San Luis Dam

San Luis Dam is a major earth-filled dam in Merced County, California, which forms San Luis Reservoir, the largest off-stream reservoir in the United States. The dam and reservoir are located in the Diablo Range to the east of Pacheco Pass and about 10 miles (16 km) west of Los Banos. San Luis Dam, a jointly-owned state and federal facility, stores more than 2 million acre feet (2.5 km3) of water for the California State Water Project and the federal Central Valley Project. Although the dam is located in the valley of San Luis Creek, the majority of its water comes from man-made aqueducts which are supplied from other rivers in Northern California.

San Luis provides water mainly for irrigation in the San Joaquin Valley, where it augments the supply for more than 1 million acres (400,000 ha) of agricultural land, although some water is also used for urban and environmental uses. The dam was built between 1963 and 1968, and filled for the first time in 1969. It provides flexibility to the state water system by capturing, via pumps and canals, the wet season (November-April) runoff that would otherwise flow from the Sacramento–San Joaquin River Delta into the Pacific Ocean. It also generates hydroelectricity by releasing the water when it is needed. However, it has indirectly impacted the environment by enabling increased water diversion from sensitive estuary regions. In recent years, a combination of drought and pumping restrictions enacted to protect endangered fish have contributed to low water levels at the San Luis Reservoir.

The dam and reservoir are visible from the Romero Overlook Visitors Center, which is located along Highway 152.

San Luis Reservoir is designed to capture excess runoff flowing out of the Sacramento–San Joaquin River Delta, the largest watershed of California, during the winter and spring rainy season so it can be used during the dry season. This water includes both natural river flows, and water that is evacuated from Shasta Lake, Lake Oroville, Folsom Lake and other reservoirs for flood control purposes. This is accomplished via a system of canals and pumping stations which divert water from the Delta and deliver it south to water users. More details