In 1953 voters within southern Ventura County united to create the Calleguas Municipal Water District for the purpose of providing the region with a reliable supply of high quality supplemental water. Today, the District serves an area of approximately 350 square miles, and communities served include the cities of Oxnard, Camarillo, Thousand Oaks, Simi Valley, Moorpark, Port Hueneme and the unincorporated areas of Oak Park, Santa Rosa Valley, Bell Canyon, Lake Sherwood, Somis, Camarillo Estates, and Camarillo Heights. Water is provided by the California State Water Project.

Calleguas MWD has undertaken such endeavors as the Las Posas Basin Aquifer Storage and Recovery project, which will enable pre-delivery of up to 300,000 acre-feet of imported water to meet drought and emergency demands. The District is also involved in a proposal for a regional recycled water and salinity control program.