The Tuolumne River (, ; Yokutsan: Tawalimnu) flows for through Central California, from the high Sierra Nevada to join the San Joaquin River in the Central Valley. Originating at over above sea level in Yosemite National Park, the Tuolumne drains a rugged watershed of , carving a series of canyons through the western slope of the Sierra. While the upper Tuolumne is a fast-flowing mountain stream, the lower river crosses a broad, fertile and extensively cultivated alluvial plain. Like most other central California rivers, the Tuolumne is dammed multiple times for irrigation and the generation of hydroelectricity.
Humans have inhabited the Tuolumne River area for up to 10,000 years. Prior to the arrival of Europeans, the river canyon provided an important summer hunting ground and a trade route between Native Americans in the Central Valley to the west and the Great Basin to the east. First named in 1806 by a Spanish explorer after a nearby indigenous village, the Tuolumne was heavily prospected during the California Gold Rush in the 1850s, and the lower valley was cultivated by American settlers over the next few decades. The city of Modesto grew up on the Tuolumne as a railroad hub, absorbing most of the population of the Tuolumne valley around the turn of the century. As agricultural production rose, farmers along the Tuolumne formed California's first two irrigation districts to better control and develop the river.
From the 1900s to the 1930s, the river was dammed at Don Pedro and Hetch Hetchy to provide water for Central Valley farmers and the city of San Francisco, respectively. The Hetch Hetchy project, located inside Yosemite National Park, incited national controversy, and has been described as having forged the modern environmental movement in the United States. As the mid-20th century progressed, demands on the Tuolumne continued to increase, culminating in the completion of New Don Pedro Dam in the early 1970s. These projects halved the amount of water flowing from the Tuolumne into the San Joaquin, greatly reducing the once-abundant runs of salmon and steelhead in both rivers.
Course
The Tuolumne originates in Yosemite National Park, high in the Sierra Nevada, as two streams. The Lyell Fork rises at the Lyell Glacier below Mount Lyell, the highest peak in Yosemite National Park, and flows north through Lyell Canyon. The Dana Fork originates between Mount Dana () and Mount Gibbs () and flows west.
After the confluence of the forks in Tuolumne Meadows, the river meanders northwest, passing under Tioga Road. Although calm for the first few miles, this quickly changes as the river drops over Tuolumne Falls and White Cascades. After briefly passing through Glen Aulin, the river enters the spectacular Grand Canyon of the Tuolumne, where it forms three more major waterfalls – California, LeConte and Waterwheel Falls. Return and Piute Creeks join the river from the north within the Tuolumne Canyon. At the end of the canyon the river widens into Hetch Hetchy Reservoir, formed by a dam built in 1923 to provide water to the city of San Francisco, in the scenic Hetch Hetchy Valley. Falls Creek forms Wapama Falls and Tueeulala Falls, two of the highest in Yosemite, as it enters the reservoir from the north. From its source at above sea level in Tuolumne Meadows, the river drops nearly a mile (1.6 km) to the Hetch Hetchy reservoir at .
Below the O'Shaughnessy Dam the Tuolumne passes Poopenaut Valley and enters a series of continuous canyons as it flows through the Sierra foothills, leaving the western boundary of Yosemite National Park and entering the Stanislaus National Forest. Most of the Tuolumne's main tributaries join within this reach, beginning with Cherry Creek from the north just below the intake of the Hetch Hetchy Aqueduct, which delivers water to San Francisco. The South Fork of the Tuolumne joins from the south a few miles downstream, near Groveland-Big Oak Flat, followed by the Clavey River from the north a few miles after that. The North Fork of the Tuolumne joins from the north near the upper end of Lake Don Pedro, which was formed in 1971 when the New Don Pedro Dam was built to provide hydroelectricity and irrigation water storage. Directly below Don Pedro Dam is the La Grange Dam, where about half the river's natural flow is diverted into the Turlock and Modesto Main Canals to irrigate over in the Central Valley.
Leaving the foothills, the Tuolumne flows approximately west across the Central Valley to its confluence with the San Joaquin River, passing Waterford, Hughson, Empire, Ceres and Modesto. The lower Tuolumne is wide and slow-flowing, dropping just from the base of La Grange Dam to its mouth at . Highway 99 and the Southern Pacific Railroad tracks both cross the river in Modesto, and the Modesto Airport is located adjacent to the river shortly upstream. Dry Creek joins the Tuolumne from the north between the airport and the railroad. The confluence of the Tuolumne and San Joaquin is located in the San Joaquin River National Wildlife Refuge, about north of Grayson in Stanislaus County.
Discharge
By volume, the Tuolumne is the largest river draining the southern Sierra, with an estimated virgin flow of per year – over . Near the river's mouth, the highest recorded annual volume was or in 1983, and the lowest was or in 1977 – a 30:1 difference. Although most of the flow in the river originates from summer snowmelt, massive floods can occur following heavy winter storms. The natural flow of the Tuolumne at the mouth is greatly reduced by diversions to both San Francisco's Hetch Hetchy municipal water system and the canals of the Modesto and Turlock irrigation districts.
The United States Geological Survey (USGS) operates, or has operated, several stream gauges along the Tuolumne River. Today, average annual river flows reach a maximum in the short stretch between Don Pedro Dam and La Grange Dam, about above the mouth, before the start of large irrigation diversions. A USGS gage recorded an average annual flow of here between 1911 and 1970, with a maximum peak of on January 31, 1911, and a minimum monthly mean of in October 1912. During the Great Flood of 1862, the river reached an estimated peak of here, after a record of rain fell in the Tuolumne River basin between November 11, 1861, and January 14, 1862, as recorded at Sonora. The gage was not in place during this time.
