CHAPTER THREE

AFFECTED ENVIRONMENT Part 2

Physical Environment

Geologic Overview

The geologic story of the Sierra Nevada can be considered in two parts: (1) the deposition and formation of sedimentary and volcanic rock over a period of hundreds of millions of years and the intrusion of granitic rocks, and (2) the uplift, erosion, and glaciation of the environment to form today’s landscape (Huber 1989).

At its foundation, the Sierra Nevada is an enormous deposit of granitic rock (U.C. Davis 1996a).  About 200 million years ago, as the granitic rocks were formed, heated, and melted, they slowly migrated toward the surface of the earth. The surface of the earth at the time was composed of massive layers of sedimentary rock deposited by ancient seas and volcanic rock that was deposited by ancient volcanic eruptions. As the granitic plutons rose, they altered some of the sedimentary and volcanic rock and created metamorphic rock.

Between 65 and 100 million years ago, magma formation slowed and a long period of erosion began in the Sierra Nevada. About 25 to 15 million years ago, mountain building activity reactivated, uplifting the Sierra Nevada to form its relatively gentle western slopes and the more dramatic, steep eastern slopes. A combination of uplift and tilt is the underlying geologic process that created the range as we see it today (Huber 1989).

As the world grew colder between two and three million years ago, the Sierra Nevada had risen high enough for glaciers and mountain ice fields to form at the higher alpine elevations. At least three major glacial periods occurred during the ice age in the Sierra Nevada. The down slope movement of the ice masses cut and sculpted valleys, cirques, and other glacially formed landforms throughout the Yosemite region and the Sierra Nevada. The last glaciation event began as late as 60,000 years ago. In the Yosemite area, this glaciation pushed fingers of ice into the major drainages on the west slopes, until it reached the maximum extent about 20,000 years ago, near Bridalveil Meadow in Yosemite Valley.

Climate

The climate of Yosemite is Mediterranean. Precipitation amounts vary from 36 inches (915 mm) at 4000 feet (1200 m) to 50 inches (1200 mm) at 8600 feet (2600 m). Most of the precipitation falls as snow between October and April. From May through September, precipitation is infrequent.

Mean daily temperatures range from 25 to 53 degrees Fahrenheit at Tuolumne Meadows at 8600 feet (2600 m). At South Entrance Station (elevation 6192 feet) mean daily temperature ranges from 36 to 67 degrees Fahrenheit. At the lower elevations, below 5000 feet, temperatures are hotter; mean daily high temperature at Yosemite Valley (elevation 3966 feet) varies from 46 to 90 degrees Fahrenheit. Frequent summer thunderstorms, along with snow that can persist into July, moderate the hot, dry summers, especially above 8000 feet. The combination of dry vegetation, low relative humidity, and thunderstorms results in frequent lightning caused fires as well (NPS 1990).

Soils

The physical, chemical (nutrient), and biotic properties of soil are important in determining function, productivity, and other characteristics of terrestrial ecosystems (DeBano et al. 1998). Soils form from the effect of climate, biologic activity, topographic position and relief, and time acting on geologic parent material. Within the park, topography is the most important factor contributing to soil differentiation. More than 50 soil types exist within the park; general or local variations depend on glacial history and the ongoing influences of weathering and stream erosion and deposition. Topography influences surface water runoff, groundwater, distribution of stony soils, and the separation of alluvial soils (Zinke and Alexander 1963). Local variations also result from differences in microclimates due to aspect and major vegetation types.

Soils of the Yosemite region are primarily derived from underlying granitic bedrock and are of a similar chemical and mineralogical composition. Except for meadow soils, most soils at high elevations were developed from 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, pebbles, cobbles, and boulders of various sizes. Alluvial soils, along streams, tend to have sorted horizons (layers) of sandy material. Colluvial soils along the edges of the Valley in areas where landslides and rockslides have occurred are composed of variously sized particles and rocks and have high rates of infiltration and permeability.

Organic content within the upper soil profile varies with the local influences of moisture and drainage. Thick sedges and grasses have contributed to the organic content of soils near ponds, lakes, and streams. Coniferous forest soils have a relatively 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-like conditions.

