The valley now filled by the San Luis Reservoir was originally inhabited by the Yokuts people; during the early 19th century it was part of Rancho San Luis Gonzaga and small-scale agriculture began in the area, supplied by groundwater and canals from the San Joaquin River. The Central Valley Project was begun in the 1930s to provide an additional water supply to the area and greatly expanded the irrigated acreage. However, the lack of local water storage limited the system's flexibility. During the dry summer months when surface water was frequently unavailable, farmers pumped vast volumes of groundwater to irrigate their crops, considerably depleting the regional aquifer. The Bureau of Reclamation recognized the need for a large storage reservoir in order to provide a year-round water supply, and began studies for this project in 1955.