Hetch Hetchy Reservoir, A reservoir lake in Yosemite National Park, California

Hetch Hetchy Reservoir is a reservoir in Tuolumne County, California, formed by the O'Shaughnessy Dam on the Tuolumne River. The dam, completed in 1923, flooded the Hetch Hetchy Valley under an average 300 feet of water. The reservoir stretches eastward for about 8 miles, and its deep blue water reflects the surrounding granite walls, including Kolana Rock and Hetch Hetchy Dome.

Native Americans lived in Hetch Hetchy Valley for thousands of years before settlers arrived in the 1850s. Congress authorized the dam on December 7, 1913, and the O'Shaughnessy Dam was completed in 1923, flooding the valley under the reservoir. Since 1934 the Hetch Hetchy Project has delivered water 167 miles west to San Francisco and Bay Area municipalities.

The reservoir takes its name from a Miwok word, reflecting the original presence of indigenous peoples in this valley. Today, visitors use the area mainly for hiking and bird watching, with the dam itself serving as a visible reminder of how humans have shaped the landscape.

Hetch Hetchy Road drops into the valley at O'Shaughnessy Dam, and every point east of the dam is roadless, open only to hikers and equestrians. The reservoir stretches about 8 miles eastward from the dam, so plan for a long walk if you follow its shore. The valley floor now lies under an average 300 ft of water.

The reservoir flooded the valley under an average 300 ft (91 m) of water, but during droughts it partly reemerges, as it did in 1955, 1977 and 1991. The submerged Tuolumne Fall, a small noisy waterfall with a natural pool, once marked the upper entrance to the valley.