The 55-mile long Los Angeles River originates in western San Fernando Valley and flows through the central portion of the City south to San Pedro Bay near Long Beach. The Los Angeles River Watershed covers approximately 834 square miles and is bounded at its headwaters by the Santa Monica, Santa Susana, and San Gabriel mountains to the north and west. The southern portion of the watershed captures runoff from urbanized areas surrounding downtown Los Angeles. Jurisdictions in the watershed include the City of Los Angeles (33%), 42 other cities (29%) and eight agencies (37%).
Much of the watershed is highly developed, with residential (36%), open space and agricultural (44%), and commercial/industrial/transportation (20%) being the predominant land uses. Overall, the watershed is approximately one-third impervious.Most portions of the Los Angeles River are completely channelized for flood protection as are many of its tributaries including Compton Creek, Rio Hondo, Arroyo Seco and Tujunga Wash. They are fed by a complex underground network of storm drains and a surface network of tributaries. Several dams and reservoirs have been constructed within the watershed for flood control and groundwater recharge. The river’s two soft-bottom reaches consist of a 3.1-mile portion running adjacent to Los Angeles and Glendale known as the Glendale Narrows and a 2.4-mile portion in the Sepulveda Basin Recreational area behind the Sepulveda Dam.
The average dry weather flow at the watershed’s most downstream monitoring station near Long Beach is 153 cubic feet per second. The average wet weather flow is two to three times higher or more during large storms.
As of March 2012, the US EPA has approved 22 TMDLs throughout the region that list the City of Los Angeles as a responsible jurisdiction. These include waterbodies within the Los Angeles River, Ballona Creek, Santa Monica Bay and Dominguez Channel Watersheds.The Los Angeles River and selected tributaries are impaired by pollutants (i.e., trash, metals, bacteria, nutrients) mainly because of the watershed’s large, dense population and the amount of impervious ground surface that prevents large quantities of runoff from infiltrating into the soils. Currently there are several TMDLs that are in the Watershed, including:
A Total Maximum Daily Load (TMDL) is the maximum amount of a specific pollutant, such as trash, bacteria, or pesticides that could be discharged into a waterbody without causing it to become impaired. Development of TMDLs, which are driven by the Clean Water Act, are an important step in cleaning up our creeks, lakes, rivers, and beaches.