The California Aqueduct is a system of canals, tunnels, and pipelines that conveys water collected from the Sierra Nevada mountains and valleys of Northern and Central California to Southern California. The Department of Water Resources (DWR) operates and maintains the California Aqueduct, including the two largest pumped-storage hydroelectric plants in California, Castaic and Gianelli. Gianelli is located at the base of San Luis Dam, which forms San Luis Reservoir, the largest off-stream reservoir in the United States. The Castaic Power Plant is located at the northern end of Castaic Lake, while Castaic Dam is located at the southern end.