Photograph by Ron Good, 1998
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"O'Shaughnessy Dam is a 312-foot-high, gravity arch concrete dam. The dam is responsible for Hetch Hetchy Reservoir, located on the main stem of the Tuolumne River in Hetch Hetchy Valley. The reservoir, with its capacity of 360,360 acre-feet, is supplied primarily by snowmelt from a watershed of 459 square miles located entirely within Yosemite National Park."
Source: Hetch Hetchy Water & Power Replacement Concepts report, February, 1988, Bureau of Reclamation, prepared on behalf of the National Park Service.
The result of this dam is the flooding of the second Yosemite, Hetch Hetchy. Other sources were available when this dam was built, and other sources are still available today.