The Los Angeles River (Spanish: Río de Los Ángeles), historically known as Paayme Paxaayt 'West River' by the Tongva and the Río Porciúncula 'Porciúncula River' by the Spanish, is a major river in Los Angeles County, California. Its headwaters are in the Simi Hills and Santa Susana Mountains, and it flows nearly 51 miles (82 km) from Canoga Park through the San Fernando Valley, downtown Los Angeles, and the Gateway Cities to its mouth in Long Beach, where it flows into San Pedro Bay. While the river was once free-flowing and frequently flooding, forming alluvial flood plains along its banks, it currently flows through a concrete channel on a fixed course, which was built after a series of devastating floods in the early 20th century.
Before the opening of the Los Angeles Aqueduct, the river was the primary source of fresh water for the city. Although the Los Angeles region still receives some water from the river and other local sources, most of the water supply flows from several aqueducts serving the area. The Los Angeles River is heavily polluted from agricultural and urban runoff.