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.
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. The Merced River is a Wild and Scenic River that cuts a breathtaking course from glacial peaks through mountain lakes, alpine and subalpine meadows, waterfalls, and gorges and supports rich and diverse riparian habitat.
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. Flooded oxbows in Cook's Meadow show where the meandering Merced River once flowed. Cook's Meadow is in the Yosemite Valley.
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.
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.
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.