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.