Certain soil types have been identified in Yosemite as highly valued resources (Yosemite Valley Plan 2000). Highly valued resource soils are found in or adjacent to meadows and riparian areas, hydric soils, and soils associated with lateral or terminal moraines. The Leidig fine sandy loam found in and around Leidig Meadow is an example of a highly valued resource soil.

Hydric soils are legally protected because they form in wetlands, which are protected by federal law. Hydric soils are found primarily in the river valleys of the Merced River and Tenaya Creek and in low meadows.

Interaction of Fire and Soil

All fire, whether natural or human-caused, changes the cycling of nutrients and the biotic and physical characteristics of soils. The magnitude and longevity of these effects depend on many factors including fire regime, severity of a particular fire, vegetation and soil type, topography, season of burning, and pre- and post-fire weather conditions. Effects can also be indirect, through changes in soil biota and changes in erosional rates. Sites that historically had frequent fires are generally better adapted to the reintroduction of fire and repeated burning.

Fire causes soil nutrients to change in form, composition, distribution, and amount. These changes are from the release of elements during combustion of fuel and organic matter. The volatilization, or release, is temperature dependant. Nitrogen, and to a lesser extent sulfur and phosphorus, are most readily lost. Other nutrients are generally lost as ash via convection or through leaching. Burning can decrease total nitrogen availability at a site while increasing nitrogen available for plant growth. Following prescribed burns in Giant Forest in Sequoia National Park, inorganic soil ammonium-nitrogen (NH -N) levels increased from 1.90 mg/k of soil under sequoias and 1.66 mg/k of soil under sugar pines to 68.63 mg/k and 62.71 mg/k respectively immediately after the fire (Haase and Sackett 1998). By five years, NH -N had returned to pre-burn levels (1.54 and 1.60 mg/k soil respectively) and by seven years had dropped below pre-burn levels (1.12 and 1.52 mg/k soil respectively). Changes in nitrate-nitrogen (NH -N) were similar except peaks occurred two years after the burn. Other nutrients (CA, Mg, K, and SO₄) also increased with SO₄ increasing by an order of magnitude (Chorover and others 1994; Williams and Melack 1997).

Biotic soil communities are complex and still poorly understood, particularly in relation to fire effects. Fire can influence soil biota directly by killing or injuring organisms, or indirectly by altering properties of the above- and below-ground soil environment. Burning generally results in declines in soil invertebrates and fungi while microorganisms such as bacteria increase in abundance. Changes in aboveground biotic communities due to changes in the fire regime may also impact soils and interact with soil nutrient status. For example, nitrogen-fixing plants are suppressed in some fire-excluded forests (Newland and DeLuca 2000). Additionally, the effects of fire on cryptogramic crusts, (important nitrogen fixers in some ecosystems) has not been explored.

Changes in physical characteristics of soil following fire are a result of complex interactions among geomorphic processes, climate, vegetation, and landforms. Fire can affect changes in organic horizons, water repellency, infiltration capacity, porosity, structure, temperature, hydrologic properties, and, most importantly, erosional processes and sedimentation rates. Fire generally increases the potential for erosion by removing vegetation and exposing mineral soil and by altering organic matter and the physical properties of soil. Generally, the more severe a fire, the greater its effects will be. These effects are further affected by soil erodibility, slope steepness, and the timing, intensity, and amount of precipitation. The magnitude of fire’s impact on soils is highly dependent on the situation and the physical and biotic properties of the area. Recent studies show that the deliberate use of prescribed fire may dramatically reduce erosion potential from wildland fires. In one study, erosion and sediment from a high intensity wildfire event was ten times higher than that measured off a low intensity prescribed burn (Wohlegmuth et al. 1999).

In most park ecosystems prior to Euro-American settlement, fire affected both the soils and geomorphic processes. The alteration of the natural fire regime by nearly a century of human intervention can be considered a significant alteration of and stressor to soil properties and processes. Understanding changes from fire suppression and restoration of fire is important. For example, there is the potential for increased erosion in areas of chaparral vegetation due to the complete removal of most aboveground biomass by fire. This differs from Sierran mixed-conifer forest where overstory vegetation is generally maintained after fire.

