The Los Angeles River (also known as the L.A. River) starts 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 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, which was built after a series of devastating floods in the early 20th century.