Hetch Hetchy Valley
Hetch Hetchy Valley, in Yosemite National Park, central Calif., on the Tuolumne River. It once rivaled Yosemite Valley in beauty and grandeur. O'Shaughnessy Dam (completed 1923; enl. 1938) turned the valley into a lake c.9 mi (14 km) long, which is used for generating power and for supplying water to San Francisco by an aqueduct 156 mi (251 km) long.