On the west side of the Central Valley, near Los Banos, is the San Luis Reservoir, the largest off-stream reservoir in the nation, covering 12,700 acres, with a capacity of more than 650 billion gallons. It was formed by a 380 foot tall dam that was completed in 1967, and is the fourth largest embankment dam in the nation. The reservoir serves as the upper reservoir for a pumped storage hydroelectric project, but its primary function is to store water for federal and state irrigation and drinking water projects. It is the main component of the San Luis Complex, which consists of two reservoirs and two pump/generator plants, and links two statewide aqueduct systems. Water is pumped into the reservoir by the William R. Gianelli Pumping-Generating Plant at the base of the dam. It lifts water 300 feet into the reservoir, and generates as much as 424 megawatts of power when water flows back down through the plant, into a canal connected to the lower reservoir, called the O’Neill Forebay.