Because of the landscape scale of some effects of fire, they could have significant impacts both inside and outside the park. Impacts and processes within the park may be considered ecologically natural, while the same process may produce effects outside the park that are considered undesirable. For example, it would be important to understand whether there are significant erosional and sedimentation risks associated with certain types of fire because of downstream structures, such as dams, flumes, and hydroelectric generation plants, on the Tuolumne and Merced Rivers.

Water Resources and Watersheds

Within the boundaries of Yosemite flow the headwaters and significant stream reaches of the Tuolumne and Merced Rivers, both of which are tributaries of the San Joaquin River basin. The park also contains approximately 3,200 lakes (greater than 100 square meters), two reservoirs, and 1,700 miles of streams, all of which help form these two large watersheds.

The Tuolumne and Merced River watersheds originate along the ragged crest of the Sierra Nevada. Waters tumble down rocky, sparsely vegetated mountainsides; course through forests underlain with granitic bedrock and strewn with boulders; and flow through nearly flat, glacially-carved valleys on their paths to the Central Valley. Areas of small lakes and meadows, typically underlain with thin, granitic soils, can be quite extensive despite the rugged landscape. Above 9,600 feet, alpine and subalpine zones have little vegetation and low soil permeability. From 8,000 to 9,600 feet, the upper montane zone has limited ability to hold soil moisture. Lower montane forests grow on thin to moderate depth soils from 4,000 to 7,000 feet.

The Tuolumne River drains the entire northern portion of the park, an area of approximately 428,115 acres (669 square miles). It flows into Hetch Hetchy reservoir, a major water supply for the City and County of San Francisco, before it leaves the park. The main stem and the South Fork of the Merced River drain the southern portion of the park, approximately 319,840 acres (499 square miles). Below Yosemite Valley, the main stem flows through the El Portal Administrative Site.

Regional Watershed Characteristics

Merced River (Main Stem) Watershed.  The main stem of the Merced River watershed drains 250,000 acres (391 square miles) of the park. Principal tributaries of the Merced River include the Merced Peak, Lyell, Triple Peak, and Red Peak Forks, as well as Echo, Sunrise, Illilouette, Tenaya, Yosemite, Bridalveil, Cascade, Grouse, Avalanche, Indian, and Crane Creeks. For the purpose of this discussion, the main stem of the Merced River is divided into three hydrologic segments: the upper Merced River, Yosemite Valley, and the Merced River gorge (which includes the El Portal Administrative Site). This division is based upon the unique watershed characteristics of the three river areas.

Upper Merced River.   The upper Merced River watershed encompasses approximately 114,840 acres (181.9 square miles) above Happy Isles in upper Yosemite Valley. Elevations range from 4,000 feet to over 13,000 feet at Mt. Lyell. Located within the watershed are the sub-basins of Merced Peak, Lyell, Triple Peak, and Red Peak Forks; Echo, Sunrise, and Illilouette Creeks; and over 1,000 lakes and ponds (Williamson et al. 1996a). The upper Merced River descends from its headwaters through a glacially carved canyon at a gradient of about 8,000 feet over 24 miles (USGS 1992). The average daily discharge rate measured at the Happy Isles gauging station is approximately 355 cfs (USGS 1998).

Yosemite Valley.  The Yosemite Valley watershed includes Yosemite Valley and its tributary areas. Tributaries include Tenaya, Yosemite, Sentinel, Ribbon, and Bridalveil Creeks. Above Pohono Bridge, the Merced River basin encompasses 205,000 acres (321 square miles) (USGS 1999). Historic discharge in the river, measured at the Pohono Bridge gauging station, has ranged from a high of about 25,000 cfs to a low of less than 10 cfs. During the last glaciation, a glacier extended to below Bridalveil Fall—leaving the nearly flat valley floor through which the river flows in a shallow channel approximately 100 to 300 feet wide in most places. The bed and banks of the channel are composed of smaller sediments and cobbles, material created and deposited by the succession of glaciers that helped form the Valley. The river alters its course periodically by eroding and re-depositing this loose material.

