The usually benign river flows through a concretized channel out the Pacific Ocean but water levels can rise quickly during heavy rains and becoming dangerous. At peak flow, the river moves 183,000 cubic feet of water per second - the equivalent of 40 million garden hoses going full blast.
"To encourage them not to tempt mother nature. It's one of those situations where you can try all you want but it only takes six inches of water to knock you off your feet," warned Los Angeles County Supervisor Don Knabe.
The Los Angeles County Urban Search and Rescue team and the Long Beach Fire Department urged people to stay out of the flood control channels and rivers. They said many people don?t realize how hazardous they are.
"We want parents to talk with their kids because the vast majority of people that get injured in the river flood system in Los Angeles and Long Beach are children," said Long Beach fireman Mike Duree.
Long Beach officials said they rescue about five people every year, involving boats, ground equipment and even helicopters. Sometimes conditions are so bad there is nothing they can do.
"There's no guarantee that when someone falls into the river or steps into the river and gets swept away that they can be rescued," said Michael Freeman with L.A. County Fire.
County officials said this doesn't mean people shouldn't enjoy the river. There are specially designated bike paths and walking areas but people need to be cautious during the winter season.
