobility of aquatic organisms. For example, sediment may not be directly lethal to fish, but sediment deposited on the streambed may disrupt the productivity and life cycles of fish and aquatic insects.

The chemistry of surface waters in the Merced River basin is characterized by low electrical conductivity (limited ions due to a lack of dissolved solids), near-neutral pH, low alkalinity, and low nutrient concentrations (NPS 1994h). Calcium and bicarbonate are the predominant ions in the waters. Within the Merced River, major ion concentrations slightly increase downstream, but levels remain relatively low and no significant changes have been observed in pH, alkalinity, or nutrient concentrations (NPS 1994h). Due to the low alkalinity of the stream water, the buffering capacity (ability to absorb water chemistry changes or additions) of the Merced River and its tributaries is limited. Occasional concentrations above drinking water and freshwater criteria have been noted within the Merced River for lead, cadmium, and mercury (NPS 1994h). Potential sources of these metals include lead gasoline, stormwater runoff from developed surfaces such as parking lots, wastewater discharge, campsites, and fuel storage facilities.

Groundwater quality is generally good in the Merced River basin and is the sole source of potable water for Yosemite Valley and El Portal. There are locations in Yosemite Valley where relatively high iron concentrations in groundwater result in a reddish deposit on the substrate surface (e.g., observed at surfacing springs near lower Tenaya Creek and several locations on the Merced River) (Williamson et al. 1996a). These iron concentrations are not a threat to water quality. Federal regulations ensure that potable water systems that rely on groundwater are continually monitored and operated within set levels for turbidity, waterborne pathogens, and other potential pollutants.

Water quality within the South Fork basin is very similar to that of the main stem of the Merced River, with near excellent conditions in most areas and some water quality stressors near human development. The Wawona Golf Course does present a potential nonpoint pollution source due to the occasional use of fertilizers and herbicides for maintenance of the course, although these products are used following strict guidelines for application and disposal. Water quality is sufficient for Wawona residents to use both surface water and groundwater as potable water. Surface water is drawn from the South Fork through the water treatment plant intake near Forest Drive.

Water quality has been affected by the extensive and concentrated visitor use of the Merced River in popular areas. High use of the streambank induces bank erosion through the loss of vegetative cover and soil compaction. Bank erosion can result in the widening of the river channel and loss of riparian and meadow floodplain areas. Water quality is then altered through increased suspended sediments due to erosion, higher water temperatures from a lack of riparian cover, and lower dissolved oxygen levels due to elevated temperatures and shallower river depths.

Human activities and the use of vehicles can distribute potential water pollutants that may collect on land surfaces and later be transported into the river or its tributaries by stormwater runoff. Recreational activities such as horseback riding, swimming, and hiking can lead to the introduction of organic, physical, and chemical pollutants into aquatic systems. Nonpoint-source runoff from roads and parking lots may potentially affect water quality by introducing organic chemicals and heavy metals. Areas where livestock are concentrated, including the High Sierra Camps, present nutrient sources, while the developed areas present sources of various pollutants associated with human waste and debris. Some pesticides are used in the park and also may enter streams, although strict federal guidelines are followed for all applications.

Stormwater runoff from developed surfaces in the park is managed in different ways. A small portion of runoff from parking lots in Yosemite Valley is diverted into the wastewater drains and treated at the El Portal Wastewater Treatment Plant. Direct runoff of oil, grease, rubber particles, metals, and other road deposits occurs from most roadways, which discharge directly or indirectly to streams and lakes throughout the park. In the wilderness areas, nonpoint-source pollutants include human and livestock wastes and sediments contributed through erosion. These sources have the potential to affect water quality in all segments of the Merced River.

In addition to local sources, water resources in the park can be affected by regional air quality pollution through atmospheric and deposition. The entire Sierra Nevada range has been designated as sensitive to acid precipitation due to its granitic substrate and the resulting low buffering capacity of its water resources. Ongoing studies are examining the effects of external and internal air pollutants on natural resources, including surface water resources.

A variety of materials has been stored in the park over the last century, often in underground storage vessels. Since 1986, over 100 underground tanks have been located and removed. The park has over 30 known contamination sites from leaking underground storage tanks. Currently, 12 sites are being cleaned up and need to be given regulatory closure. The park also contains a number of old landfill and surface dumpsites. Regardless, these underground nonpoint pollution sources represent potential contaminant sources for the degradation of water quality.

Point sources of pollution include discharges from pipes or other devices where the discharge can be traced to a single point or location. Facilities in Yosemite Valley and El Portal are connected to a wastewater collection system that terminates at the El Portal Wastewater Treatment Plant. Treated wastewater is discharged to percolation and evaporation ponds at the treatment facility. Water quality impacts from wastewater may occasionally occur as a result of sewerline blockage and wastewater backup and overflow. A tertiary wastewater treatment plant serves most of the public and private sources in Wawona, with the treated wastewater used to irrigate the Wawona Golf Course. During winter months, the treated wastewater is discharged to the South Fork when storage capacity is insufficient and disposal to the golf course is unfeasible due to snow cover.

Fire is a natural component of the Sierra Nevada and Yosemite National Park. The recurrence of fire shapes the ecosystems of the park, with many common plants exhibiting specific fire-adapted traits. The National Park Service has adopted a Fire Management Plan (NPS 1990a), which has clear guidelines about when and where to allow natural and prescribed fires to burn. The effects of fire are potentially great on water quality. Fires are a disturbance that can increase sediment contributions to aquatic systems, alter runoff patterns, and thereby influence concentrations of chemical and biological constituents in water bodies.

Groundwater occurs in Yosemite National Park in four general types of settings: large alluvial valleys such as Yosemite Valley; small deposits of alluvium, colluvium, and glacial till; porous geologic formations; and fractured rocks. The shallow aquifers of alluvial deposits tend to be highly responsive to recharge and withdrawals. The deep aquifers within the fractured rock are mostly unresponsive to any yearly hydrologic change, although these deep systems have not been fully studied.

The surface water and groundwater function as one unit in Yosemite Valley and El Portal. Recharge of the shallow groundwater aquifers reaches its peak during spring snowmelt. In Yosemite Valley, the entire meadow system may be saturated to the forest edge, resulting in restricted tree growth that defines the forest/meadow boundaries and extensive Valley wetlands. In El Portal, steeper terrain and river gradients have played a role in limiting the extent of groundwater-supplied wetlands. In addition, historical development has caused impacts to the few remaining wetland systems.

In 1985, the National Park Service ceased the use of surface water in Yosemite Valley and the El Portal area (diversions from the Merced River) and began drawing from newly drilled groundwater wells (NPS 1991a). Groundwater is used in both Yosemite Valley and El Portal for potable water supplies. Four wells in Yosemite Valley have the capacity to produce approximately 1,400 gallons per minute (gpm). In El Portal, six wells support a capacity of approximately 240 gpm.

In the Wawona area, the groundwater flows through upper unconsolidated fills and lower fractured rock aquifers that have not been defined. The primary aquifer at Wawona is the fractured granitic rocks in the South Fork basin. Fractured granitic rock aquifers typical of the Sierra Nevada can be highly variable for groundwater flow and supply. Drilling tests in the Wawona area have indicated a local, shallow groundwater flow system sustained by groundwater from deeper fractures (USGS 1995a). Groundwater in the local, shallow aquifer likely does not circulate deeper than approximately 250 feet below the surface. Short-term pumping tests on domestic wells in 1995 indicated that the median yield of wells is less than 5 gallons per minute from the shallow aquifer in Wawona (USGS 1995a). Currently, approximately 100 wells in Wawona supply about 265 private residences and a store (USGS 1995a).

The Wild and Scenic Rivers Act requires that each component of the National Wild and Scenic Rivers System be administered in such a manner as to protect and enhance the values that led to their inclusion under the act.

The Clean Water Act requires the National Park Service, in implementing its management activities, to comply with all federal, state, interstate, and local requirements regarding the control and abatement of water pollution.

