High Quality Content by WIKIPEDIA articles! 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.