Local water supplies consist primarily of groundwater aquifers within Los Angeles County and local surface water due to rainfall in the mountain and valley watersheds.
Groundwater represents a significant portion of local supplies in the County. Most groundwater basins in the County are adjudicated (via a court decision) and producers within these basins follow management guidelines established by their respective adjudications. Groundwater basin recharge can occur via existing and restored natural channel bottoms or percolation of rainwater (natural recharge), however natural recharge is typically insufficient to maintain basin water levels and current pumping levels due to the extent of impervious surfaces and the presence of clay soils in parts of the County. Many agencies rely on artificial recharge, by diverting local supplies from rivers or creeks when flow conditions are optimal, to spreading grounds (or basins) which typically contain sandy soils that promote infiltration.
Local Surface Water
Los Angeles River
The Los Angeles River flows 51 miles from the union of Bell Creek and Arroyo Calabasas in the San Fernando Valley, then southeast through the City of Burbank and eventually southward to Long Beach. Originally, the Los Angeles River was the primary water source for the City of Los Angeles. Today, the river is primarily fed from storm water, effluent from wastewater treatment plants, urban runoff, base flow from the Santa Monica and San Gabriel Mountains, and groundwater inflow in the Glendale Narrows.
Water agencies that have water diversion rights within the Los Angeles River watershed include the City of Pasadena and the City of Los Angeles. The City of Pasadena has rights up to 25 cubic feet per second (cfs) of Arroyo Seco runoff, though the yield of the Arroyo Seco is highly variable depending on weather and rain patterns, and uses its diversions for both direct use and groundwater recharge. Pasadena both uses its rights for recharge of the local groundwater basin, and treats for direct use. The City of Los Angeles has full rights to flows in the Los Angeles River, and uses its diversion rights for groundwater recharge at various locations in the San Fernando Valley.
San Gabriel River
The San Gabriel River flows 75 miles southwest from the San Gabriel Mountains, then southward from the Whittier Narrows to its ocean discharge at the City of Seal Beach. Unlike the Los Angeles River, due to more favorable soil conditions the San Gabriel River has a natural bed for most of its length, although the banks are armored with rip rap and concrete for flood control purposes. The river is fed by storm water, base flow from the San Gabriel Mountains, dry weather urban runoff and effluent from wastewater treatment plants. Significant quantities of surface water naturally recharge groundwater via the permeable bottom in the San Gabriel River and are also used for groundwater recharge in several locations. During the dry season, the presence of dams and other diversions can result in river flow that is sometimes discontinuous, as some river reaches are dry, while other reaches have flow. Conversely, the regulated flows released from the dams in some cases could also extend the periods of river flow.