Merced River Gorge.  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 Merced River gorge watershed includes the area from Pohono Bridge through the El Portal Administrative Site. At the western end of Yosemite Valley, where the river transitions into the steep river gorge, Cascades Diversion Dam collects suspended sediments and bedload discharging from the Valley. Tributaries along the gorge include Cascade, Tamarack, Wildcat, Grouse, Avalanche, Indian, Crane, and Moss Creeks. The riverbed and banks are largely composed of boulders and cobbles, ranging in size from a few inches to several yards in diameter. Much of the riverbank has been developed and hardened for road and facility protection. Because of the steep gradient and development, the river channel usually only shifts during periods of large floods. There are no flow gauges in the gorge.

South Fork Watershed.  The headwaters of the South Fork originate near Triple Divide Peak at an elevation of approximately 10,500 feet. The South Fork flows westward over granitic bedrock to Wawona and then flows northwest over an area underlain by sedimentary rocks at a 3,500-foot elevation (USGS 1995a) and into the Merced River downstream from El Portal. Chilnualna, Big, Alder, and Bishop Creeks are major tributaries to the South Fork. The watershed area of the South Fork at Wawona is approximately 63,000 acres (98 square miles) and about 154,000 acres (approximately 70,000 acres within the park) by the time it reaches the main stem. Upstream from Wawona, tributaries enter the steep-walled 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), building substantial gravel bars within the channel. The average annual flow at its confluence with the Merced River is 356 cfs (USGS 1989). Between 1958 and 1968, upstream of the Big Creek confluence, the average annual flow was 174 cfs.

Tuolumne River Watershed.  The Tuolumne River originates in the peaks above Tuolumne Meadows and is the major drainage system for the northern part of Yosemite. The river and its tributaries drain in excess of 669 square miles of the park. The Tuolumne has two principal sources: the Dana Fork, which drains the west-facing slopes of the 13,053-feet-high Mount Dana, and the Lyell Fork, which begins at the base of the glacier on Mount Lyell at an elevation of 13,114 feet. Confluence of the two forks occurs at the eastern end of Tuolumne Meadows. The Tuolumne River continues through Tuolumne Meadows and the associated park developments at an elevation of 8,600 feet. It then cascades on its westward decent through the Grand Canyon of the Tuolumne, and enters the eastern end of Hetch Hetchy Reservoir, still within the park, at an elevation of about 4,000 feet. Return, Paiute, Rancheria, and Falls Creeks enter the Tuolumne River upstream of the reservoir and along the reservoir’s shores. At O’Shaughnessy Dam, which impounds the Tuolumne, water is diverted through Canyon Tunnel to the Kirkwood Powerhouse. Water that is not diverted continues downstream in the Tuolumne River channel, reaching the park boundary about six miles downstream, near the Mather Ranger Station.

      Hetch Hetchy and Lake Eleanor Reservoirs.  These two reservoirs are in Yosemite, within the Tuolumne watershed and are part of a massive system of water and power production operated by the City and County of San Francisco. Hetch Hetchy is on the main stem of the Tuolumne River and Lake Eleanor is on Eleanor Creek, upstream of its confluence with Cherry Creek. Cherry Creek joins the Tuolumne River downstream of the park’s western boundary. Hetch Hetchy is dammed by the 430-foot-tall O’Shaughnessy Dam and has a storage capacity of 360,360 acre-feet. It is the primary water source for about 2.5 million residents of the San Francisco Bay area. Lake Eleanor’s maximum volume of 27,100 acre-feet was created by building the 70-foot-tall Lake Eleanor Dam in 1918.

      Middle Tuolumne River.  The Middle Tuolumne River drains a small portion of the park’s extreme western edge, south of Hetch Hetchy Reservoir and northwest of the Tioga Road. The headwaters are between 7,000 and 8,000 feet in elevation. Cottonwood Creek is a major tributary. The Middle Tuolumne River exits the park at an elevation of 5,000 feet and joins the South Fork Tuolumne River downstream of the park.

      South Fork Tuolumne River.  The South Fork Tuolumne River drains a small portion of the western edge of the park. The headwaters begin between White Wolf and Yosemite Valley at elevations between 8,000 and 8,500 feet. The South Fork Tuolumne River exits the park at an elevation of 4,500 feet, just north of Hodgdon Meadow and upstream of its confluence with the main Tuolumne River.

