San Luis Reservoir stores water for the Metropolitan Water District of Southern California (MWD) which is one of the primary providers of water for the Greater Los Angeles Area. This water reaches Southern California via the East and West Branches of the California Aqueduct. It also provides water for the Central Coast cities like San Luis Obispo and Santa Barbara via the Coastal Branch of the California Aqueduct. Local water agencies and irrigation districts are allowed to "bank" water in San Luis Reservoir for later use, by purchasing it during the rainy season at lower prices when it is not immediately needed. However, regulations only allow such carryover storage when San Luis is not full, since state and federal water takes priority. Once the reservoir fills, any carryover water that has not been used is effectively "lost", i.e. transferred to the CVP and SWP supplies.