Chapter IV: Affected Environment

Introduction

This chapter presents topics included in the analysis of the Merced Wild and Scenic River Revised Comprehensive Management Plan/Supplemental Environmental Impact Statement (Revised Merced River Plan/SEIS) 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 the public scoping period. This chapter also provides a discussion of topics that were dismissed from further analysis.

Existing conditions are described based on the most recent analyses completed for each topic area. 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 V, Environmental Consequences.

Because this document is a supplemental environmental impact statement to the original Merced Wild and Scenic River Comprehensive Management Plan and Final Environmental Impact Statement (Merced River Plan/FEIS), impact topics discussed below have been updated to reflect current conditions, where appropriate. In addition, some of the impact topics related to biological resources, cultural resources, and recreation in the El Portal area were updated to accurately reflect additional research conducted to identify and locate the Outstandingly Remarkable Values associated with these resources in the El Portal Administrative Site.

Impact Topics Considered in this Plan

Natural Resources

The federal and state Endangered Species Acts (and associated legislation), Clean Water Act, Clean Air Act, and NEPA 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 the El Portal Administrative Site 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 could result from implementation of Revised Merced River Plan/SEIS 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

Cultural Resources

The National Historic Preservation Act, the Archeological Resources Protection Act, Native American Graves Protection and Repatriation Act, and NEPA 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. Many historic and archeological sites, museum collections, historic buildings and structures, cultural landscape resources, and traditional cultural properties exist within the park and the El Portal Administrative Site and could be affected by the alternatives.

Analysis was performed for the following cultural resource topics:

·       Archeological Resources

·       Traditional Cultural Resources

·       Historic Sites, Structures, and Landscapes

Visitor Experience

Stewardship of Yosemite National Park requires the consideration of two integrated purposes: (1) to preserve Yosemite's unique natural and cultural resources and scenic beauty, and (2) to make these resources available to visitors for study, enjoyment, and recreation. Different options for implementing a user capacity program and river boundaries in El Portal considered in the Revised Merced River Plan/SEIS could 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

·       Scenic Resources

Social Resources

Analysis of social resources examines the effects of 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 elements of the user capacity program presented in the alternatives considered in this Revised Merced River Plan/SEIS could affect how visitors circulate within and/or 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. NEPA requires that socioeconomic impacts of the Revised Merced River Plan/SEIS be addressed. The Revised Merced River Plan/SEIS could affect socioeconomic activity within the park and in the surrounding gateway communities.

Analysis was performed for the following social resource topics:

·       Land Use

·       Transportation

·       Socioeconomics

·       Park Operations and Facilities

Impact Topics Dismissed from Further Analysis

Environmental Justice

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:

(1)  is predominately borne by a minority population and/or a low-income population, or

(2)  will be suffered by the minority population and/or low-income population and is appreciably more severe or greater in magnitude than the adverse effect that will be suffered by the non-minority population and/or non-low-income population.

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 Revised Merced River Plan/SEIS 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 Revised Merced River Plan/SEIS would be equally applied to all visitors, regardless of race or socioeconomic standing. In addition, it is expected that minority populations comprising a portion of Yosemite visitation come from areas outside the immediate Yosemite area, such as the Central Valley, San Francisco, and Los Angeles, and have a variety of other recreation options available to them besides Yosemite National Park. Although levels of park employee housing in various areas may be affected by decisions made under the Revised Merced River Plan/SEIS, employee housing decisions are not expected to 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.

Prime and Unique Agricultural Lands

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 direct or indirect effects to downstream agricultural lands.

Public Health and Safety

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

Museum Collection

The Yosemite Museum collection is not presented as a separate topic. This Revised Merced River Plan/SEIS, as a programmatic document, does not specifically call for any data collection activities. Future projects undertaken in the river corridor could require data collection. Any effect from these projects on the Yosemite Museum collection would be addressed under future compliance documents.

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 park includes lands within Mariposa, Tuolumne, and Madera Counties and shares a boundary with Mono County.

Yosemite National Park is located about 200 miles east and 4 hours by car from San Francisco, and about 320 miles northeast and 6 hours by car 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, the U.S. 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 Road bridge in the El Portal Administrative Site. The main stem of the Merced River flows a total of 140 miles from its headwaters to its 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 Nevada. Major east-west watersheds that dissect the Sierra Nevada 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 (UC Davis 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 (UC Davis 1996).

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 that were formerly 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 (UC Davis 1996).

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 (UC Davis 1996).

Humans are an integral part of Sierra Nevada ecosystems, having lived and sustained themselves in the region for at least 10,000 years. Indigenous populations were widely distributed throughout the range at the time of European immigrations. Archeological evidence indicates that American Indians practiced resource management through localized harvesting, pruning, irrigation, burning, and vegetation thinning. Immigration of Euro-American settlers in the mid-1800s began a period of increasingly intense resource extraction and settlement (UC Davis 1996).