Influence of Fire on Watersheds

Through changes in soil and vegetative characteristics, fire influences the rate at which water flows and the volume of water in watersheds. Fire can be destructive to watershed processes, but when natural processes are allowed to occur, fire helps maintain watersheds. Fire affects several major attributes of watersheds, including water yield, peak flows, sediment yield, nutrient yield, and stream system response.

The proportion of a watershed that is burned and the proximity of the burned area to a stream channel largely determine the effects of fire on streams. A stream draining a watershed of which over 90% of the land has burned will show much greater effects than a stream emanating from a similar watershed in which only the upper slopes and ridge tops were burned. Fire intensity is often highly variable over a landscape, and patches of unburned or lightly burned vegetation (especially near streams) can reduce the adverse effects of intensely burned, upslope areas (Kattelmann 1996).

Although fire is a natural part of many ecosystems, high-intensity fire can produce some of the most extensive changes in watershed conditions of any disturbance. Intense fire kills vegetation, volatizes organic matter in the litter layer, and often forms a layer in the soil that reduces infiltration of water into deeper soil layers. The combined effect of these changes increases water yield and overland flow, possibly increasing peak flows months, or years, later. High-intensity fire may also create the conditions for shallow debris flows. Under the conditions of bare soil, increased overland flow, and lack of vegetation and litter, soil particles are transported into streams, increasing sediment loads.

Water Yield.  Although the National Park Service does not manage Yosemite National Park to maximize water yield, it is a major indicator of the relative influence of fire in a watershed. Because of Hetch Hetchy Reservoir, water yield is of interest to the City and County of San Francisco.

Fire affects water yield primarily by killing vegetation and reducing the amount of water intercepted by plants, however, it also affects snow accumulation and melt rates. Plant transpiration is virtually stopped wherever a high-intensity fire has burned (Kattleman 1996). The daily cycle of plant water uptake affects hourly stream flow and this daily cycle can be changed completely by catastrophic fire. Seasonal water yield may also be affected by fire. Snow accumulation and melt rates may change after a fire. For example, melt rates would increase if more light reached the forest floor, while snow accumulation rates could either increase (small openings), or decrease (large openings). These changes may increase annual runoff in the first years after a fire.

Peak Flows.  Peak flows can be expected to increase after large (relative to the watershed) fires because of increases in soil moisture caused by reduced plant transpiration, decreased soil infiltration, and higher rates of snowmelt (Kattelmann 1996).

Infiltration is usually the most important factor affecting peak flows. It is decreased in two ways. Removal of vegetation and the litter layer exposes bare mineral soil to raindrops, which can physically force the solid particles closer together and disperse soil aggregates into surface pores, thereby reducing the infiltration capacity. Secondly, fires can vaporize organic compounds in the litter layer, some of which move into the soil until the vapor condenses and forms a layer that is water repellent, or hydrophobic. These hydrophobic layers tend to be more coherent under very hot fires, where a thick litter layer and/or organic horizon is present, and in course textured soils, such as the decomposed granitics found in Yosemite. The continuity of these layers determines their overall impact on hill-slope water movement. Although the water repellent layers tend to break down in a year or two, those formed in soils that are hydrophobic even without fire may be more persistent. Under some conditions, a hydrophobic layer forms on the surface of the soil and acts as a binder and sealant, maximizing overland flow while minimizing erosion. Studies in the western United States have shown dramatic increases in peak flows following wildland fires.

Sediment Yield.  Sediment yields increase markedly after some fires, particularly if riparian vegetation was burned (Kattelmann 1996). This increase in sediments happens through several processes. Erosion from the land surface usually increases after a fire, especially if overland flow increases—sediments may then wash into streams. In the absence of streamside vegetation, banks become less stable and soil particles move into the channels from dry ravel erosion (the particle-by-particle transport of material down slope due to gravity). Increases in total discharge and peak flows cause channel erosion as well. Debris torrents may scour streams if extreme climatic events follow the fire. If a fire is particularly hot, woody debris that helped stabilize the channel may be consumed, increasing water velocity and stream-bank erosion.

