The Los Angeles River is the heart of an 871-square mile watershed. The watershed encompasses the Santa Susanna Mountains to the west, the San Gabriel Mountains to the north and east, and the Santa Monica Mountains and Los Angeles coastal plain to the south.
The Los Angeles River Watershed has diverse patterns of land use. Forest or open space covers the upper half of the watershed, while the remaining watershed is highly urbanized with commercial, industrial, or residential uses. There are 22 lakes within its boundaries. In addition, there are a number of spreading grounds in the watershed including sites at Dominguez Gap, the Headworks, Hansen Dam, Lopez Dam, and Pacoima Dam. The Los Angeles River is hydraulically connected to the San Gabriel River through the Rio Hondo, although this occurs primarily during large storm events.
Many people don't even realize there is a river in Los Angeles. It's usually remembered as the cement channel where two Terminators had a high-speed chase in the movie T2, or the staging ground for a giant ant invasion in Them!. But our river is more than a backdrop for movies and traffic. It is presently a shadow of it's former self, but areas of great beauty still exist.
The Los Angeles River is not like other rivers in the United States. At only 52 miles long, the L.A. River is 45 times shorter than the Mississippi, but drops 795 feet in elevation from the headwaters in the San Fernando Valley to its end in Long Beach. That's 150 feet more than the Mississippi drops in its entire 2350 miles, meaning our river is short but steep.
In times of peak flow, the river carries 183,000 cubic feet of water per second out to the Pacific Ocean (the equivalent of 40 million garden hoses going full blast) - 14 times the flow of NY's Hudson River. The LA River has no "average" flow, varying widely from a bare trickle in drought years to a raging torrent in years of heavy rain.
The Los Angeles River Watershed has impaired water quality in the middle and lower portions of the basin due to runoff from dense clusters of commercial, industrial, residential, and other urban activities. The impairments include pH, ammonia, metals, coliform, trash, algae, oil, pesticides, and volatile organics.
Of course, one of the most different things about the LA River is the fact that much of it is encased in concrete. Confining the river to a concrete channel began in 1938, as an effort to control the devastating floods that periodically swept through the city. It took 30 years and 3.5 million barrels of concrete to channelize the river and its tributaries, and when it was done, it wasn't called a river anymore. It was renamed the Los Angeles River Flood Control Channel.