O'Shaughnessy Dam

The O'Shaughnessy Dam is a gravity dam on the Tuolumne River within Yosemite National Park in California, creating a large reservoir behind its concrete wall. The structure spans the river and forms a storage basin set between steep canyon walls.

Construction began in 1919 and finished in 1923, with the dam initially built to a lower height. Between 1935 and 1938, the structure was raised to increase storage capacity.

The dam's construction sparked environmental debates that continue today, marking a turning point in American conservation and water management priorities.

The reservoir supplies drinking water to the San Francisco Bay Area and feeds two hydroelectric plants downstream. Access roads lead into the valley from the north, with visits subject to special rules within the protected national park zone.

Water from the system requires only basic filtration because strict watershed protection preserves its natural purity. This high quality makes additional treatment steps unnecessary.