As completed, O'Shaughnessy Dam is long, spanning the valley at its narrow outlet. The dam contains of concrete. The Hetch Hetchy Reservoir created by the dam has a capacity of , with a maximum area of and a maximum depth of . From Hetch Hetchy Reservoir, the water flows through the Canyon and Mountain Tunnels to Kirkwood and Moccasin Powerhouses, which have capacities of 124 and 110 megawatts, respectively. An additional hydroelectric system comprising Cherry Lake, Lake Eleanor and the Holm Powerhouse is also part of the Hetch Hetchy Project, adding another 169 megawatts of generating capacity. The entire system produces about 1.7 billion kilowatt hours per year, enough to meet 20% of San Francisco's electricity needs.