1934 and 1938 saw a further pair of devastating floods that in part brought an end to the area 's citrus industry . In the Los Angeles flood of 1938 , the Santa Ana again burst its banks and flooded Anaheim and Orange in up to of water , stripping away thousands of acres of rich topsoil and destroying many of the citrus groves . Almost 60 people were killed in the disaster and about of land were flooded , despite the fact that the flow in the river was only one-third of that of the 1862 flood . With the extreme damage from the floods , the U.S. Army Corps of Engineers made the decision to dam and concrete the river beginning in the 1940s , and declared it as the greatest flood hazard in the U.S. west of the Mississippi River . Prado Dam , built in 1941 , was designed to capture floodwaters from the Inland Empire about upstream from the river 's mouth . The dam 's impoundment , Prado Flood Control Basin , was designed to handle a 70-year flood
Los Angeles flood of 1938
Los Angeles flood of 1938
Santa Ana River