A derrick is a lifting device. The timbers pictured below were harvested to build derricks in the Hetch Hetchy Valley in preparation for construction of the O’Shaughnessy Dam. The men and horses stand on the Valley floor. Wapama Falls can be seen across the river.

The Hetch Hetchy Valley began as a V-shaped river canyon cut out by the ancestral Tuolumne River. About one million years ago, the extensive Sherwin glaciation widened, deepened and straightened river valleys along the western slope of the Sierra Nevada, including Hetch Hetchy.

During the last glacial period, the Tioga Glacier formed from extensive icefields in the upper Tuolumne River watershed; between 10,000 and 110,000 years ago Hetch Hetchy Valley was sculpted into its present shape by repeated advance and retreat of the ice. When the glacier retreated for the final time, sediment-laden meltwater deposited thick layers of silt, forming the flat alluvial floodplain of the valley floor.

In 1912, John Muir described the valley dimensions as, “about three and a half miles long, and from a fourth to half a mile wide. The lower portion is mostly a level meadow about a mile long, with the trees restricted to the sides and the river banks, and partially separated from the main, upper, forested portion by a low bar of glacier-polished granite across which the river breaks in rapids.”

Work on the Hetch Hetchy Project began in 1914. Construction of O’Shaughnessy Dam began in 1919, with groundbreaking on August 1st when workers of the Utah Construction Company of San Francisco began clearing the trees in Hetch Hetchy Valley. The dam was completed in May of 1923. Finally, on May 24, 1923, the reservoir filled for the first time.