The Los Angeles River has its (occasional) headwaters in the San Fernando Valley and runs 50 miles to Long Beach.
It has concrete banks which help prevent seepage and overflowing. Los Angeles receives very little rainfall compared to other cities, so most of the time the Los Angeles River is either a dry concrete riverbed, or it has a few inches of water that is brackish, highly contaminated, and unsafe for drinking purposes (or for any useful purposes). When dry, the river bed is often used by bicyclists and as a location for Hollywood movie car chases. When it does rain in Southern California, the river can fill quickly, can move rapidly, and can become hazardous for anyone who ventures near it.
