The river was originally an alluvial river that ran freely across a flood plain that is now occupied by Los Angeles , Long Beach and other townships in Southern California . Its path was unstable and unpredictable , and the mouth of the river moved frequently from one place to another between Long Beach and Ballona Creek . In the early 19th century , the river turned southwest after leaving the Glendale Narrows , where it joined Ballona Creek and discharged into Santa Monica Bay in present Marina del Rey . However , this account is challenged by Col. J. J. Warner , in his '' Historical Sketch of Los Angeles County '' : " ... until 1825 it was seldom , if in any year , that the river discharged even during the rainy season its waters into the sea . Instead of having a river way to the sea , the waters spread over the country , filling the depressions in the surface and forming lakes , ponds and marshes . The river water , if any , that reached the ocean drained off from the land at so many places , and in such small volumes , that no channel existed until the flood of 1825 , which , by cutting a river way to tide water , drained the marsh land and caused the forests to disappear . " Prior to the Great Flood of 1862 , it was joined by the San Gabriel River in present-day Long Beach , but in that year the San Gabriel carved out a new course to the east , and has discharged into Alamitos Bay ever since
Great Flood of 1862
Great Flood of 1862
Los Angeles River