Preparation and adoption of water quality control plans (Basin Plans) is required by the California Water Code (Section 13240) and supported by the federal Clean Water Act. Section 303 of the Clean Water Act requires states to adopt water quality standards that "consist of the designated uses of the navigable waters involved and the water quality criteria for such waters based upon such uses." The Basin Plans are regulatory references for meeting the state and federal requirements for water quality control.

The General Management Plan (NPS 1980a) restates the park mission in the following management objectives that are applicable to the protection of water resources:

• Conduct continuing research to gather and analyze information necessary for managing natural resources

• Identify and perpetuate natural processes in park ecosystems

• Permit only those types and levels of use or development that do not significantly impair park natural resources, and direct development and use to environments least vulnerable to deterioration

• Limit unnatural sources of air, noise, visual, and water pollution to the greatest degree possible

Executive Order 11988 on floodplain management and the National Park Service Floodplain Management Guidelines (1993) provide guidance for the protection of natural floodplain values and of life and property in the National Park System. The National Park Service must avoid construction of facilities in a floodplain if alternative locations are available. Where no alternatives exist, policies allow construction of structures, such as day-visitor parking lots, picnic areas, and campgrounds, if risks to human life and property are studied and then minimized or mitigated through design. The Floodplain Management Guidelines require medical facilities, schools, and fuel storage areas to be placed outside the 500-year floodplain.

In 1993, the National Park Service ended the practice of removing fallen trees from the river within Yosemite Valley. Previously, fallen trees were removed for bridge protection and to reduce hazards to rafters. Today, fallen trees are considered beneficial for streambank protection, aquatic organisms, and overall health of the riparian and aquatic corridor.

The National Park Service has primary responsibility for watershed inspection and monitoring along the Merced River within Yosemite National Park. The National Park Service also has the responsibility of preparing a Floodplain Statement of Findings when there is the potential for adverse impacts to floodplains.

Section 404 of the federal Clean Water Act requires that a permit be issued for discharge of dredged or fill materials in "waters of the United States," including wetlands. The U.S. Army Corps of Engineers administers the Section 404 permit program, with oversight and veto powers held by the U.S. Environmental Protection Agency. Also, the Merced River is considered a navigable waterway and is subject to provisions of the Rivers and Harbors Act of 1899. Therefore, the Corps is responsible for issuing permits for construction or other work in or affecting the river (33 CFR § 325.8[a] and 329).

The Central Valley Regional Water Quality Control Board (RWQCB) has jurisdictional authority over water quality in the state of California. The RWQCB's jurisdictional authority is derived from Sections 301, 401, 402, and 404 of the Clean Water Act and the Porter-Cologne Act. Section 401 of the Clean Water Act requires that the discharge of dredged or fill material into waters of the United States does not violate state water quality standards. Applicants for Section 404 (Clean Water Act) or Section 10 (Rivers and Harbors Act) permits must obtain a water quality certification or waiver from the state.

The application of beneficial use designations and water quality objectives within the RWQCB's Water Quality Control Plan (or Basin Plan), as developed under the federal Clean Water Act, provides regulatory guidance for the protection of water quality. The RWQCB has designated the Merced River with the beneficial uses of municipal, domestic, industrial, and agricultural water supply; wildlife habitat; freshwater habitat; and contact and noncontact water recreation.

The RWQCB also issues and manages permits under Section 402 of the Clean Water Act, which requires a National Pollutant Discharge Elimination System permit for the discharge of pollutants from any point source into waters of the United States. The RWQCB issues these permits for the wastewater treatment plants in El Portal and Wawona.

Wetland data presented in this section are descriptive and programmatic in nature. The intent is to provide general descriptions, functions, and values of wetland and water-dependent communities within the Merced River corridor. Details concerning actual extent (location on the ground, acreage) and jurisdictional determination are not included herein and are left for more specific planning and implementation documents. Refer to the Vegetation section for vegetative descriptions, the Wildlife section for data relating to wildlife and aquatic species, and the Rare, Threatened, and Endangered Species section for information on protected species of plant and wildlife.

Wetlands are ecologically productive habitats that support a rich array of both plant and animal life. They sustain a great variety of hydrologic and ecological functions vital to ecosystem integrity. These functions include flood abatement, sediment retention, groundwater recharge, nutrient capture, and high levels of plant and animal diversity (USFS 1995). Wetlands and riparian areas are relatively rare compared to the entire landscape. When wetlands are converted to systems that are intolerant of flooding (drained agricultural lands, filled developed lands), their storage capacity decreases and downstream flooding increases (National Academy Press 1993, as in NPS 1997g). Modification of even small wetland areas induces effects that are proportionally greater than elsewhere in an ecosystem (Graber 1996).

Although there are several definitions for the term "wetland," the two used herein relate to National Park Service and U.S. Army Corps of Engineers conventions. These definitions are presented below.

The National Park Service classifies and maps wetlands using a system created by the U.S. Fish and Wildlife Service, which is often referred to as the Cowardin classification system (USFWS 1979). This system classifies wetlands based on vegetative life form, flooding regime, and substrate material. Wetlands, as defined by the U.S. Fish and Wildlife Service and adopted by the National Park Service, are lands transitional between terrestrial and aquatic systems, where the water table is usually at or near the surface or the land is covered by shallow water. For purposes of this classification, wetlands must have one or more of the following attributes:

• The land supports predominantly hydrophytes, at least periodically. Hydrophytes are plants that grow in water or on a substrate that is at least periodically deficient in oxygen as a result of excessive water content.

• The substrate is predominantly undrained hydric soils. Hydric soils are wet long enough to periodically produce anaerobic conditions.

• The substrate is saturated with water or covered by shallow water at some time during the growing season of each year (USFWS 1979).

Under Section 404 of the Clean Water Act, the U.S. Army Corps of Engineers issues permits for the discharge of dredged or fill material into "waters of the United States" (33 Code of Federal Regulations [CFR] 323.3). Wetlands are a subset of waters of the United States and receive jurisdictional protection under Section 404 of the Clean Water Act. Waters of the United States (also regulated under Section 404 of the Clean Water Act) include features such as streams, rivers, bays, lakes, inlets, mudflats, washes, sloughs, sand flats, territorial seas, tributaries, and impoundments. Wetlands are defined under the Clean Water Act as, "Those areas that are inundated or saturated by surface water or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions (33 CFR 328.3[b])."

The Cowardin system and the U.S. Army Corps of Engineers both use the three wetland parameters to define wetlands: hydrophytic vegetation, hydric soil, and wetland hydrology. However, the Cowardin system defines more habitat types as wetlands than does the Corps definition. The Cowardin system also recognizes that many unvegetated sites (e.g., mudflats, stream shallows, saline lakeshores, playas, deepwater) or sites lacking soil (e.g., rocky shores, gravel beaches) are wetland habitats. The reason these sites lack hydrophytic vegetation and/or hydric soil is due to natural chemical or physical factors. Although the Corps does not consider these sites to be wetlands, they are still subject to regulations under Section 404 of the Clean Water Act as other waters of the United States.

The U.S. Army Corps of Engineers also has jurisdiction over navigable waters of the United States under Section 10 of the Rivers and Harbors Act of 1899. Navigable waters of the United States are those waters that are subject to the ebb and flow of the tide and/or are presently used, or have been used in the past, or may be susceptible for use to transport interstate or foreign commerce. A determination of navigability, once made, applies laterally over the entire surface of the water body and is not extinguished by later actions or events that impede or destroy navigable capacity (33 CFR 329.4). No portion of the Merced River^[5] within Yosemite National Park is designated as navigable waterway under Section 10 of the Rivers and Harbors Act.

Data on wildlife presented in this section are descriptive and programmatic in nature and are intended to provide general habitat descriptions, functions, and values in addition to species presence and use of those habitats within the Merced River corridor. Details concerning individual or population locations or size are not included herein and are left for more specific planning documents. Peer-reviewed scientific studies have been conducted since the 1950s (CDFG 1999a), and reasonably accurate descriptions of park fish and wildlife resources have been developed based on field reconnaissance, literature review, the professional knowledge and judgment of park staff, wildlife-habitat relationships models^[11] (CDFG 1999a), records of observations, published references on Sierra Nevada wildlife, and studies of selected species. In particular, California Wildlife Habitat Relationship models have been used for predicting impacts within the park (Chow et al. 1994).