Natural Resources

Geology, Geohazards, and Soils

Regional Geology and Geologic History

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. 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 above mean sea level 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 plutons of melted rock. About 200 million years ago, 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 million years ago and 65 million years ago, 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 million years ago, 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 million years ago, 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 3 million years ago and 2 million years ago, 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 its maximum extent about 20,000 years ago. The Tioga glacier extended westward as far as Bridalveil Meadow and, when it receded, 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).

Bedrock of Yosemite

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.

Upper Main Stem Geology

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 Geology

Yosemite Valley is primarily granitic in composition and glacially carved, with its floor ranging from 3,800 to 4,200 feet above sea level. 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 one-half mile wide to around three-quarters 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 ridge-shaped landforms known as moraines. A medial moraine at the east end of Yosemite Valley was created when glaciers extending from the Upper Merced and Tenaya canyons merged at the confluence of the two canyons. Two other 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 farthest extent. After the last glacier melted, water flow was dammed by morainal material to form 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 features of the geologic Outstandingly Remarkable Value.

Merced River Gorge and El Portal Geology

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 the 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 a feature of the 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 remnants of ancient sedimentary and volcanic rocks that were deformed and metamorphosed, in part by granitic intrusions (Huber 1989). This metamorphosed sedimentary rock (which includes banded chert) was once part of the ocean floor that covered the region about 200 million years ago (Huber 1989). The transition from igneous to metasedimentary rocks is identified as a feature of the geologic Outstandingly Remarkable Value in the El Portal segment of the river.

South Fork Geology

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 westward, the South Fork contacts granitic rocks, metamorphic rocks near Gravelly Ford, and granite (similar to that found in Yosemite Valley) 8 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.

Geohazards

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

Regional Seismicity

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 1997). However, Yosemite can undergo seismic shaking associated with earthquakes on fault zones on 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 to the west, the volcanically active area in the Mono Craters-Long Valley Caldera area to the east, and the various faults within the Owens Valley fault zone, also to the east (CDMG 1996).

The Foothills fault zone, which includes the Melones Fault and Bear Mountain Fault, extends in a north-south direction within the foothills of the Sierra Nevada, approximately 30 to 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 1996).

The Mono Lake fault is located approximately 35 miles northeast of Yosemite Valley within the Mono Craters-Long Valley Caldera region. Since 1980, this area has experienced considerable seismic activity. 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. A 5.7 magnitude earthquake on the Mono Lake fault in October 1990 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). In September 2004, a swarm of earthquakes, with two greater than magnitude 5, occurred in the Adobe Hills north of Long Valley and just east of Mono Lake; the epicenter of the swarm is in the vicinity of the Hunton Valley fault system (CISN 2004).

The Owens Valley fault, located approximately 100 miles southeast of Yosemite Valley, has experienced movement within the last 200 years, and the California Geological Survey considers this fault active (CDMG 1997). The most notable earthquake felt in Yosemite National Park was the Owens Valley earthquake of March 26, 1872. The Owens Valley earthquake is estimated to have had a magnitude of 7.6 and was one of the largest earthquakes in U.S. history (USGS 1990a). 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. Ground shaking typically is expressed in terms of peak ground acceleration 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 ground shaking, but structural damage would be negligible to slight in buildings constructed according to modern building standards.

Rockfall

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 extremely large and catastrophic 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 (angular rock fragments) slopes along each side of the 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 500 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:

The Eagle Rock, a short distance up the Valley, had given way, and I saw it falling in thousands of the great boulders I had been studying so long, pouring to the Valley floor in a free curve luminous from friction, making a terribly sublime and beautiful spectacle--an arc of fire fifteen hundred feet span, as true in form and as steady as a rainbow, in the midst of stupendous roaring rock storm.

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). These larger rockfalls may result in blowdown, or sudden wind gusts associated with large slabs of rock hitting the ground, and which have the potential for threats to human safety and property damage. An example of this type of event occurred associated with a large rockfall at Happy Isles in 1996. The U.S Geological Survey and the National Park Service have cooperated in documenting potential geologic hazards in developed areas, including areas most susceptible to rockfalls (USGS 1999c). The National Park Service has developed Yosemite Valley Geologic Hazard Guidelines (Appendix C in NPS 2000e) with the intent to better protect park visitors and staff by avoiding placement of structures in areas with a high potential for rockfall impact.

Upper Main Stem and Yosemite Valley Geohazards

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 east 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, 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.

An inventory of historical rockfalls in Yosemite National Park identified 519 rockfalls that occurred between 1857 and early 2004, according to published and unpublished accounts and field studies of recent rockfalls. Of these, about 330 occurred within Yosemite Valley, and most of the others occurred in the Merced River gorge or along El Portal Road. Report authors note that many more than 500 rockfalls undoubtedly occurred during this period, but some went unnoticed or unreported because of the small size of individual rockfalls or the lack of impact on trails, roads, structures, or utilities (USGS 2004a). 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-sized rockfalls has been determined from an analysis of historical events (Wieczorek et al. 1995).

