The 1920's were a period of dramatic economic and demographic change in Los Angeles, the city growing at an unprecedented rate. An influx of new residents - and the accompanying boom in real estate development - brought with it an upsurge in demand for city services such as water and sewage. The boom also turned the river from a water-providing asset to a nuisance - and worse - whenever heavy rainfall became the inevitable flash flood. In the late 1930's the Army Corps of Engineers began to channelize the river in order to control its watershed, thereby creating a drainage system for the city.
Above, a slideshow with captions illustrating the reason for and construction of the LA River concrete channel.