More complete information is available on species that present a distinct management challenge, for example, bats (Pierson and Rainey 1993), spotted and great gray owls (Roberts et al. 1988; Wildman 1992), and bears (Harms 1980; Graber 1981, 1996; Graber and White 1983, 1987).

Yosemite National Park, one of the largest and least-fragmented habitat blocks in the Sierra Nevada, supports a diverse and abundant assemblage of wildlife. Its importance in protecting the long-term survival of certain species and the overall biodiversity^[12] of wildlife in the Sierra Nevada was recognized in the reports prepared as part of the Sierra Nevada Ecosystem Project (University of California, Davis 1996). The Sierra Nevada Ecosystem Project included

assessments of the Sierra Nevada headwaters of 23 major river basins in addition to the Merced River, from Eagle Lake in the north to the Mojave River in the south. As part of these assessments, much of the main stem of the Merced River corridor and the South Fork corridor were identified as an "aquatic diversity management area" (University of California, Davis 1996).

The Merced River corridor also plays an essential ecological role in linking wildlife habitats across the park's landscape and elevational gradients; this fact forms an important part of the framework for this analysis. For wildlife populations to be viable, resources and environmental conditions must be sufficient for foraging, resting, cover, and dispersal of animals. Arrangement, types, and amounts of resources must be sufficient for the needs of reproductive individuals on daily, seasonal, and yearly scales. Habitat must also be well distributed over a broad geographic area to allow breeding individuals to interact spatially within and among populations, and a stable, relatively undisturbed riparian corridor supplies a mechanism for this kind of ecological connection.

Approximately 85 native mammal species in six families inhabit Yosemite. There are 17 species of bats, 9 of which are either state or federal species of special concern (see the Rare, Threatened, and Endangered Species section of this document). Many of these bat species are dependent upon riparian and meadow habitats for foraging, and large trees or rock crevices for roosting. Ungulates include large numbers of mule deer. Bighorn sheep formerly populated the Sierra crest, but are now extinct in the Merced River watershed. Carnivores include black bears, bobcats, coyotes, raccoons, mountain lions, ringtails, weasels, and grey foxes. Yosemite's largest mammal, the grizzly bear, was extirpated from the region and from the state in the 1920s. Other mammal species that survive but are extremely rare are the fisher, wolverine (possibly extinct), and Sierra Nevada red fox.

Yosemite's wide range of elevations and habitats support a diversity of bird species: 150 species regularly occur in the park, and approximately 80% of these are known or suspected to breed within park boundaries. Familiar to park visitors include black-headed grosbeak, red-winged blackbird, western tanager, and American robin. A majority of Yosemite's bird species migrate from the park in winter, but among the more conspicuous species that remain year-round are the common raven, Steller's jay, mountain chickadee, and American dipper. There are many additional year-round park-resident bird species that are less easily detected, such as western tanager, white-throated swift, and yellow-rumped warbler.

Several bird species have probably been reduced in Yosemite Valley by human activity, but are present in less disturbed areas. Yosemite Valley meadows are suitable habitat for great gray owls, and the species persists in other meadows, though sightings in Yosemite Valley are rare. Willow flycatchers no longer nest in Yosemite Valley. probably due as much to parasitism by brown-headed cowbirds as to destruction of riparian and meadow habitat. but were recorded in Wawona as recently as 1998.

On a wider scale, apparent population declines have been detected in numerous other bird species in the Sierra Nevada, including Yosemite National Park. Possible causes for these declines include grazing, logging, fire suppression, development, recreational use, pesticides, habitat destruction on wintering grounds, and large-scale climate changes.

Compared to most mountain regions of the west, Yosemite has a particularly large number of native reptiles and amphibians: 14 snakes (one poisonous), 7 lizards, 1 turtle, 2 toads, 1 tree frog, 3 true frogs, and 5 salamanders (including newt and ensatina). Most of these species have been found in Yosemite Valley.

Amphibians in Yosemite National Park have suffered population declines similar to those seen in the rest of the Sierra Nevada (Drost and Fellars 1996). Red-legged frogs likely were found in Yosemite Valley in the past but are now apparently extinct there. Significant factors in their disappearance probably include reduction in perennial ponds and wetlands, and predation by bullfrogs. At higher elevations, mountain yellow-legged frogs and Yosemite toads are still present in a number of areas, but are severely reduced in population and range. Foothill yellow-legged frogs have disappeared completely from the park, if not the entire Sierra Nevada. Research continues to identify the causes of Sierra Nevada-wide amphibian declines; possible causes include habitat destruction, non-native fish, pesticides, and diseases.

Most fish currently found in the Merced River and its tributaries in Yosemite have been introduced. Prior to trout stocking for sport fishing, native fish in Yosemite were probably limited to the rainbow trout and the Sacramento sucker, both of which were present only in the lower portions of the Merced River (i.e., Yosemite Valley and below). The last period of glaciation eliminated all fish from the high country, and waterfalls remaining on all rivers after the glaciers retreated prevented repopulation by upstream migration. Fishes native to the Merced River downstream of El Portal include Sacramento pikeminnow (squawfish), hardhead, California roach, and riffle sculpin.

Although the upper watershed of the Merced River was stocked with a variety of non-native trout in the earlier part of the century, Yosemite streams are subject to tremendous fluctuations in flow; these fluctuations, combined with severe climatic conditions, low nutrient availability associated with snow melt over granitic watersheds, and lack of spawning habitat, have restricted natural sustainability of introduced fish in a majority of Yosemite's lakes. Fishery surveys conducted in the mid-1970s found 62 lakes with self-supporting fish populations and 195 that supported little or no natural reproduction. Approximately 550 miles of streams are thought to support fish (NPS 1977).

Until very recently, trees that fell into the Merced River in nonwilderness areas were considered hazardous to bridges and humans and were removed. This practice deprived fish and other aquatic organisms of important habitat and has altered natural river dynamics. Elimination of riparian vegetation by human trampling and bank stabilization devices in many areas along the Merced River have also reduced nutrients (fallen leaves) in aquatic ecosystems, which has further affected the food chain. The loss of soil from riverbanks caused by the lack of riparian vegetation has also led to the creation of broad, shallow stretches of the river that support few fish (NPS 1990a, USFWS 1992b). Roads, ditches, utilities, and other structures in meadows have likely altered meadow hydrology, affecting water and nutrient flows into aquatic ecosystems. Fallen trees are now allowed to remain in the river because of their value to aquatic and riparian ecosystems.

As with vegetation, the introduction of non-native species has had significant adverse effects on native wildlife species in Yosemite National Park. As a result, the park has identified non-native species as an important concern in recent planning documents. Non-native wildlife includes white-tailed ptarmigan, wild turkey, brown-headed cowbird, and the bullfrog. Feral pigs have recently been sighted near the park and could establish ranges in park ecosystems.

Introductions of fishes into the Merced River drainage of Yosemite National Park probably began with transfers of Lahontan cutthroat trout, coastal rainbow trout, and California golden trout from nearby waters. Rainbow trout is the only trout species native to the Merced River; rainbow trout introduced through stocking from other waters and fish hatcheries have now hybridized with, and/or have displaced, the original strain. Other species of trout not native to California, including brook trout, brown trout, and arctic grayling, have also been introduced into the Merced River drainage. Brown trout seems to have become well established and outnumbers rainbow trout in many areas. Brook trout are found in the main stem, and in large numbers in lakes and small streams of the Merced River watershed. The arctic grayling, reportedly only stocked in one headwater lake, may no longer be found in the Merced River drainage (Moyle et al. 1996). Fish introductions in higher-elevation lakes and streams, all of which were naturally fishless, have likely severely altered those ecosystems. Such introductions of fish are strongly suspected of being a primary factor in declines of native amphibian species in the Sierra Nevada (Drost and Fellers 1996). The National Park Service discontinued fish stocking in Yosemite National Park in 1991.