Merced River Gorge and El Portal Geohazards

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 7 miles between Pohono Bridge to the park boundary. The gorge area has had many rockfall incidences, including rockfalls that have occurred along El Portal Road. Of 519 historical rockfalls identified in a recent inventory, most of the approximately 190 rockfalls that did not occur within Yosemite Valley occurred in areas near the Valley such as along El Portal Road in the Merced River gorge (USGS 2004a). The high incidence of rockfalls is partly due to the steep, narrow configuration of the gorge, riverbank undercutting, and historic human activity such as the construction of El Portal Road. These events have been well documented (USGS 2004a) 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.

Soils

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 1978). 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 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 (NPS 1978). 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 (NPS 1978).

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 1978).

Upper Main Stem Soils

Soils specific to the upper main stem of the 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.

Yosemite Valley Soils

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 (SCS 1992). Each soil type has specific characteristics that influence plant growth, water movement, and land use capabilities, etc. Land use limitations are commonly associated with frequent flooding, a 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.

Merced River Gorge and El Portal Soils

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.

Upper South Fork Soils

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.

Lower South Fork Soils

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.

Hydrology, Floodplains, and Water Quality

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.

Hydrology

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 the 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.

Watersheds and Drainage

Regional Watershed

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 Tuolumne and Merced River systems originate along the crest of the Sierra Nevada mountains and have carved 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 of the park, or 326,000 acres (511 square miles).

Merced River Basin

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 its confluence with the San Joaquin River (USGS 1992b). Principal tributaries of the Merced River within the park boundaries and the El Portal Administrative 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 on the unique watershed characteristics of the three river areas. Discharge flows within the different hydrologic segments 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, Simonsen et al. 1996). 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 and holds it 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 1992b). 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, and the average annual total discharge is approximately 257,400 acre-feet (USGS 1998a).

The Merced High Sierra Camp has a seasonal water system that draws surface water from the Merced River. This water system serves tent cabins, a kitchen/store, shower facilities, flush toilets, and a backpacker campground. Approximately 50 to 150 persons can be served by this water system on a daily basis, which is operational from the early part of July through the early part of September. The system has a design capacity of approximately 3,000 gallons per day and is permitted through the California Department of Health Services.

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 1999b). Historic discharge in the river, measured at the Pohono Bridge gauging station, has ranged from a high of about 25,000 cubic feet per second to a low of less than 10 cubic feet per second. The mean daily discharge rate is about 600 cubic feet per second, 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 1992b). 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 2- and 5-year flow within the existing channel banks (NPS 1997b). 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 1991). 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 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 can inundate low-lying areas or overflow channels. Although the Happy Isles Bridge was removed in late 2001 and early 2002, the remaining 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 gauging station. Tributaries within the gorge are relatively minor, although Cascade Creek flows into the Merced River as the river enters the steepest part of the gorge.

In late 2003 and early 2004, the Cascades Diversion Dam was removed from the Gorge segment of the river. The Cascades Diversion Dam was 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 hydroelectric power plant that is no longer in use. The removal of the dam allowed the accumulation of sediments that were retained behind the dam to be redistributed down-river during periods of higher river flows. The river is in the process of re-establishing its normal channel and bank in this area, although it will likely be a few years before normal river dynamics are fully restored in this area.

South Fork Basin

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 South Fork's confluence 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 metasedimentary rocks at a 3,500-foot elevation (USGS 1996a). 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 its confluence with the main stem of the Merced River several miles downstream from the western park boundary (USGS 1992b). The average annual flow at its confluence with the Merced River is 356 cubic feet per second, with a maximum recorded flow of 46,500 cubic feet per second and a minimum recorded flow of 2.2 cubic feet per second (USFS 1989).[2] At Wawona, upstream of the Big Creek confluence, the average annual flow was 174 cubic feet per second between 1958 and 1968, with an estimated maximum flow of 15,000 cubic feet per second in December 1955.[3] The 100-year flow volume of the river through the South Fork Bridge cross-section is estimated at 13,563 cubic feet per second. 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.

Precipitation

Merced River Basin

The overall climate in the Merced River basin 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.

South Fork Basin

In Wawona (elevation 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.

Alluvial Processes

Yosemite National Park is composed of and underlain by various granite rock types; therefore, weathering, erosion, and transport of sediment can be a very slow process in the park. Unfractured granite is impermeable and weathers very slowly; however, granite weathers much more readily when the various granites are buried in 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.

Merced River Basin

Sedimentation is a significant natural process within Yosemite Valley. The Merced River has a very low gradient within the Valley, approximately 0.1% or 6.25 feet per mile (NPS 1992c). 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, the El Capitan moraine was reduced in elevation by blasting to decrease flooding in Yosemite Valley. This moraine served as a hydraulic control for the Merced River in Yosemite Valley and influenced the rate and distribution of sediment deposition. The reduction of flooding may have allowed encroachment of the forest into meadow areas near the river through 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 the El Capitan moraine removal. The dam provided a condition where sediment that normally moved through the river system would settle and become trapped.

The National Park Service has recently completed two restoration projects along the Merced River to restore the river's natural free-flowing conditions and sediment transport patterns. In late 2003 and early 2004, the park removed the Cascades Diversion Dam. As part of this project, park staff removed or repositioned about 4,000 to 5,000 cubic yards of sediments trapped behind the dam. Park staff are currently removing a smaller dam upstream of Happy Isles that was part of an obsolete water diversion system.

