The Los Angeles River is a major river in Los Angeles County, California. Its headwaters are in the Simi Hills and Santa Susana Mountains, and it flows nearly from Canoga Park through the San Fernando Valley, Downtown Los Angeles, and the Gateway Cities to its mouth in Long Beach. While the river was once free-flowing and frequently flooding, forming alluvial flood plains along its banks, it is currently notable for flowing through a concrete channel on a fixed course, which was built after a series of devastating floods in the early 20th century.