The sensitive balance of aquatic ecosystems in Yosemite Valley has been severely disrupted by the presence of bullfrogs, which are voracious, non-native predators. The full impact of bullfrogs on native species in the park is unknown, but bullfrog predation was probably a factor in the disappearance from Yosemite Valley of red-legged frogs. Recent observations suggest that bullfrogs occupy standing and slow-moving water throughout the Valley.

Brown-headed cowbirds have recently increased in the Sierra Nevada (Laymon 1987), threatening native bird species. Cowbirds are nest parasites that lay their eggs in the nests of other birds, usually songbirds. The cowbird eggs hatch before the eggs of the host species, and the larger, more vigorous cowbird young then eject the eggs or young of the host species or outcompete the host's young for food. This parasitism can have a devastating effect on the populations of some songbird species and is implicated in the disappearance of willow flycatchers from Yosemite Valley and of other riparian species statewide (Laymon 1987). The spread of cowbirds into the Sierra Nevada has been associated with human disturbance and activities; brown-headed cowbirds are common in Yosemite and can be found in large numbers at the park's stables and corrals, campgrounds, and residential areas.

A list of wildlife species believed to occur within the Merced River corridor would include nearly all of the wildlife species believed to occur within the park as a whole (NPS 1999q), because the corridor passes through nearly all of the habitat types found within the park. Table III-1 provides a display of the predominant habitat types known to be present, along with representative species.

In the broadest sense, the presence and abundance of wildlife species at any site or area depend on the structure of the habitat available in that area. Habitat types broadly correlate with vegetation types (or plant associations/communities) or general stream classifications. For many wildlife species there is an additional requirement for special habitat attributes, such as cliffs, caves, rocks, lakes or rivers, or other abiotic (nonliving) elements. In addition, many species have explicit habitat requirements for one or more elements of the biotic environment, such as large trees, large snags (standing dead trees), large downed logs, high degrees of canopy closure, or, for fish, pools, riffles, and undercut banks.

As described in the preceding section, the vegetation of Yosemite National Park is roughly stratified altitudinally and also affected by local topography. The highest mountain slopes show barren rock walls and herbaceous or shrubby plant life, which give way to open subalpine and montane coniferous forests farther down the canyon, then to more dense lower montane coniferous forest in Little Yosemite Valley. In Yosemite Valley, the vegetation pattern is highly modified and was probably largely dominated by meadows 150 years ago. From Pohono Bridge downstream, the Merced River gorge functions as a mountain canyon, and the vegetation shifts from lower montane coniferous forest near the Valley to hardwood and chaparral near El Portal. Similar changes occur along the South Fork. These changes in habitat structure correlate broadly with changes in the composition and abundance of wildlife species present in these altitudinal zones.

Overlaid on the overall elevation pattern is a local topographic effect. Where the river flows through low-gradient reaches, the valleys tend to be broad and relatively flat and are dominated by denser and taller-stature forests than in areas with steeper channel reaches. Thus, locations like Little Yosemite Valley, Yosemite Valley, and the Wawona area tend to have taller and more extensive forests than steeper sections. The broad valleys in the flat reaches also tend to be associated with lakes, saturated soils, and wetlands such as meadows. These wetter areas are important wildlife habitat elements and are associated with a number of the sensitive species known to occur in the park (see the Rare, Threatened, and Endangered Species section of this document).

Wild and Scenic Rivers Act. The Wild and Scenic Rivers Act requires that each component of the National Wild and Scenic Rivers System be administered in such a manner as to protect and enhance the values that led to their inclusion under the statute. Rivers designated under this act must be administered to limit other uses that would detract from defined values, while not substantially interfering with public use and enjoyment of these values.

Yosemite General Management Plan. The General Management Plan (NPS 1980a) restates the park mission in the following management objectives:

• Conduct continuing research to gather and analyze information necessary for managing natural resources

• Restore altered ecosystems as nearly as possible to conditions that would exist had natural ecological processes not been disturbed

• Protect threatened and endangered plant and animal species and reintroduce, where practical, those species eliminated from the natural ecosystems

• Identify and perpetuate natural processes in park ecosystems

• Permit only those types and levels of use or development that do not significantly impair park natural resources, and direct development and use to environments least vulnerable to deterioration

• Limit unnatural sources of air, noise, visual, and water pollution to the greatest degree possible

Resources Management Plan for Yosemite National Park. Approved in 1993 (NPS 1993c), the Resource Management Plan addresses specific issues such as the role of fire in the ecosystem, non-native-plant control, forest pest control, horse and mule grazing, protection of threatened and endangered plants, human/bear conflicts, other wildlife and fisheries management programs, and the park's research program.

Yosemite Vegetation Management Plan, Fire Management Plan, and Wilderness Management Plan. A more detailed discussion of these plans is provided in the preceding section. The goal of these plans is to provide guidelines for management of natural processes and events and to preserve or restore vegetation structure and diversity to that would occur independent of human influences. These plans are local expressions of the biological resources management policies of the National Park Service.

The overall guidance for the management of biological resources is contained in the following statement:

Yosemite Human/Bear Management Plan. The goal of the Human/Bear Management Plan (NPS 1997r) is to "restore the natural ecology, distribution, and behavior of black bears through control of human activities." To this end, the plan directs specific actions and responsibilities to reduce the potential for bear/human interaction.

Fisheries Rules and Regulations. In general, Yosemite National Park has adopted the same fishing regulations as apply to the California Department of Fish and Game management region that contains the park, and requires a valid California fishing license. California Department of Fish and Game maintains jurisdiction over areas outside of Yosemite National Park, where it enforces rules regarding hunting and fishing. The National Park Service has exclusive jurisdiction in the park concerning fishing and hunting, but, in most cases, sets fishing regulations that are consistent with those set by the California Department of Fish and Game for the management region in which the park is located. Fishing licenses are available for sale at Yosemite Village and Tuolumne Meadows. Licenses can also be purchased in Wawona and El Portal. In 1992, the National Park Service instituted special fishery regulations for the Merced River valley reach between Happy Isles and Pohono Bridge. In this reach, the fishery is catch-and-release only for rainbow trout (zero limit). Bait fishing is not allowed in this reach, with the requirement for artificial lures and flies only with barbless hooks in single, double, or triple configuration. The catch of brown trout in Yosemite Valley is limited to 5 fish per day and 10 in possession. Further restrictions are imposed on "chumming" for trout, digging for worms or grubs, and using bait fish. either dead or alive. in any of the park's waters.

Regulatory Agencies

The Federal Endangered Species Act of 1973, as amended, requires all federal agencies to consult with the U.S. Fish and Wildlife Service before taking actions that could jeopardize the continued existence of species that are "listed" or proposed to be listed as threatened or endangered, or could result in the destruction or adverse modification of critical or proposed critical habitat. The first step in the consultation process is to obtain a list of protected species from the U.S. Fish and Wildlife Service.

In addition, Council of Environmental Quality Regulations for Implementing the National Environmental Policy Act (Section 1508.27) also require considering whether the action may violate federal, state, or local law or requirements imposed for the protection of the environment. For this reason, species listed under the California Endangered Species Act or accorded "special status" (i.e., considered rare or sensitive) by the California Department of Fish and Game are included in this analysis.

Also included in this analysis are "park rare" species. Park rare species^[13] are those that have no other status (either state or federal), have extremely limited distributions in the park and may represent relict populations from past climatic or topographic conditions, may be at the extreme extent of their range in the park, or represent changes in species genetics. They are included in this analysis because they could be affected (due to proximity to human use zones, or susceptibility of individual plants or populations to loss from natural or unnatural events) and their existence is considered when evaluating consequences for any proposed management action.

The Sierra Nevada contains 33 bird species, 19 mammals, 4 reptiles, and 13 amphibians considered at risk (i.e., through listing as endangered, threatened, or of special concern by the state or federal government), which is roughly 17% of the Sierra Nevada terrestrial fauna (University of California, Davis 1996). Three species have been extirpated from the range since the time of Euro-American settlement: Bell's vireo, California condor, and grizzly bear. The declines can be attributed to several factors, in varying proportions: habitat loss, disturbance or

hunting by humans, environmental toxins, climatic change, and competition from non-native species. However, two of the most charismatic species associated with the park, the bald eagle and the peregrine falcon, are showing signs of recovery. The bald eagle was proposed for "delisting" on July 6, 1999; the peregrine was formally delisted on August 25, 1999.