Localized bank erosion is apparent in the wilderness areas above Nevada Fall where hiking trails parallel the river through Little Yosemite Valley. In east Yosemite Valley, localized loss of riverbank vegetation coupled with the backwater effect of undersized bridges has led to extensive bank erosion and channel widening. Downstream of Yosemite Valley, bank stability and sediment transport are affected by the alteration of the channel and floodplain due to historic roads and development.

South Fork Basin

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 from the placement of bridges and roads along streambanks.

Floodplains

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 from flooding. 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, humanmade 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 from 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 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 caused by 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 January 1997 flood resulted from heavy, warm rains and melting snow, with rain at elevations up to 10,000 feet (NPS 1997i). Rain alone occasionally causes peak discharge events that are usually local in nature but sometimes cover a large area.

Merced River Basin

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 (main stem - Wilderness) 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 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 on the function of the floodplain. Loss of vegetation and soil compaction in highly visited areas, channel confinement by riprap, and bridges can also influence functions of the floodplain. Historic development has altered the hydrologic response of the Yosemite Valley watershed. Past land uses and related 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. Since designation of the river in 1987, park staff have taken measures to reverse these effects through removal of some infrastructure in the floodplain and restoration of some meadows and riverbanks.

National Park Service field staff surveyed the extent of the January 1997 flood inundation in Yosemite Valley and El Portal immediately after the event. Flood flow rates during the January 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 January 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 cubic feet per second as measured at Pohono Bridge. These floods were the result of rain-on-snow events during which rain fell on winter snow pack and caused 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 cubic feet per second (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 cubic feet per second at Happy Isles and 25,000 cubic feet per second at Pohono Bridge (Eagan 1998). The January 1997 flood was estimated to have a recurrence interval of 90 years (NPS 1997b) 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 and caused downed trees, movement of landslide talus into streams, channel erosion, and substantial changes in channel morphology (NPS 1997b).

Merced River Gorge and El Portal Watershed. From where the Cascades Diversion Dam was formerly located and 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 and small levees have altered the floodplain by restricting flow during flood events and forming a barrier to channel migration. During extreme flood events, the Merced River has shown the capability to undermine or spill over and damage the roadway.

South Fork Basin

The South Fork has a limited floodplain (except in the Wawona area) because of the steep topography through which the river flows. The only significant floodplain in the South Fork basin is in the Wawona area, which is 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 dry season, 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 from the river when flow drops to critical levels (NPS 1987a).

Frazil Ice Flooding

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 to become slush and then press 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 in February 1996.

Nonflood Alterations of the Floodplain

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 (NPS 1991a). 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

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 (CVRWQCB 1998).

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, Spoto et al. 1996).

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.

Merced River Basin

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 of lead, cadmium, and mercury above drinking water and freshwater criteria have been noted within the Merced River (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 (USGS 1999a).

Groundwater quality is generally good in the Merced River basin and it 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, Simonsen et al. 1996). 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.

South Fork Basin

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 from 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.

Bank Erosion

Water quality has been affected by localized areas where visitor use of the Merced River is concentrated. 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 caused by erosion, higher water temperatures from a lack of riparian cover, and lower dissolved oxygen levels due to elevated temperatures and more shallow river depths.

The National Park Service has recently completed multiple riverbank restoration projects to restore degraded riverbanks along the Merced River. Between 1991 and 1995, 11 riverbank restoration projects were completed spanning from Little Yosemite Valley to Devil's Elbow in Yosemite Valley. Crews removed riprap and historic dump deposits close to the river's edge, recontoured riverbanks, decompacted soil, and planted appropriate vegetation. In 2002, the National Park Service completed an additional riverbank restoration project in Yosemite Valley at the confluence of Eagle Creek and the Merced River. As a result, crews restored numerous highly degraded riverbanks with sparse vegetation and erodable soil to stable riverbanks with well-established native vegetation.

Nonpoint Pollution Sources

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 contributing organic chemicals and heavy metals to land surfaces. 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 is discharged directly or indirectly into the main stem and South Fork Merced Wild and Scenic River or other streams and lakes throughout the park. In the Yosemite Wilderness, 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 pollution through atmospheric deposition. The entire Sierra Nevada range has been designated as sensitive to acid precipitation because of 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.

Underground Tanks and Abandoned Landfills

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

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, and the treated wastewater is used to irrigate the Wawona Golf Course. During the winter, the treated wastewater is discharged to the South Fork when storage capacity is insufficient and disposal to the golf course is not feasible because of snow cover.

Fires

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 2004b), which has clear guidelines about when and where to allow natural and prescribed fires to burn. The effects of fire on water quality are potentially great. 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 waterbodies.

Groundwater and Water Supply

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 groundwater 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.

Merced River Basin

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 become 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 1991g). Groundwater is used in both Yosemite Valley and El Portal for potable water supplies. Three wells in Yosemite Valley have the capacity to produce approximately 2,800 gallons per minute (gpm) (NPS 2004n). In El Portal, six wells support a capacity of approximately 220 gpm (NPS 2004z).

