South Coast hydrologic region

The South Coast hydrologic region of California is one of the 10 major hydrologic regions of the state as identified by the California Department of Water Resources. The South Coast hydrologic region has a surface area of about 11100 mi2, and includes all of Orange County, major portions of Los Angeles, Riverside, San Bernardino, San Diego, and Ventura counties, and a small portion of Santa Barbara County. The state-defined South Coast hydrologic region is roughly equivalent to the federally-defined Southern California Coastal water resource subregion. The South Coast hydrologic region has 78 state-recognized alluvial groundwater basins and subbasins.

Significant geographic features include the Transverse Ranges and the Peninsular Ranges. Major rivers in the region include the Los Angeles River, San Gabriel River, San Diego River, San Luis Rey River, Santa Ana River, Santa Clara River, Santa Margarita River, and Ventura River.

The climate in the region ranges from Mediterranean to subtropical steppe. Annual precipitation in the region ranges from more than 40 in in the mountains to less than 10 in in some valleys, with an overall average of 17.6 in for the region.

Alluvial groundwater basins in the South Coast hydrologic region
The South Coast hydrologic region overlies an aquifer with a number of identified alluvial groundwater basins and subbasins. An alluvial groundwater basin is essentially the top level of an aquifer, above less-permeable rocks below, such as limestone and shale. Alluvium is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings.