Nine communities in California’s Central Valley are planning a regional drinking water supply system which will convert some agricultural surface water to municipal use. The drinking water supply program is being implemented under the auspices of the Turlock Irrigation District (TID) and nine communities. This paper describes the purpose and features of this regional water supply program. The study area is depicted on Figure 1. Formed in 1887, TID was the first irrigation district created in California under legislation passed in 1886. TID has broad powers in water supply and power generation and sales. TID does not have a responsibility to provide a drinking water supply, although its powers include the ability to supply treated and untreated surface water using a raw supply derived from the Tuolumne River.