South Fork Basin

In the Wawona area, the groundwater flows through upper unconsolidated fills and lower fractured rock aquifers that have not been defined. The primary aquifer that supplies potable water to private wells in Wawona comes from 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 1996a). 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 gpm from the shallow aquifer in Wawona (USGS 1996a).

Currently four potable water distribution systems and multiple private wells supply water to the Wawona Area. The National Park Service is responsible for operating one of the distribution systems that supplies surface water from the South Fork Merced Wild and Scenic River to National Park Service and concessioner employee residences, the Wawona Hotel, the Wawona Campground, and 30 private residences. The National Park Service's potable water production system is regulated under a permit issued by the Regional Water Quality Control Board and is designed to draw 480 gpm. The three remaining water distribution systems are owned by private homeowners and are regulated under permits issued by Mariposa County (NPS 2004aa).

Wetlands

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. For vegetative descriptions, refer to the Vegetation section of this chapter; for data relating to wildlife and aquatic species, refer to the Wildlife section; and refer to the Rare, Threatened, and Endangered Species section for information on protected species of plant and wildlife.

Wetland Classification and Definition

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. 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 (Corps) 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 transitional lands 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 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])." Streams, creeks, rivers, and natural drainages that are regulated under Section 404 of the Clean Water Act are defined as "other waters of the United States" and are referred to as such in this document. For purposes of this document, wetland waters of the United States and other waters of the United States are referred to collectively as waters of the United States, unless noted otherwise. Additionally, both waters of the United States and Cowardin wetlands are referred to as wetlands.

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, or 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 waterbody 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.

Regional Context

Aquatic and riparian systems are the most altered and impaired habitats of the Sierra Nevada (UC Davis 1996). Dams and diversions throughout most of the range 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 (UC Davis 1996). Within the mountains, broad valleys with wide riparian areas were often reservoir sites, and much of the best former riparian habitat in the Sierra Nevada is now under water. The extent of the inundation across the range becomes apparent when one realizes that virtually all flatwater[6] on the western slope of the Sierra Nevada below 5,000 feet is artificial (UC Davis 1996). Wetlands in the Sierra Nevada have been drained since the earliest settlers attempted to reclaim meadows and other seasonally wet areas. Mountain meadows were commonly drained with the intent of improving forage conditions and to permit agriculture (Hughes 1934, as in NPS 1997g; UC Davis 1996).

Riparian Wetlands

Wetlands within the Merced River corridor are broadly classified as riparian in nature and include aquatic, riparian, meadow, and floodplain communities. The riparian zone is the plant community adjacent to a river or stream channel and serves as the interface between the river and the surrounding meadows, floodplain, and upland plant communities. It may be best described as the zone of direct interaction between land and water (Swanson et al. 1982, as in NPS 1997g; Gregory et al. 1991, as in NPS 1997g; Cummins 1992, as in UC Davis 1996).

Riparian areas are characterized by the combination of high species diversity, high species density, and high productivity. Continuous interactions occur among riparian, aquatic, and upland terrestrial ecosystems through exchanges of energy, nutrients, and species (Mitsch 1986, as in NPS 1997g). Compared to other wetland and aquatic types, riparian areas are open, with large energy, nutrient, and biotic interchanges between aquatic systems on the inner margin and upland terrestrial ecosystems on the upland margin. Riparian ecosystems are further distinguished from other ecosystem types, as described below.

·       Riparian ecosystems have a linear form as a consequence of their proximity to rivers, streams, and lakes.

·       Energy and material from the surrounding landscape converge and pass through riparian ecosystems in much greater amounts than with any other ecosystem.

·       Riparian ecosystems connect upstream and downstream ecosystems.

·       Floodwater and subsequent groundwater levels are the main determinants of the type and productivity of the vegetation found in the riparian zone.

·        Floodwater also brings nutrient-rich sediment to the floodplain, exports organic and inorganic material from the floodplain, and serves as a primary agent for long-term aggregation and degradation of the floodplain (Mitsch 1986, as in NPS 1997g).

Riparian ecosystems play a critical role in a variety of ecosystem processes. Situated at the interface between terrestrial and aquatic ecosystems, these ecosystems act to buffer hydrology and erosional cycles, control and regulate biogeochemical cycles of nitrogen and other key nutrients, limit fire movements, and create unique microclimates for animal species (Rundel and Stuner 1998). Large trees within the riparian zone provide shade to keep water temperatures cooler in the summer. Thick vegetation along the river channel helps to stabilize soils, which tend to be easily eroded in the absence of vegetation because of their coarse texture.

The diversity and structural complexity of riparian vegetation creates a wide variety of habitats for animals. Both terrestrial and aquatic wildlife depend on riparian ecosystems with their year-round availability of water, nutrients, food sources, and organic matter. In addition to these critical components of food resources, riparian ecosystems provide wildlife with a structural complexity that includes mosaics of shade and sun, shelter, and protected corridors between adjacent plant communities. It is not surprising, therefore, that riparian ecosystems are centers of high biodiversity (Rundel and Stuner 1998).

