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 their water sinking into the soil before reaching the coast.
Periodically, heavy rain caused the Los Angeles River to flow hard and fast enough to push its way into Santa Monica Bay, at what is now Marina del Rey. This dramatically changed in 1825, when an epic flood from Big Tujunga Canyon in the San Gabriel Mountains instead forced the Los Angeles River towards San Pedro Bay, at what is now Long Beach today. The river has emptied to the sea there ever since.
In 1938, in response to devastating floods early in that year, the U.S. Army Corps of Engineers began lining the Los Angeles River in concrete, in order to control flooding and channel the river in a predictable (although not natural) path.