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Yosemite National Park experiences a Mediterranean climate with typically long, hot summers and mild winters. Precipitation amounts vary from 36 inches (915 mm) at 4,000 feet (1200 m) elevation to 50 inches (1200 mm) at 8,600 feet (2600 m).
Yosemite has a variety of surface water features, some of which are a major attraction for park visitors. Some of the tallest waterfalls in the world are found in Yosemite Valley, including Yosemite Falls (with a total drop of 2,425 feet) and Ribbon Fall (1,612 feet). The Tuolumne and Merced River systems originate along the crest of the Sierra Nevada in the park and have carved river canyons 3,000 to 4,000 feet deep. The Tuolumne River drains the entire northern portion of the park, an area of approximately 680 square miles. The Merced River begins in the park’s southern peaks, primarily the Cathedral and Clark Ranges, and drains an area of approximately 511 square miles. Hydrologic processes, including glaciation, flooding, and fluvial geomorphic response, have been fundamental in creating landforms in the park.
Water quality throughout Yosemite National Park is considered to be good and is generally above state and federal standards. An inventory of water quality performed by the National Park Service indicated pristine conditions in many parts of the park, with some water quality degradation in areas of high visitor use. The surface water quality of most park waters is considered by the State of California to be beneficial for wildlife habitat, freshwater habitat, and for canoeing, rafting, and other recreation, as indicated in the 1998 Central Valley Regional Water Quality Control Board’s Water Quality Control Plan.
Yosemite National Park has had 11 winter floods since 1916 that have caused substantial damage to property. All of these floods took place between November 1 and January 30. The largest floods 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 the Pohono Bridge gauging station in Yosemite Valley.
Wetlands are areas that are saturated with surface water or near-surface groundwater for much of the year. They usually have no clearly defined ecological boundaries, but are a gradient between deepwater habitats, such as lakes and rivers, and upland terrestrial ecosystems, such as conifer forests.
Yosemite National Park experiences a Mediterranean climate with typically long, hot summers and mild winters. Precipitation amounts vary from 36 inches (915 mm) at 4,000 feet (1200 m) elevation to 50 inches (1200 mm) at 8,600 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 8,600 feet (2,600 m). At South Entrance near Wawona (elevation 6192 feet), mean daily temperature ranges from 36 to 67 degrees Fahrenheit. At the lower elevations below 5,000 feet, temperatures are hotter; the mean daily high temperature at Yosemite Valley (elevation 3,966 feet) varies from 46 to 90 degrees Fahrenheit. At elevations above 8,000 feet, the hot, dry summer temperatures are moderated by frequent summer thunderstorms, along with snow that can persist into July. The combination of dry vegetation, low relative humidity, and thunderstorms results in frequent lightning-caused fires as well.
The Yosemite National Park has had 11 winter floods since 1916 that have caused substantial damage to property. All of these floods took place between November 1 and January 30. The largest floods 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 the Pohono Bridge gauging station in Yosemite Valley. These floods were caused by warm winter rains falling on snow at elevations up to 8,600 feet (e.g., Tuolumne Meadows), partially melting the accumulated snow pack. The 100-year floodplain is the area that is inundated by a 100-year flood, or the annual peak flow that has a 1% chance of being equaled or exceeded in any given year. Prediction of the 100-year floodplain is necessary in order to comply with Executive Order 11988 (Floodplain Management) and with the NPS Floodplain Management Guideline. In order to predict the 100-year floodplain, it is necessary to perform a flood frequency analysis of the nearest gauging station data to determine the flow rate of a 100-year flood. This flow rate, along with topographic cross sections, is used by models to predict the inundation (or floodplain), flow velocities, and inundation depths of a 100-year flood event. The accuracy of these predictions is higher for areas near gauging stations, for areas with gauging stations that have been operating for many years, and for areas with more precise topographic cross-section data.