Riparian communities are among the most affected in Yosemite Valley because of their proximity to water, the effects of trampling, and the placement of above- and below-ground infrastructure, including lift stations, bridges, and underground sewer lines within riparian zones. The National Park Service has initiated ecological restoration projects designed to protect these sensitive communities and riverbanks from unnaturally high rates of erosion and encourage the re-establishment of vegetative cover. Visitors are directed to areas that can accommodate heavy visitor use without long-term impacts, such as to point bars and gravel bars along meandering river segments.

Wetland Classes

Specific wetland classes identified within the river corridor include riverine (rivers, creeks, and streams), palustrine (shallow ponds, marshes, swamps, sloughs), and lacustrine (lakes and deep ponds).

Using the Cowardin classification system, specific wetland and deepwater classes within the Merced River ecosystem consist of:

·       Riverine upper perennial - main channels of the Merced River and South Fork

·       Riverine intermittent - intermittent tributaries to the Merced River and South Fork

·       Palustrine emergent - emergent wetland (marsh, meadow) habitat along the Merced River and South Fork subject to various flooding regimes

·       Palustrine forested - riparian forest habitat along the Merced River and South Fork subject to various flooding regimes

·       Palustrine scrub shrub - riparian scrub (e.g., willow) habitat along the Merced River and South Fork and their tributaries subject to various flooding regimes

·        Lacustrine limnetic - naturally occurring deep-water lakes (e.g., Merced Lake, Washburn Lake) along the Merced River

Additional areas within the Merced River ecosystem are mapped as undesignated and may be considered potential wetland (USFWS 1995). In some of these areas, there is development in the wetland or fill soils on top of wetland soils. One undesignated block is the 16 acres of braided stream channel of lower Yosemite Creek.

The following discussion provides general descriptions for each wetland class identified within the Merced River ecosystem.

Riverine Upper Perennial

Riverine upper perennial habitat within the corridor includes the open and flowing water of the Merced River and the South Fork. It is the permanently flooded rock-, cobble-, or sand-bottom channel with little to no in-stream vegetation. Occasional sandbars form within and at the channel edge and typically support willows and emergent (grasses and herbs) vegetation. Based on the National Park Service guidelines, the majority of the main stem of the Merced River and the South Fork would be classified as wetland. Channel portions that lie at a depth of 2 meters below low water would be considered deepwater. The main channel of the Merced River and the South Fork of the Merced River would likely be considered as jurisdictional by the U.S. Army Corps of Engineers under Section 404 of the Clean Water Act, not as wetlands but as other waters of the United States.

Riverine Intermittent

Numerous riverine intermittent drainages (other waters of the United States) are tributary to the main stem Merced River and the South Fork. Almost all riverine intermittent drainages within the river corridor are classified as Cowardin wetlands and waters of the United States. These drainages often have a nonsoil substrate that is saturated and/or covered by shallow water at some time during the growing season. These wetlands are typically narrow and encompass the lowest portion of creekbeds. Very little wetland vegetation is found in these areas because of the intermittent nature of the flows within the drainage channels. All above-ground drainages within the river corridor are subject to the National Park Service protection policies under Executive Order 11990. These drainages are classified as other waters of the United States and would be subject to Sections 401 and 404 of the Clean Water Act.

Palustrine Emergent

Palustrine emergent habitat includes alpine, subalpine, and montane meadows and seeps. Soils are generally deep and peaty, remaining saturated year-round or on a seasonal basis. Vegetation is dominated by grasses, sedges, rushes, and perennial herbs. The meadows in Yosemite National Park play a particularly critical role in the Merced River ecosystem. High spring flows create wet areas in side channels, low-lying wetlands, meadows, and cutoff channels. These areas support the concentration of organic matter, nutrients, microorganisms, and aquatic invertebrates throughout the relatively dry summer. When the flush of winter or spring flooding occurs, this stored aquatic biomass is washed into the main river channel, forming the base of the aquatic food chain. Examples of this wetland type include Wawona Meadow, El Capitan Meadow, and meadows adjacent to Washburn and Merced Lakes. These communities are typically considered wetlands under the Cowardin system and typically meet the U.S. Army Corps of Engineers' wetland criteria. Thus, palustrine emergent wetlands are subject to the National Park Service protection policies under Executive Order 11990 and Section 404 of the Clean Water Act.

Palustrine Forested

Palustrine forested wetlands are the riparian forest habitats along the main stem of the Merced River and South Fork that are regularly inundated by normal high-water or flood flows. Palustrine forests within the upper reaches of the main stem of the Merced River and South Fork consist mainly of evergreen pines and firs, with occasional aspens. In Yosemite Valley, where the river is broad, shallow, and slow-moving, deciduous cottonwoods, willows, and alders dominate the riparian corridor. Substrate under the palustrine forest community varies from rock, gravel, sand, clays, loams, and mud. Palustrine forests (riparian forests) are classified as wetlands based on the National Park Service guidelines (USFWS 1995). These areas are classified as either wetland or other waters of the United States by the U.S. Army Corps of Engineers, depending on site-specific vegetation, soils, and hydrologic conditions, and would be subject to Section 401 and/or 404 of the Clean Water Act.