The Sierra Nevada is also rich in plant diversity. Of California's 7,000 plant species, about 50% occur in the Sierra Nevada. Of these, more than 400 are found only in the Sierra Nevada and 200 are rare. As a group, Sierra Nevada plants are most at risk where habitat has been reduced or altered or when restricted to rare local geologic formations and their derived unique soils. This is true in the El Portal area, for example, which supports a number of state-listed rare species that are sustained in a unique contact zone of metamorphic and granitic rock.

Critical habitat has not been designated for any federally listed species that is known or has potential to occur within the Merced Wild and Scenic River planning corridor.

A Notice of Intent to Prepare an Environmental Impact Statement was sent to the U.S. Fish and Wildlife Service on August 20, 1999. On September 9, 1999, project staff met with a representative from the Sacramento office of the U.S. Fish and Wildlife Service. The U.S. Fish and Wildlife Service provided a draft letter listing species of concern, based on USGS 7.5-minute quadrangles that they thought would encompass the immediate project area, as well as a summary list. A final, augmented list was provided by U.S. Fish and Wildlife Service a week later (USFWS 1999) and included all of the lands potentially affected by the proposed action (Chapter V).

The National Park Service prepared a Biological Assessment in accordance with Section 7 of the Federal Endangered Species Act of 1973, as amended, and implementing regulations (19 USC 1536[c], 50 CFR 402.14[c]), National Environmental Policy Act requirements (USC 4332[2][c]), and direction provided in the 1988 National Park Service Management Policies (4:11). The Biological Assessment was submitted to the U.S. Fish and Wildlife Service for official review and comment in January 2000. A Final Biological Assessment based on the Merced River Plan/FEIS was submitted to the U.S. Fish and Wildlife Service in June 2000. Copies of the Biological Assessment are on file at Yosemite National Park.

An overriding assumption of the Biological Assessment was that each site-specific action that could occur under the proposed action would be analyzed as required by the National Environmental Policy Act and the Endangered Species Act and that all federal laws would be complied with during implementation. Since the decision made under this EIS is programmatic, no specific commitment of resources is made by the decision. Therefore, a Biological Evaluation and/or Biological Assessment would be made for all site-specific projects, as warranted. Some site-specific projects have the potential to adversely affect threatened or endangered species. Therefore, site/project-specific assessments and determinations, in accordance with the provisions of the Endangered Species Act and in cooperation with the U.S. Fish and Wildlife Service, could be required for future actions.

Table III-2 presents information on federally listed threatened or endangered species; species of concern (former federal category 2 species); state-listed threatened, endangered, and rare species; and species that are locally rare or threatened that are known to be or could be present within the Merced River corridor based on data gathered from the National Park Service, the U.S. Fish and Wildlife Service, and California Natural Diversity Database (CNDDB).

A total of 56 special-status wildlife species and 58 special-status plant species (114 total) have been considered in the evaluation of this plan. This includes 8 federally listed species of wildlife, 35 species of wildlife and 6 species of plants listed as federal species of concern, 13 species of wildlife and 4 species of plants listed by the State of California as rare, threatened, endangered or species of special concern, and 48 species of plants listed by Yosemite National Park as rare. Refer to the Biological Assessment (NPS 2000) on file at Yosemite National Park for additional information.

Surveys specific to this planning effort to identify individuals or populations of special status species within the corridor have not been performed. Data presented herein are based on field reconnaissance, literature review, the professional knowledge and judgment of park staff, records of observations, published references, and studies of selected species.

The Wild and Scenic Rivers Act requires that each component of the National Wild and Scenic Rivers System be administered in such a manner as to protect and enhance the values that led to its inclusion under the statute. Rivers designated under this act must be administered to limit other uses that would detract from defined values, while not substantially interfering with public use and enjoyment of these values.

The General Management Plan (NPS 1980g) restates the park mission in the following management objectives:

• Conduct continuing research to gather and analyze information necessary for managing natural resources

• Restore altered ecosystems as nearly as possible to conditions that would exist had natural ecological processes not been disturbed

• Protect threatened and endangered plant and animal species and reintroduce, where practical, those species eliminated from the natural ecosystems

• Identify and perpetuate natural processes in park ecosystems

• Permit only those types and levels of use or development that do not significantly impair park natural resources, and direct development and use to environments least vulnerable to deterioration

• Limit unnatural sources of air, noise, visual, and water pollution to the greatest degree possible

The plan proposed boundary changes and acquisitions, extensive changes to developed sites, and removal of cars from Yosemite Valley as a long-term goal.

Approved in 1993 (NPS 1993d), the Resources Management Plan addresses specific issues such as the role of fire in the ecosystem, non-native-plant management, forest pest management, horse and mule grazing, protection of rare, threatened, and endangered species, human/bear conflicts, other wildlife and fisheries management programs, and the park's research program.

A more detailed discussion of these plans is provided in the preceding section. The goal of these plans is to manage for natural processes and events, to preserve or restore vegetation structure and diversity, or, in the case of wilderness, that would occur independent of human influences. These plans are local expressions of the biological resources management policies of the National Park System (NPS 1999a).

The overall guidance for the management of biological resources is contained in the following statement:

It is the purpose of the Federal Endangered Species Act, as amended, to provide protection for animal and plant species that are currently in danger of extinction (endangered) and those that may become so in the foreseeable future (threatened).

Section 7 of the act requires federal departments and agencies to ensure that all federally associated activities within the United States do not have adverse impacts on the continued existence of threatened or endangered species or on designated areas (critical habitats) that are important in saving those species. Section 9 of the Federal Endangered Species Act (16 USC 1531 et seq.) prohibits the "taking"^[14] of listed species, including their habitat, except by authorized permit. If incidental take^[15] might occur from a project (i.e., if individuals of a listed species would be inadvertently harmed, harassed, or collected, or would suffer significant habitat modification), consultation with the U.S. Fish and Wildlife Service is required.

The objective of the Fish and Wildlife Coordination Act is to provide that wildlife conservation receive equal consideration and be coordinated with other features or water resources development programs. Sections 1 and 2 of the act mandate that fish and wildlife receive equal consideration with water resources development programs throughout planning, development, operation, and maintenance. Whenever a federal agency proposes to impound, divert, channelize, or otherwise alter or modify any stream, river, or other body of water for any purpose, the agency must first consult and coordinate its actions and projects with the U.S. Fish and Wildlife Service and the affected state fish and game agency wherein the impoundment, diversion, or other control facility is to be constructed. This consultation and coordination process addresses ways to conserve wildlife resources by preventing loss of and damage to such resources as well as to further develop and improve these resources.

No person within the United States or any place subject to the jurisdiction thereof, shall possess, sell, purchase, barter, offer to sell, transport, export, or import at any time or in any manner any bald eagle or any golden eagle, alive or dead, or any part, nest, or egg thereof. The Secretary of the Interior can permit the taking, possession, and transportation of specimens thereof for scientific or exhibition purposes or for the religious purposes of American Indian tribes if the action is determined to be compatible with the preservation of the bald eagle or golden eagle.

The Migratory Bird Treaty Act regulates or prohibits taking, killing, possession of, or harm to migratory bird species listed in Title 50 Code of Federal Regulations (CFR) Section 10.13. This act is an international treaty for the conservation and management of bird species that may migrate through more than one country and is enforced in the United States by the U.S. Fish and Wildlife Service. Hunting of specific migratory game birds is permitted under the regulations listed in Title 50 CFR 20. The act was amended in 1972 to include protection for migratory birds of prey (raptors).

The California Endangered Species Act expanded upon the original plant protection act and enhanced legal protection for plants and wildlife. The California Endangered Species Act parallels the policies of the Federal Endangered Species Act. The state legislation was written to protect state endangered and threatened plant and animal species whose continued existence in California is in jeopardy. The California Endangered Species Act and Sections 2050 and 2097 of the Fish and Game Code prohibit "take" of plant and animal species designated by the California Fish and Game Commission as either endangered or threatened.