Nutrient Yield.  During a fire, some materials are volatilized into the atmosphere, while the remainder is left as ash on and near the soil surface in forms that are readily mobile. Thus, fires provide an opportunity for nutrients that have been stored in vegetation and soils to move into streams (Kattelmann 1996). Concentrations of nitrates and other ions in streams usually increase dramatically after a fire, although the absolute amounts often remain almost negligible or at least within water quality standards. After some fires, potential is high for large nutrient losses from soil erosion carrying nutrients into streams.

Stream System Response.  Both physical and biological features of streams change over time. In a fire maintained system, after a fire, initially the channel may agrade and widen in response to higher flows of water and sediment. As vegetation becomes re-established, the channel usually returns to pre-fire size within several years. In the Sierra Nevada, vegetation community similarity, density, and taxa richness will be comparable between burned and unburned reaches in one to three years after a fire (Kattelmann 1996).

Water Quality

An inventory of water quality in Yosemite revealed excellent water quality in most of the park, although some water quality degradation is occurring in areas of high visitor use (NPS 1994). Water quality is generally above state and federal standards. The surface water quality of most park waters is considered valuable by the State of California for wildlife and freshwater habitat and recreation [Central Valley Regional Water Quality Control Board’s Water Quality Control Plan (Basin Plan)].

Surface water that drains 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 diluted (lacking in dissolved solids), making the ecosystem sensitive to human disturbances and pollution (Clow et al. 1996).

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

Merced River Watershed.  The chemistry of surface waters in the Merced River watershed 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 1994). 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 1994). 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.

Water quality within the South Fork watershed is very similar to that of the main stem of the Merced River. Water quality is excellent in most areas although some water quality stressors have been exhibited near human development.

Tuolumne River Watershed.  Water quality of the Tuolumne River watershed is similar to that of the Merced River watershed, and generally appears to be of high quality (NPS 1994). The quality of the Tuolumne River water above Hetch Hetchy Reservoir can be attributed to the river’s free flow, its location high in the watershed, its confluence of a low order of streams, and its position in an area of minimal development. Because of the reservoir’s use as a water supply, the park has taken a preventive approach to watershed health and the maintenance of high water quality.

Fire and Water Quality

High-intensity Wildland Fire.  The riparian systems in Yosemite are resilient and typically return to their previous condition after low-intensity fire events. High-intensity wildland fire, on the other hand, can reduce or remove protective riparian vegetation that regulates stream temperature; traps and transforms nutrients, chemicals, and sediment; and moderates the flow of organic materials (stems, leaves, insects, microorganisms, etc.). Catastrophic fire also increases the amount of flowing water on exposed bare soils, causing erosive overland flow (sheetflow), rills, or gullies, and substantially increasing the sediment load into streams. This accelerated loss of soil adversely affects terrestrial and aquatic ecosystems—it depletes the land of nutrients and overloads streams with sediments. 

Prescribed Fire.  Because prescribed fires burn under controlled fuel moisture and weather conditions, time of day, and spatial patterns of ignition, the impacts to soils and vegetation are considerably less than with high-severity wildland fires. Prescribed fires generally retain a portion of the duff layer that helps to prevent soil erosion. In contrast to the impacts from high-severity wildland fire, infiltration rates are not greatly reduced, therefore, prescribed fire treatments tend not to exacerbate overland flow. Without overland flow, movements of soil into stream channels is limited to soil creep and ravel on steep slopes, at rates only slightly higher than areas not receiving prescribed fire.

Mechanical Treatments.  The potential effects of mechanical treatments on water quality decreases as the distance from streams increases. Research in Yosemite shows that, within 300 feet of a stream, activities that compacted more than 5% of the area significantly reduced the population of sediment intolerant aquatic invertebrates (McGurk and Fong 1995). Activities that prescribe stream buffers or limits to ground disturbance can control amounts of sediment reaching the aquatic ecosystem. The risk of accelerated erosion or alteration of soil conditions from mechanical fuel reduction treatments varies depending on factors such as total acres treated, method of treatment, type of equipment used, amount and type of materials yarded or piled, soil type, soil moisture conditions, degree of slope, and history of past disturbance. The primary potential source area for sediment would be ephemeral channels and skid roads, and their immediate vicinity.