Palustrine Scrub Shrub

Along the Merced River, palustrine scrub shrub is only found in the riparian corridor. This habitat type occurs sporadically along the banks of the main stem of the Merced River, the South Fork, and at lake margins. It is regularly inundated by normal high-water or flood flows. This habitat is dominated by various willows and often intergrades with meadow (palustrine emergent) and riparian (palustrine forest) communities. These communities are typically considered wetlands under the Cowardin system, would be subject to the National Park Service protection policies under Executive Order 11990, and typically meet the U.S. Army Corps of Engineers' wetland criteria. These areas may meet the Corps' criteria of a wetland or other waters of the United States, depending on site-specific vegetation, soils, and hydrologic conditions, and may be subject to Sections 401 and/or 404 of the Clean Water Act.

Lacustrine Limnetic

Lacustrine limnetic refers to naturally occurring deepwater lakes, such as Merced and Washburn Lakes. Both lakes were formed along the Merced River by glacial activity. In-lake vegetation is typically limited to rooted aquatic grasses, floating vascular plants, and algae. Meadow (palustrine emergent) and riparian (palustrine forest and palustrine scrub shrub) communities generally border lake margins.

These lakes provide important habitat for fish, amphibians, reptiles, and other aquatic species. Substrate varies from rock, gravel, sand, and mud. Lacustrine limnetic (deepwater lakes and ponds) are classified as deepwater habitat based on the Cowardin system (USFWS 1995). These areas are typically classified other waters of the United States by the U.S. Army Corps of Engineers and would be subject to regulation under Section 404 of the Clean Water Act.

Merced River Wetland and Aquatic Habitats

As the Merced River leaves its headwaters, it alternates between areas with very steep gradients, high velocities, and no floodplains (such as the reach between Nevada Fall and Happy Isles) and areas with low gradients, slow velocities, and wide floodplains (such as Yosemite Valley). These river reaches function in very different ways with regard to nutrient cycling, though they are part of the same river. Plant and animal life in the steeper river sections depend on nutrients and organic materials that are carried within the main river channel. Plant and animal life in low-gradient reaches consume nutrients and organic materials that come laterally from adjacent floodplains during annual flood events. Thus, the lateral connection between the floodplain and the river, and the downstream connection within the river corridor, are essential to maintaining the natural system balance for the aquatic, riparian, and meadow communities.

Wilderness Segment of the Main Stem Wetland and Aquatic Habitats

The upper Merced River watershed is characterized by steep canyons, broad interstream areas of glacially smoothed granite, lakes and meadows, and thin, granitic soils. The upper river segments have a narrow riparian band commonly dominated by pines, firs, and aspens. The riparian zone is controlled by stream gradient, slope, sedimentation, and aspect. High-elevation tributaries to the Merced River (e.g., the Merced Peak Fork, the Triple Peak Fork) are sparsely vegetated by scattered patches of alpine riparian scrub and alpine willow thickets. As the river descends and the gradient becomes more gentle, lodgepole pines, aspens, willows, and alders become more prevalent. Willows often colonize where sandbars collect at the margins of or within the river channel (e.g., at Merced Lake High Sierra Camp). Riparian species often intergrade with upland coniferous forest at or near the river's upper banks.

Merced and Washburn Lakes were formed where the Merced River canyon was carved by glaciers. In-lake vegetation is typically limited to rooted aquatic grasses, floating vascular plants, and algae. Meadow communities border lake margins, providing important wildlife habitat.

Although human intrusion into the wilderness reaches of the Merced River has been ongoing for thousands of years, the upper reaches of the Merced River and its associated wetland communities remain intact and relatively free from disturbance. Riparian communities of the upper Merced River zone are generally intact, except in a few locations where human use is intense (for example, in the vicinity of the Little Yosemite Valley Backpackers Campground, Moraine Dome Backpackers Campground, and Merced Lake High Sierra Camp and Backpackers Campground). Riparian vegetation at these locations has been degraded by trampling and erosion, resulting in loss of natural structure, diversity, and productivity (USFS 1993). These impacted areas are but a fraction of the wetland and aquatic habitats in the Wilderness segments. Riparian areas to the north of the Merced River within Little Yosemite Valley experience relatively heavy use (along major trail routes and campsites) and are low in species diversity. Forests south of the river receive almost no use and are more rich and pristine in nature.

Yosemite Valley Wetland and Aquatic Habitats

Wetlands in Yosemite Valley are formed in low-gradient land adjacent to the Merced River, its tributaries, or other bodies of water that are, at least periodically, influenced by flooding or high water tables. These wetlands would be broadly identified as riverine upper perennial (e.g., Merced River), palustrine (e.g., riparian, tributaries, shallow ponds, meadows, marshes), and undesignated (USFWS 1995).