State listing of plant species began in 1977 with the passage of the Native Plant Protection Act. The act directed the California Department of Fish and Game to carry out the Legislature's intent to "preserve, protect, and enhance endangered plants in this state." The act gave the California Fish and Game Commission the power to designate native plants as endangered or rare, and to require permits for collecting, transporting, or selling such plants. When the California Endangered Species Act was passed, it expanded upon the Native Plant Protection Act and enhanced legal protection for plants. To align with federal regulations, the California Endangered Species Act adopted the categories "threatened" and "endangered" species. It grandfathered all "rare" animals into the act as threatened species, but did not do so for rare plants. Thus, there are three listing categories for plants in California: rare, threatened, and endangered.

Sections 3511 (birds), 4700 (mammals), 5050 (reptiles and amphibians), and 5515 (fish) of the California Fish and Game Code designate certain species as "fully protected." Fully protected species, or parts thereof, may not be taken or possessed at any time without permission by the California Department of Fish and Game. Section 3503 of the California Fish and Game Code affords protection to bird nests and birds of prey (orders Falconiformes or Strigiformes).

The National Park Service has primary responsibility for the protection of rare, threatened, and endangered species on National Park Service lands.

The U.S. Fish and Wildlife Service has primary responsibility for protecting the nation's federally listed threatened and endangered species. It is the purpose of the Federal Endangered Species Act, as amended, to provide protection for animal and plant species that are currently in danger of extinction (endangered) and those that may become so in the foreseeable future (threatened).

The California Department of Fish and Game has primary responsibility for protecting the state's biological resources. California's legal requirements to protect rare species of plants and wildlife are woven from a number of pieces of legislation – most notably the Native Plant Protection Act (1977), California Endangered Species Act (1984), and California Fish and Game Code (various sections).

The primary factors that determine air quality are the locations of air pollutant sources, the types and amounts of pollutants emitted, meteorological conditions, and topographic features. Atmospheric conditions such as wind speed, wind direction, and air temperature gradients interact with the physical features of the landscape to determine the movement and dispersal of air pollutants.

The state of California is divided into air basins that are defined partly by their meteorological and topographical characteristics. The portions of the Merced River and South Fork that traverse Yosemite National Park are located within two air basins: Mountain Counties Air Basin and San Joaquin Valley Air Basin. Generally, the uppermost reaches of the Merced River and South Fork lie within San Joaquin Valley Air Basin, and the lower reaches lie within Mountain Counties Air Basin. Figure III-4 shows these two air basins along with the other California air basins.

The portions of the Merced River and South Fork that traverse the park lie within the Sierra Nevada mountain range, which roughly parallels the eastern boundary of California and extends from the Cascades Range in the north to the Tehachapi Mountains in the south. Cooler climates with more wind are, in general, characteristic of the mountains, as contrasted with the nearby valleys. Mountain climatic zones are characterized by considerable vertical wind motion and by winds and temperatures different from those in the valleys. During the warm portion of the year, wind circulation in the mountain zones is generally upslope, with only brief periods of downslope winds at night. During the cold season, wind circulation in the absence of storm activity is generally downslope, with brief periods of upslope winds on south-facing slopes.

While air quality in a given air basin is usually determined by emission sources within the basin, it also can be affected by pollutants transported from upwind air basins by prevailing winds.^[16] For instance, the California Environmental Protection Agency concluded that all of the ozone exceedances in 1995 in the southern portion of Mountain Counties Air Basin (i.e., Tuolumne and Mariposa Counties) were caused by transport of ozone and ozone precursors from San Joaquin Valley Air Basin (California Environmental Protection Agency 1996). Air quality in Mountain Counties Air Basin also is significantly affected by pollutant transport from the metropolitan Sacramento area and the San Francisco Bay Area. In contrast, San Joaquin Valley Air Basin is considered both a source and a receptor of pollutant transport.

As a general matter, regulation of air pollution is achieved through both national and state ambient air quality standards and emissions limits for individual sources of air pollutants.

The federal Clean Air Act requires the U.S. Environmental Protection Agency (U.S. EPA) to identify National Ambient Air Quality Standards (national standards) protective of public health and welfare. Currently, U.S. EPA has established national standards for ozone, carbon monoxide, nitrogen dioxide, sulfur dioxide, particulate matter (PM-10 and PM-2.5), and lead. California has adopted more stringent standards for most of the criteria air pollutants (referred to as State Ambient Air Quality Standards, or state standards) and has adopted ambient air quality standards for some pollutants for which there are no corresponding national standards. Both sets of standards (national and state) apply throughout California.

Under amendments to the federal Clean Air Act, U.S. EPA has classified air basins, or portions thereof, as either "attainment" or "nonattainment" for each criteria air pollutant, based on whether or not the national standards have been achieved. In 1988, the state legislature passed the California Clean Air Act, which is patterned after the federal Clean Air Act to the extent that areas are required to be designated as "attainment" or "nonattainment" (but for the state standards rather than the national standards). Thus, areas in California have two sets of designations: one set with respect to the national standards and one set with respect to the state standards.

The portions of the Merced River and South Fork that flow through Yosemite National Park lie in Mariposa and Madera Counties, which are located in Mountain Counties Air Basin and San Joaquin Valley Air Basin, respectively. Table III-3 shows the current attainment/nonattainment status of the applicable subregions within these two air basins. As shown in table III-3, the Mountain Counties Air Basin portion of the corridor (i.e., within Mariposa County) is designated as nonattainment for state ozone and PM-10 standards but is designated attainment or unclassified for the other state standards and all of the national standards. The San Joaquin Valley Air Basin portion of the corridor (i.e., within Madera County) is designated as nonattainment for both state and national ozone and PM-10 standards.

The federal Clean Air Act and the California Clean Air Act require plans to be developed for areas designated as nonattainment (with the exception of areas designated as nonattainment for the state PM-10 standard). Such plans are to include strategies for attaining the standards. Air quality plans and associated control measures that are developed to achieve the national standards are referred to as State Implementation Plans (SIPs).

No air quality plans have been developed in the Mariposa County portion of the Mountain Counties Air Basin. Although Mariposa County is designated as nonattainment for the state ozone standard, a plan has not been required under the California Clean Air Act due to the overwhelming influence of pollutant transport on ozone conditions in the county. Also, while the Yosemite National Park portion of Mariposa County is designated nonattainment for the state PM-10 standard, the California Clean Air Act does not impose planning requirements on state PM-10 nonattainment areas.

In the Madera County portion of San Joaquin Valley Air Basin, three air quality plans apply, two related to ozone and one related to the national PM-10 standard. The applicable ozone air quality plans include the federal Ozone Attainment Demonstration (i.e., the ozone SIP) (San Joaquin Valley Unified Air Pollution Control District 1994) and the State Ozone Air Quality Attainment Plan (San Joaquin Valley Unified Air Pollution Control District 1998). The applicable PM-10 air quality plan is the federal PM-10 Attainment Demonstration Plan (i.e., the PM-10 SIP). This PM-10 SIP predicts attainment of the national annual PM-10 standard by 2001 and the national 24-hour-average PM-10 standard by 2006 (San Joaquin Valley Unified Air Pollution Control District 1997).

Under the federal Clean Air Act Amendments of 1990, federal agencies must make a determination of conformity with the applicable SIP before taking any action on a proposed project. In 1993, U.S. EPA published a rule (referred herein as the "general conformity rule"), that indicates how most federal agencies, including the National Park Service, are to determine whether a conformity determination is required, and if so, how to make such a determination (U.S. Environmental Protection Agency 1993a). The rule establishes "de minimis" emissions thresholds that are used to determine whether a conformity determination is required. If emissions increases would exceed the applicable de minimis thresholds due to a proposed action, then the rule establishes specific criteria through which a federal agency must demonstrate that the proposed action would conform to the SIP, despite the greater-than-de-minimis increase in emissions.

