For much of its history, the Los Angeles Basin was mostly a vast wetland with islands of forested land and dense shrub. The Los Angeles and San Gabriel Rivers watered the basin with most of the water sinking into the soil before reaching the coast.
Periodically, the Los Angeles River would flow hard and fast enough to push its way into Santa Monica Bay at what is now Marina del Rey. This changed significantly, however, when, in 1825, an epic flood from Big Tujunga Canyon in the San Gabriel Mountains forced the Los Angeles River to begin emptying into San Pedro Bay (at Long Beach) where the river ends today.
See Rivers of Los Angeles County
In 1938, in response to devastating floods early in the year, the U.S. Army Corps of Engineers began lining the Los Angeles River in concrete so as to control flooding and channel its water in the same predictable path.