Floodplain Characteristics of Yosemite Valley, El Portal and Wawona
The floodplain of the Merced River in Yosemite Valley is well developed in some sections, such as in meadow areas in Yosemite Valley. In other areas the floodplain is lacking due to narrowing of canyon/valley walls, such as the gorge, or incision of the channel into moraine deposits, such as west Yosemite Valley moraines. In Yosemite Valley, 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, flood waters back up against the dense vegetation and tend to be deep and slow (low velocity). From the El Capitan moraine downstream, the river channel is steeper and confined in the narrow river canyon, the floodplain is narrow, and flow velocities are high.
The broad, well-developed floodplain that occurs in Yosemite Valley between Housekeeping Camp and the El Capitan moraine serves many hydrologic functions, including dissipation of flood water energy as water spreads out over the flat, expansive plain. The meadows in Yosemite Valley occur primarily in the floodplain and are maintained and rejuvenated by periodic floodwaters. The roads across Stoneman, Ahwahnee, Cook’s, Sentinel, and El Capitan Meadows have varying degrees of influence on the function of the floodplain.
The river channel in El Portal is narrow and steep, though less steep than in the gorge segment immediately upstream, and flow velocities are very high. The river channel can shift laterally during large floods.
In Wawona, an elongated alluvial valley, the river meanders less than in Yosemite Valley, but the river channel can shift laterally during large floods. Development in Wawona has altered the floodplain. Surface water diversions affect the Wawona floodplain through reduction of the water table during dry periods such as drought and in the fall before the onset of winter rains. Water diversion is governed by the Wawona Water Conservation Plan, which includes provisions for reduction and/or cessation of withdrawals when stream flow drops to critical levels.
Frazil Ice Flooding
Waterfalls in the park occasionally produce a late winter and early spring phenomenon called frazil ice at the base of the fall. Small ice crystals develop in turbulent super-cooled stream water when the air temperature suddenly drops below freezing. These ice crystals join into slush and become pressed together as more crystals form. Frazil ice lacks the erosional force of regular stream ice, but it can cause streams to overflow their banks and change course. Frazil ice sometimes reaches a depth of more than 20 feet along Yosemite Creek at the Lower Yosemite Fall Bridge. A 1954 flow of frazil ice completely filled the streambed of the creek and covered the footbridge near Lower Yosemite Fall with many feet of ice. More recently, a frazil ice event covered the Yosemite Falls footbridge on February 27, 1996.
Non-Flood Alterations of the Floodplain
Although floods are significant to ecosystems because they can induce large changes in channel morphology and the floodplain landscape, low stream-flow characteristics are also important. Low stream flow during the summer can affect the surrounding floodplain as riparian and wetland communities undergo a drying phase. Diversion of river flows for human consumption can upset this normal balance and induce further reduction of riparian communities and destabilization of stream banks. Prior to 1985, potable water in Yosemite Valley was produced almost entirely from surface water diverted from the Merced River upstream of Happy Isles. It is estimated that up to 54% of the low stream-flow discharge may have been diverted for park facilities. This practice has been terminated in Yosemite Valley, and all potable water is now taken from groundwater wells; however, water continues to be drawn from the South Fork in Wawona to augment groundwater supplies.
Development in Floodplains
Executive Order 11988 (Floodplain Management) and the NPS Floodplain Management Guideline provide guidance for the protection of life and property in conjunction with natural floodplain values in the National Park System. This guidance applies to both existing facilities and proposed facilities, and requires the National Park Service to avoid locating facilities in floodplains if alternative locations are feasible. Where no alternative exists, and with a formal statement of findings, properly mitigated facilities can be located in floodplains. Each action (or facility) is assigned to one of three classes, depending on its use, and each class has a different regulatory floodplain. Actions of a given class can occur within the regulatory floodplain if properly mitigated. The regulatory floodplain for Class I actions, such as administrative facilities, residential areas, warehouses, and maintenance buildings, is the 100-year floodplain. The regulatory floodplain for Class II actions, such as medical facilities, emergency services, schools, irreplaceable records, museums, and fuel storage areas, is the 500-year floodplain.
