The Los Angeles and San Gabriel Rivers both originate in mountainous areas including a large portion of the Angeles National Forest. They flow through the mountains into the San Fernando and San Gabriel Valleys, both of which are underlain by rich groundwater basins. The rivers then continue on over the coastal plain of Los Angeles into the Los Angeles/Long Beach Harbor and into the Pacific Ocean.

Due to pressures of urbanization both rivers have been highly modified with dams and concrete channeling resulting in a loss of habitat and human access to the rivers. Diversions of water for use in groundwater recharge, significant discharges of wastewaters including sewage treatment plant reclaimed waters, and non-point source contributions such as urban runoff have dramatically changed the natural hydrology of the rivers. The watersheds are host to a wide range of wildlife species (especially in the upper more pristine areas), many of which are now listed on federal and state endangered and threatened species lists.