High Quality Content by WIKIPEDIA articles! The Los Angeles River (also known as the L.A. River) is a river that starts in the San Fernando Valley, in the Simi Hills and Santa Susana Mountains, and flows through Los Angeles County, California, from Canoga Park in the western end of the San Fernando Valley, nearly 48 miles (77 km) southeast to its mouth in Long Beach. Several tributaries join the once free-flowing and frequently flooding river, forming alluvial flood plains along its banks. It now flows through a concrete channel on a fixed course.