Air Quality

Yosemite National Park is classified as a mandatory Class I area under the Federal Clean Air Act (42 USC 7401 et seq.). This most stringent air quality classification is aimed at protecting national parks and wilderness areas from air quality degradation. The Act gives federal land managers the responsibility for protecting from adverse air pollution impacts on air quality and related values, including visibility, plants, animals, soils, water quality, cultural and historic structures and objects, and visitor health.

Yosemite National Park lies within three California counties: Tuolumne and Mariposa which are within the Mountain Counties Air Basin, and Madera which is within the San Joaquin Valley Air Basin—part of the San Joaquin Valley Unified Air Pollution Control District. Yosemite Valley is in Mariposa County, which is regulated by the Mariposa County Air Pollution Control District.

National Ambient Air Quality Standards.  The federal Clean Air Act, as amended in 1990, requires the Environmental Protection Agency (EPA) to identify national ambient air quality standards to protect public health and welfare. Standards have been set for six pollutants: ozone (O₃), carbon monoxide (CO), nitrogen dioxide (NO₂), sulfur dioxide (SO₂), particulate matter less than 10 microns (PM₁₀), and lead (Pb). In 1997, the EPA released revised national ambient air quality standards for ozone and for particulate matter less than 2.5 microns (PM_2.5). In the spring of 1999, a U.S. Court of Appeals panel remanded the new standards to the EPA for further consideration. However, in early 2001, the Supreme Court upheld the EPA’s authority to set these new, more stringent standards.

While the EPA’s authority to set the new eight-hour ozone standard was upheld, the Supreme Court ordered it to rework its policy for implementing the new ozone standard in non-attainment areas. Although the Court of Appeals prohibited the EPA from implementing the eight-hour ozone standard, it did note that the Clean Air Act required the EPA to finalize area designations within specific timeframes. The California Air Resources Board updated the proposed area recommendations with the most current air quality monitoring data and transmitted California's recommendations to the EPA in July 2000. These recommendations include non-attainment designations for the federal eight-hour standard for the Mountain Counties and San Joaquin Air Basins.

The pollutants are called criteria pollutants because the standards satisfy criteria specified in the Act. An area where a standard is exceeded more than three times in three years can be considered a non-attainment area subject to planning and pollution control requirements, which are more stringent than in areas that meet standards. Table 3.7 presents the federal and California ambient air quality standards. Table 3.8 shows the California and federal air quality standards attainment designation for the counties containing portions of Yosemite National Park. 

While air quality in an air basin is usually determined by emission sources within the basin, pollutants transported from upwind air basins by prevailing winds can also affect it. For example, the California Environmental Protection Agency concluded that the ozone exceedences in 1995 in the southern portion of the Mountain Counties Air Basin (i.e. Tuolumne and Mariposa Counties) were caused by transport of ozone and ozone precursors from the San Joaquin Air Basin. Air quality in the Mountain Counties Air Basin also is affected by pollutant transport from the metropolitan Sacramento and San Francisco areas.

Table 3.7   Federal and California Ambient Air Quality Standards

California Ambient Air Quality Standards.  To protect public health and welfare, the California Air Resources Board has set stricter ambient air quality standards than national standards. Under the 1988 California Clean Air Act, air basins were designated as attainment, non-attainment, or unclassified for the state standards.

State Implementation Plan.  State implementation plans define control measures that are designed to bring areas into attainment. Currently, Mariposa and Tuolumne Counties are in attainment or are unclassified for all national ambient air quality standards, but Madera County is in non-attainment for the PM₁₀ and ozone national ambient air quality standards. Basic components of a state implementation plan include legal authority, an emissions inventory, an air quality monitoring network, control strategy demonstration modeling, emission limiting regulations, new source review provisions, enforcement and surveillance strategies, and other programs necessary to attain standards.

Table 3.8   Status of Ambient Air Quality Designations

                 A = Attainment   N = Non-attainment   U = Unclassified