O'Shaughnessy Dam

The O'Shaughnessy Dam is a curved gravity dam on the Tuolumne River in the Hetch Hetchy Valley of California's Sierra Nevada. The dam is located in Yosemite National Park, and creates the Hetch Hetchy Reservoir. It is named for former San Francisco chief engineer and the original chief engineer of the Hetch Hetchy Project Michael M. O'Shaughnessy.

The dam provides water and electricity to 2.4 million people in the city of San Francisco, San Mateo County, Alameda County, and the San Joaquin Valley. The power-generation facilities and transmission lines are concealed to protect the valley's famous scenery.

The hydroelectric plant generates about 500 megawatts, or 1 percent of California's power use during the summer. The reservoir holds 360,000 acre feet (0.444 km3) of water, of which about one-third is diverted into Canyon Tunnel and the Hetch Hetchy Aqueduct to the San Francisco Bay Area.

The O'Shaughnessy Dam is near Yosemite's western boundary, but the long, narrow, fingerlike reservoir stretches eastward for about 7.8 miles (12.5 km).