In contrast to air quality plan requirements and the general conformity rule, which relate to nonattainment areas, the federal Clean Air Act also includes provisions designed to prevent industrial growth from causing a significant deterioration in areas designated as attainment. This section of the federal Clean Air Act is known as Prevention of Significant Deterioration (PSD). PSD regulations apply to new or expanded industrial plants (i.e., PSD covers stationary sources, not mobile sources). PSD regulations also establish concentration-based increments that are not to be exceeded due to operation of the plant. These increments vary depending upon the classification of the area affected by emissions from the plant. For instance, the lowest, or most stringent, increment (least extent of allowable air quality degradation) applies to "Class I" areas, which are to be kept in especially pristine condition. Yosemite National Park is a Class I area, as are other national parks and national wilderness areas. The El Portal Administrative Site is located within a "Class II" area, in which less stringent standards apply.

Under PSD, the federal Clean Air Act establishes the following national visibility goal: "prevention of any future, and the remedying of any existing, impairment of visibility in Class I areas which impairment results from man-made air pollution." To further this goal, U.S. EPA recently established regional haze regulations (U.S. Environmental Protection Agency 1999a). By addressing regional haze, these new regulations take a comprehensive approach to improving visibility, since regional haze reflects the various contributions of a multitude of emissions sources (including mobile, stationary, and area) spread over a wide geographic area. The ultimate goal of the new regulations is to restore natural visibility conditions at Class I areas, such as Yosemite National Park, within 60 years. Under the regulations, all states will be required to develop implementation plans that demonstrate reasonable progress towards this goal.

As a general matter, the National Park Service seeks to perpetuate the best possible air quality in parks because of its critical importance to visitor enjoyment, human health, scenic views, and the preservation of natural systems and cultural resources (NPS 1988a). To accomplish this goal, the National Park Service endeavors to ensure that air pollution sources within the park comply with federal, state, and local rules and regulations, and participates in reviewing SIPs and applications to operate major stationary sources that lie outside the park but have the potential to affect air quality inside the park (NPS 1991e). At Yosemite National Park, the General Management Plan calls for markedly reducing traffic congestion within Yosemite Valley to reduce the exposure of visitors to the fumes and odors associated with motor vehicle exhaust (NPS 1980b). The General Management Plan also calls for the National Park Service to limit unnatural sources of air pollution to the greatest extent possible.

U.S. EPA is principally responsible for implementing the federal Clean Air Act. Some of the actions undertaken by U.S. EPA under the federal Clean Air Act include establishment of national standards, PSD regulations, visibility protection regulations, and national motor-vehicle emissions standards; review and approval of SIPs; and supervision of the states. implementation of SIP requirements.

California Environmental Protection Agency's Air Resources Board is the state agency responsible for reviewing regional plans for inclusion in the California SIP, submitting SIP revisions to U.S. EPA, and establishing state standards. The state Air Resources Board also has primary responsibility for regulating mobile- and area-source emissions and for overseeing the activities of regional and local air districts, called Air Pollution Control Districts (APCDs) or Air Quality Management Districts (AQMDs).

In addition to having primary responsibility for preparing air quality plans for the areas within their jurisdiction, APCDs and AQMDs are also responsible for regulating stationary sources. Stationary sources are regulated through a permitting process in which applicants must secure an Authority to Construct and a Permit to Operate from the applicable APCD or AQMD, under a process termed New Source Review, prior to operation of new or modified equipment that may affect air quality. Under the federal Clean Air Act, federal facilities are subject to local air quality rules and regulations to the same extent as other governmental and private entities.

Within Yosemite National Park, the Merced River and South Fork flow through Mariposa County and Madera County, which lie in the jurisdictions of Mariposa County Air Pollution Control District (MCAPCD) and San Joaquin Valley Unified Air Pollution Control District (SJVUAPCD), respectively.

MCAPCD exercises permit authority over new or expanded stationary sources within Mariposa County through its Rules and Regulations. In addition, MCAPCD Rules and Regulations address visible emissions (rule 202), public nuisance (rule 205), and open burning (regulation III).

SJVUAPCD exercises permit authority over new or expanded stationary sources within San Joaquin Valley Air Basin (which includes Madera County) through its Rules and Regulations. Both federal and state ozone plans rely heavily upon stationary-source control measures set forth in SJVUAPCD's Rules and Regulations. In addition to stationary-source requirements, the SJVUAPCD's federal PM-10 SIP also relies on control of area sources, known as "fugitive" dust sources, specifically construction sites, paved and unpaved roads, and material-handling activities. To regulate such sources, SJVUAPCD has adopted regulation VIII (Fugitive Dust Prohibitions) as part of its Rules and Regulations. The rules contained in regulation VIII implement control measures contained in the federal PM-10 plan.

Federal, state, and local agencies operate a network of monitoring stations throughout California to provide data on ambient concentrations of air pollutants. Table III-4 summarizes recent monitoring data from the monitoring stations in the project vicinity. Three of the stations that contributed to table III-4 are located in Yosemite National Park (Turtleback Dome, Wawona, and Yosemite Valley Visitor Center) and one is located outside of the park in the Sierra National Forest (Jerseydale). Wawona, Yosemite Valley Visitor Center (in Yosemite Village), and Jerseydale are approximately 4,000 feet above sea level, and Turtleback Dome is approximately 5,300 feet above sea level. As shown in table III-4, exceedances of state and national standards for ozone and state standards for PM-10 are recorded on occasion within the park and in the park vicinity.

Table III-4 shows that exceedances of the state 24-hour-average PM-10 standard occur approximately 11% of the time in Yosemite Village. No exceedances of the less stringent national standard of 150 micrograms per cubic meter have been recorded over the past four years. PM-2.5 data have been collected at Turtleback Dome as part of the visibility network established under the Interagency Monitoring of Protected Visual Environments (IMPROVE) program.^[21] IMPROVE data from Turtleback Dome indicate that PM-2.5 concentrations are lowest during winter and highest during summer. Over the 1994 through 1998 period, the 90th percentile, 24-hour-average concentration (i.e., 90% of the values are lower and 10% are higher) ranged from 4.2 micrograms per cubic meter during winter to 14 micrograms per cubic meter during summer. In contrast, the new 24-hour-average PM-2.5 standard is 65 micrograms per cubic meter. The average of the annual averages during that period was 4.3 micrograms per cubic meter; in contrast, the new annual-average PM-2.5 standard is 15 micrograms per cubic meter.

Visibility conditions are commonly expressed in terms of three mathematically related metrics: visual range, light extinction, and deciviews. Visual range is the maximum distance at which one can identify a black object against the horizon and is typically described in miles or kilometers. Light extinction, which is inversely related to visual range, is the sum of light scattering and light absorption by particles and gases in the atmosphere and is expressed in terms of inverse megameters (Mm^-1), with large values presenting poorer visibility. Unlike visual range, the light extinction coefficient expresses the relative contribution of one particulate constituent (e.g., sulfates or nitrates) versus another to overall visibility impairment. The deciview metric was developed because changes in visual range and light extinction are not proportional to human perception. For example, a five-mile change in visual range can be either very apparent or not perceptible, depending on the baseline level of ambient pollution. The deciview metric provides a linear scale for perceived visual changes over the entire range of conditions, from clear to hazy, analogous to the decibel scale for sound. Under many scenic conditions, a change of 1 deciview is considered to be perceptible by the average person. A deciview of zero represents pristine conditions.

Current visibility impairment in Yosemite National Park ranged from 5 deciviews for the clearest 20% of days during the 1995 to 1997 period to 18 deciviews for the haziest 20% of days during that period (U.S. Environmental Protection Agency 1998). In contrast, the corresponding range of deciview values was 5 (clearest 20%) to 13 (haziest 20%) and 15 to 29 in Rocky Mountain National Park and Great Smoky Mountains National Park, respectively. On the haziest days in Yosemite National Park, sulfates are responsible for approximately half of the visibility impairment. Nitrates, organic carbon, elemental carbon, and crustal matter are responsible for the remainder in roughly equal measure.