Further downstream, the USGS gage at Modesto – about above the mouth – recorded an annual mean of between 1940 and 2013. The highest peak was on December 9, 1950, and the lowest daily mean was on August 6, 1977.
Watershed
Physiography The Tuolumne River watershed can be divided into three distinct physiographic regions. From the headwaters to below Hetch Hetchy Reservoir, the river drains the Sierra highlands, which are characterized by solid, glacier-carved granitic bedrock, thin poor soils and areas of heavy forestation. Most of the snowpack that feeds the Tuolumne accumulates in this region. Streams are generally high-gradient, clear and rocky, but large alpine meadows provide riparian and wetland habitat, which contribute sediment to the watershed's runoff. There are also a large number of natural lakes and tarns in the upper watershed. The Sierra crest harbors a number of glaciers, including Lyell and Maclure Glaciers, the largest in Yosemite National Park. Melting ice feeds the upper reaches of the Tuolumne River, maintaining water flow in the late summer when many other streams in the region are dry. The Lyell Glacier is the second largest in the Sierra Nevada, after Palisade Glacier at the headwaters of Big Pine Creek a tributary of the Owens River. Both Lyell and Maclure glaciers have shrunk significantly since the late 1800s when measurements were first made.
Between Hetch Hetchy Reservoir and the upper end of Lake Don Pedro the Tuolumne flows through the Sierra foothills, where it has a lower gradient but continues to maintain a fast current. Stream pools and gravel bars figure more prominently in the channel, allowing the growth of riparian habitat. By the time the river reaches Lake Don Pedro it has accumulated almost 90 percent of its total flow as precipitation run-off, snowmelt and groundwater base flow. Below Don Pedro Dam the river flows across the large alluvial plain of the San Joaquin Valley, formed by millions of years of sediment deposits eroded from the Sierra. Historically this section of the river was subject to frequent course changes and formed thousands of acres of seasonal wetlands during the spring floods. Since the introduction of large-scale agriculture the Tuolumne River course has been fixed between an extensive system of levees. More than in the San Joaquin Valley are irrigated using Tuolumne water.
About , or 34 percent, of the Tuolumne River watershed is inside Yosemite National Park. The rivers whose watersheds border that of the Tuolumne are the Stanislaus River and West Walker River in the north, the Merced River in the south, the East Walker River to the northeast, the Mono Lake basin (Mill Creek, Lee Vining Creek and Rush Creek) to the east, and the headwaters of the San Joaquin River proper to the southeast.
Tributaries Major tributaries of the Tuolumne and pertinent data are listed in the below table. Order is from the headwaters downstream.
NameCoordinatesElevationDistancefrom mouthLengthWatershedDischargeBankImageDana Fork Tuolumne River ft(2,618 m) mi(240 km) mi(8 km)RLyell Fork Tuolumne River ft(2,618 m) mi(240 km) mi(11 km)LReturn Creek ft(1,891 m) mi(224 km) mi(23 km)RPiute Creek ft(1,324 m) mi(209 km) mi(32 km)RRancheria Creek ft(1,157 m) mi(195 km) mi(39 km)RFalls Creek ft(1,153 m) mi(192 km) mi(37 km)RCherry Creek ft(659 m) mi(167 km) mi(68 km) mi2(607 km2) cfs(19.7 m3/s)RSouth Fork Tuolumne River ft(441 m) mi(155 km) mi(50 km) mi2(425 km2) cfs(4.7 m3/s)LClavey River ft(359 m) mi(147 km) mi(68 km) mi2(407 km2) cfs(7.2 m3/s)RNorth Fork Tuolumne River ft(260 m) mi(130 km) mi(58 km) cfs(1.5 m3/s)RWoods Creek ft(241 m) mi(109 km) mi(45 km)RDry Creek ft(14 m) mi(16 km) mi(101 km)R
Land use Almost 80 percent of the Tuolumne River watershed lies above New Don Pedro Dam and is primarily forested with some alpine meadow and grassland zones. The primary land use in the lower Tuolumne River watershed is agriculture, but there are also significant urbanized areas.SJRG supplemental Although only about (5.4 percent) of the watershed itself are farmed, river water is used to irrigate almost five times that area outside the watershed boundaries. About (1.3 percent) of the watershed are urbanized, mostly in the Greater Modesto area. About 550,000 people inhabit the Tuolumne River watershed as a whole, with approximately 210,000 living in Modesto.
Engineering and flood control Like most other Sierra Nevada rivers, the Tuolumne has been extensively dammed to provide flood control, water supply, and hydroelectric power. The uppermost dam is the O'Shaughnessy, which impounds the Hetch Hetchy Reservoir, providing water and power to the City of San Francisco. The Hetch Hetchy hydroelectric system diverts large volumes of water into tunnels that feed two hydroelectric plants, the Kirkwood and the Moccasin. The power system is efficient because of the Tuolumne's great drop, over , between Hetch Hetchy and the outlet of Moccasin Powerhouse. However, it also causes dewatering of significant portions of the Tuolumne River, especially during low water summers when all but a minimal fish release is diverted through the power plants. Not all of the flow used by the power plants is returned to the river. About per year flows through the gravity fed Hetch Hetchy Aqueduct to serve San Francisco and its customer municipalities. Due to the pristine quality of the water, the Hetch Hetchy system is one of only a few serving major cities in the United States that does not require filtration.