O'Shaughnessy Dam is a 430-foot (131 m) high concrete arch-gravity dam in Tuolumne County, California, in the United States. It impounds the Tuolumne River at the lower end of Hetch Hetchy Valley in Yosemite National Park, about 160 miles (260 km) east of San Francisco. The dam and reservoir are the source for the Hetch Hetchy Aqueduct, which provides water for over 2 million people in San Francisco and other municipalities of the west Bay Area.