By definition, noise is human-caused sound and is considered to be unpleasant and unwanted. Whether a sound is considered unpleasant depends on the individual "listening" to the sound and what the individual is doing when the sound is heard (i.e., working, playing, resting, sleeping). While performing certain tasks, people expect and, as such, "accept" certain sounds. For instance, if a person works in an office, sounds from printers, copiers, and typewriters are generally acceptable and not considered unpleasant or unwanted. By comparison, when resting or relaxing, these same sounds are not desired. The desired sounds during these times are referred to as "natural quiet," a term used to describe ambient (outdoor) natural sounds without intrusion of human-caused sounds. Natural quiet can be essential in order for some individuals to achieve a feeling of peace and solitude.

Natural sounds within Yosemite National Park and adjacent to the Merced River are not considered to be noise. These sounds result from natural sources such as waterfalls, flowing water, animals, and rustling tree leaves. Existing noise within the park results from mechanical sources, such as motor vehicles, generators, and aircraft and from human activities, such as talking and yelling.

Noise results from automobiles, recreational vehicles, and trucks (motor vehicles) accessing the park via El Portal Road, Wawona Road, the Big Oak Flat Road, and Tioga Road. Near the Yosemite Valley Visitor Center, noise results from vehicles on Northside Drive, Southside Drive, and roadways to and from camping areas. Noise from motor vehicles is obviously "loudest" immediately adjacent to the roadways but, due to generally low background sound levels, can be audible a long distance from the roads. Atmospheric conditions (such as wind, temperature, humidity, rain, fog, and snow) and topography (e.g., echo from canyon walls) can significantly affect the presence or absence of motor vehicle noise in various areas of the Merced River corridor. Logically, noise levels from this source will be "loudest" where and when activity levels are the greatest and nearest to the area.

As part of a report to Congress (NPS 1994e), the National Park Service conducted a visitor survey in Yosemite National Park. Of the visitors surveyed, 55% reported hearing aircraft sometime during their visit. The report notes that recognition of noise from aircraft was highly variable from location to location and, logically, that visitor impacts were greater for activities where visitors removed themselves from automotive transportation and areas where other visitors were present. In Yosemite, a majority of the complaints came from wilderness trail users. Measurements made in 1993 at four locations within the park (Rafferty Creek, the Soda Springs area in Tuolumne Meadows, Mirror Lake, and Glacier Point) indicated that aircraft were audible 30% to 60% of the time during each of the measurement periods (six hours at each site). Most overflights are associated with high-altitude jet aircraft. The National Park Service also uses aircraft in its management activities. These aircraft are generally helicopters that are used for firefighting, search and rescue, medical, law enforcement, and other special operations (NPS 1993a).

Other mechanical sources of noise within the park and near the Merced River include roadway construction equipment, generators, radios, and park maintenance equipment (i.e., mowers and chainsaws). The frequency of use and the location of these sources vary both by season and reason for use.

Generally, the federal government sets standards for transportation-related noise sources that are closely linked to interstate commerce, such as aircraft, locomotives, and trucks; for those noise sources, state governments are preempted from establishing more stringent standards. The state governments set noise standards for those transportation-related noise sources that are not preempted from regulation, such as automobiles, light trucks, and motorcycles. Noise sources associated with industrial, commercial, and construction activities are generally subject to local control through noise-related plans and policies.

As a general matter, the National Park Service seeks to preserve the natural quiet and the natural sounds associated with the physical and biological resources of the park and to prevent or minimize unnatural sounds that adversely affect park resources or values or visitor enjoyment of them (NPS 1988a). National Park Service guidelines note that limitations on wilderness use may be based in part on the potential for adverse effects, such as noise, on visitor aesthetic experience, in addition to effects on the natural resources themselves (NPS 1991e). At Yosemite National Park, the General Management Plan calls for markedly reducing traffic congestion within the Valley to reduce the exposure of visitors to the noise associated with motor vehicles (NPS 1980b). The General Management Plan also calls for the National Park Service to limit unnatural sources of noise to the greatest extent possible.

Current sound levels adjacent to the main stem and South Fork of the Merced River vary by location and also by season (the volume of water in the rivers being lower in the fall and higher in the spring). Current noise levels are also influenced by the number of visitors to the park and by the proximity of mechanical noise sources.

Sound-level measurements were obtained at various locations adjacent to the Merced River (from the headwaters of the Merced River to the base of Vernal Fall), within Yosemite Valley, and in the Wawona area. Measurements were obtained with a Larson Davis dosimeter (Model 700). The dosimeter was calibrated with a Larson Davis sound-level calibrator. At each measurement location, observations of the background level were made over a period ranging from one to five minutes. In addition, observers noted the sources contributing to the background level and noted any sources that caused intrusive levels above the typical background sound level. Appendix F includes a table that describes the measurement locations, the measurement results, and the associated sources. Appendix F also includes a figure that shows where the measurements were taken.

Sound levels at the highest elevations of the Merced River corridor (between the Merced and Triple Peak Forks) measured 35 dBA. Also in the headwaters area, approximately 2 to 2.5 miles southeast of Washburn Lake, sound levels ranged form 39 to 41 dBA, with the influence of aircraft noise (the maximum observed levels with the aircraft were 43 and 56 dBA). At and near Washburn Lake, sound levels ranged from 31 to 36 dBA, with very little influence of sound from the river. At a lower elevation, between the soda springs and Washburn Lake, sound levels on the trail ranged from 35 to 42 dBA. In the Bunnell Cascades and the soda springs areas, sound levels ranged from 54 to 56 dBA. These sound levels primarily resulted from river water washing over granite cascades in both areas. Away from the river, in the Little Yosemite Valley Campground area, sound levels measured 40 dBA (in an area with no human activity). Near the waterfalls (Vernal and Nevada) sound levels varied from 61 to 76 dBA, with some influence from people talking during each measurement period.

Within Yosemite Valley, sound levels ranged from 44 to 47 dBA along the Lower Yosemite Fall Trail, with maximum observed levels of 66 dBA when people passed the monitor on the trail. Notably, there was no water in Yosemite Creek when the monitoring was performed. At Swinging Bridge, sound levels measured 50 dBA, with noise from people constituting the greatest source of sound within the area. At Sentinel Bridge, sound levels measured 59 dBA. This area experiences noise from vehicular traffic, but speeds are generally slow. Overall, the greatest source of sound was the numerous buses traversing the bridge. Near Happy Isles, sound levels measured 59 dBA, with most of the sound resulting from people on the trails and using facilities nearby. Within the camping area (Upper Pines Campground), sound levels varied from 32 dBA when human activity levels were at the lowest (early in the morning) to 55 dBA when activity levels increased during the day.

West of the Valley Visitor Center area, the river was calm in El Capitan Meadow and no people were present during the monitoring. Measured sound levels within this area were 39 dBA. At Devils Elbow, water was flowing through the river, but the sound of the river was minimal due to the lack of rocks and rapids. Sound levels in this area were 44 dBA, with a maximum observed level of 67 dBA when a bus passed on nearby Northside Drive. Finally, at Cascades Diversion Dam, measured sound levels were 49 dBA, with a recorded maximum level of 63 dBA when a bus passed on Northside Drive.

In Wawona, sound levels were measured in the middle of the old Wawona bridge on Wawona Road, and west of the covered bridge near the Pioneer Yosemite History Center. Sound levels in these areas were 50 and 44 dBA, respectively, with maximum observed levels of 59 dBA near the bridge.

Although "noise" is not specifically addressed in the classification criteria for the National Wild and Scenic Rivers System, the presence of noise can reduce a visitor's enjoyment and degrade the immediate environment adjacent to a river. Depending on the area, noise sources adjacent to the main stem and South Fork of the Merced River include motor vehicles, generators, aircraft, and human activity, such as talking and yelling.

Measured sound levels indicate that the background (minimal) sound level in the study area is 31 to 32 dBA (measured near Washburn Lake and in the Upper Pines Campground). In river areas where water flow is minimal, sound levels averaged 37 dBA. In areas with flowing water, sound levels averaged 44 dBA. In areas of cascading water, sound levels averaged 55 dBA. Finally, in waterfall areas, sound levels averaged 68 dBA. Logically, sound levels associated with the river itself increased as the flow of water increased and in areas where whitewater and waterfalls were present.