Within Yosemite Valley, the Merced River supports riparian, aquatic, and meadow communities. Riparian zones in Yosemite Valley are characterized by broadleaf deciduous trees such as white alder, black cottonwood, big-leaf maple, white fir, mountain dogwood, and willow species. Vegetation along moving water is regularly disturbed by the deposition and removal of soil and the force of floodwaters. Vegetation in this zone readily colonizes on newly formed river-edge deposits. Big-leaf maple riparian forests grow on moist, gravelly soils in protected spots at the base of cliffs and on alluvial soils bordering streams. Meadows, such as El Capitan Meadow, are characterized by grasses, sedges, rushes, and herbs.

Wetlands within Yosemite Valley have undergone systematic alteration since the middle of the 19th century as they were grazed, farmed, and used as recreational sites and corridors for travel. One of the earliest impacts to wetlands in Yosemite Valley occurred in 1879 with the blasting of El Capitan moraine in the west Valley. This action lowered the base hydrologic level and caused the Merced River to downcut several feet (Milestone 1978g; NPS 1992c). Vegetation in adjacent wetlands was probably altered, and wetland function would have been further compromised by actions designed to dewater these areas. Impacts to wet meadows would have been most severe immediately upgradient of the blast (El Capitan Meadow) and dissipated in the vicinity of Yosemite Lodge. The blasting of the moraine would have had minimal impact on Sentinel, Cook's, Stoneman, and Ahwahnee Meadows.

Other alterations that took place in the early 20th century include drainage ditches that were constructed to dewater wet meadows to reduce mosquito breeding areas and provide open land for grazing and agriculture. Many of these drainage ditches have not been filled in and still dewater wet meadows in Yosemite Valley. Road construction has involved drainage measures and diversion of surface water adjacent to many of the Valley's wetlands.

Although changes are qualitatively evident to wetlands in many parts of Yosemite, quantitative evidence to support these observations is rare. Wetland impacts through time have been documented to a degree for one type of wetland, the meadows in Yosemite Valley. Approximately 800 acres[7] of Yosemite Valley meadows existed in 1866, as mapped by geologist J. D. Whitney (Hoffman 1866). Vegetation maps from 1994 classified approximately 370 acres in Yosemite Valley as meadow (NPS 1994e), roughly 50% of the 1866 meadow acreage. Because meadows were not burned for well over 60 years and water-flow patterns have been altered by development, dense stands of conifers cropped up in previously open meadows. The National Park Service is actively burning remaining meadows on a 5-year rotational cycle. Restoration of wetland communities along the Merced River within Yosemite Valley to mid-19th century conditions is ongoing through a variety of management programs, including prescribed burning, non-native plant eradication, and increasing inundation levels through restoration of natural drainage patterns.

Wetland restoration activities in Cook's Meadow, Stoneman Meadow, and Sentinel Meadow span several decades since the National Park Service initiated the first project in 1987. The National Park Service constructed raised boardwalks in all three meadows to allow surface water to flow across meadows. Construction of the boardwalks also set the stage to remove thousands of linear feet of social trails (informal foot trails) throughout the meadows. In 2002, restoration workers removed an elevated historic dirt road in Cook's Meadow that functioned as a dam and blocked natural water flows. Park restoration crews also filled in human-constructed drainage ditches and redirected altered water flows. Resource Management and Science staff have also coordinated extensive invasive plant eradication projects in Yosemite Valley meadows.

Merced River Gorge and El Portal Wetland and Aquatic Habitats

The Merced River gorge extends from Pohono Bridge through the El Portal Administrative Site. Within this area, the Merced River has a much steeper gradient compared to Yosemite Valley and consists mostly of seasonally continuous rapids through the El Portal Administrative Site. The riverbed and banks are largely composed of boulders and cobbles, ranging in size from a few inches to several feet in diameter.

The Merced River gorge is lined with a narrow band of riparian vegetation along the river course. These communities include blue oak woodland, interior live oak woodland, foothill pine/oak woodland, interior live oak/chaparral, and riparian woodland. El Portal does not have the deep loam deposits that characterize Yosemite Valley. Flooding has been an important aspect of the development of riparian communities along the Merced River and its tributaries that intersect drier adjacent vegetation types of El Portal. Localized seasonal flooding creates debris flows in tributary channels, thus furthering a diversity of scour and depositional soils for riparian species. On the Merced River, natural flooding and vegetative patterns have been influenced by the construction of levees and application of riprap to confine the river.

Early to mid-20th century development in what is now the El Portal Administrative Site has affected some of the oxbows, river terraces, and seasonal river channels that were a part of the riparian wetlands of the area. Many of the sites that would be characterized as palustrine have been impacted to some degree. For example, Odger's Pond and the Abbieville wetland appear to be oxbows or backwater channels that were cut off from the main stem of the Merced River during construction of Highway 140 in the 1920s (ESA 2004a). These areas continue to maintain palustrine wetland characteristics and riparian vegetation even though they are no longer directly connected to the Merced River. The remaining wetland areas that appear on the USFWS (1995) wetland inventory are riverine perennial wetlands and are in proximity to the