Excepted actions are exempt from the NPS Floodplain Management Guideline if risks to human life and property are studied and then minimized or mitigated through design. Examples of excepted actions are bridges, flood control facilities, picnic areas, trails, roads, day-visitor parking facilities, and campgrounds. If a non-exempted action is proposed, a formal statement of findings is required. The statement of findings includes a description of the site-specific flood risk, describes why the action must be located in the floodplain, and describes how the action will be designed or modified to minimize harm to floodplain values or risk to life or property.
Hydrology 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.
Watersheds and Their Characteristics
Merced River Watershed – Main Stem
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. The upper Merced River descends from its headwaters through a glacially carved canyon at a gradient of about 8,000 feet over 24 miles. The average daily discharge rate measured at the Happy Isles gauging station is approximately 355 cubic feet per second (cfs).
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). 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.
Merced River Watershed – South Fork
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 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. 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 descent 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.
Wetlands are areas that are saturated with surface water or near-surface groundwater for much of the year. They usually have no clearly defined ecological boundaries, but are a gradient between deepwater habitats, such as lakes and rivers, and upland terrestrial ecosystems, such as conifer forests. Wetlands are differentiated from deepwater habitats by the presence of rooted emergent plants, and distinguished from uplands by nearly continuous soil saturation and the presence of water-tolerant (hydrophytic) vegetation communities and characteristically textured and colored wetland (hydric) soils. These wet soils may have rust colored mottling or be very black in appearance.
Wetlands in Yosemite occur in valley bottoms throughout the park, and are often hydrologically linked to nearby lakes and rivers through seasonal flooding and groundwater movement. Meadow habitats, distributed at elevations from 3,000 feet to 11,000 feet in the park, are generally wetlands, as are the riparian habitats found on the banks of Yosemite’s numerous streams and rivers.
The park contains three major types of wetland: Riverine, Lacustrine, and Palustrine These wetland types are described below and are also discussed on the U.S. Fish and Wildlife's National Wetlands Inventory Web Site. Each of these types of wetlands varies in geographic distribution, duration of saturation, vegetation community, and overall ecosystem function. All three types of wetlands provide rich habitat for plant and animal species, delay and store seasonal floodwaters, minimize downstream erosion, and improve water quality.
Riverine wetlands are found within river and stream channels and are strongly influenced by seasonal runoff patterns. When inundated, riverine wetlands provide habitat for water-tolerant plants such as willows, and aquatic animals such as tadpoles and immature fish.
Lacustrine wetlands generally occur on river floodplains and along lakeshores and are influenced by seasonal variations in groundwater levels. These wetlands are relatively rare in the park, but support an abundance of warm-water loving plant and animal species.
Palustrine wetlands are typically distinguished from riverine and lacustrine systems by the presence of very dense covers of trees, shrubs, or emergent plants. This wetland type includes wet meadows, densely vegetated riparian habitats, and shallow ponds. They provide cover and forage for wildlife traveling between upland and aquatic habitats.
Protection and Restoration
Since the 1970s the United States has made substantial progress toward protecting and restoring wetland habitats. Yosemite National Park complies with a 1990 Presidential Executive Order that mandates 'no net loss' of wetlands on federally-managed lands, and requires federal agencies to map and protect all existing wetlands.
In 1996 the National Fish and Wildlife Service delineated and classified Yosemite’s wetlands, through analysis of aerial photographs and topographic maps, as a part of the National Wetlands Inventory Web Site (NWI). Although the NWI maps cover the entire park, they have not been rigorously ground-truthed and only delineate wetlands larger than five acres in size.
The park restores to natural conditions wetlands that have been drained or filled in the past. Most recently in Yosemite Valley, the Cook’s Meadow restoration project involved filling old drainage ditches that were draining the meadow and removing an old roadbed that was inhibiting water flow. These actions are currently being monitored with vegetation transects and mapping of surface water to determine how successful the project was in restoring the wetland.