Record amounts of rain caused the Los Angeles River to go from a small stream to a raging river in just a matter of hours, forcing some drivers to ditch their flooded cars, reports CBS News correspondent Ben Tracy.
The river is now looking more like wild Colorado rapids, after water levels rose from less than an inch deep Tuesday morning to several feet by midday.
Along the nearby San Gabriel River, sheriff deputies warned the homeless to seek higher ground.
"We're not here to arrest them, we're here to make sure they don't get injured or killed in this El Niño storm," said Lt. Geoffrey Deedrick of the LASD Community Partnerships Bureau.
Overnight in San Diego, lifeguards rescued two homeless men stranded in the rising San Diego River.
"We were hitting logs and debris and current. At one point, we got swept in the current down with them and got spun around," said Chuck Davey of the San Diego Lifeguard Swift River Team.
As much as three inches of rain fell across parts of Southern California Tuesday, snarling traffic on muddy roadways and sending drivers scrambling for safety.
"I saw cars going through it, so I thought 'Oh, I'm fine' until I felt my car slightly lift off and I just felt all the water and then I was like, 'I can't move,'" said one driver, Alexxa Maitia.
A tornado in a Los Angeles County reportedly also damaged roofs and blew out windows in at least eight commercial buildings, downing power lines and crashing them into windshields.
In Los Angeles neighborhoods that were only recently fighting droughts and wildfires, the heavy rain is now threatening dangerous mudslides.
Resident Ed Heinlein has constructed retaining walls around his home, but they may not be enough.
"The likelihood is that the house will be shoved into the street and the county will be coming to pick it up," said Heinlein.
In the Sierra Nevada Mountains, snow is the concern -- as much as two feet is forecast to fall Wednesday. But that's not stopping truckers from plowing through.
"We're pretty much not happy campers doing this, but we've got to do it just to get the freight on the other side," said one truck driver, Chris Ifo.
The risk of landslides is high even though less rain is forecast to fall. That's because as the ground gets saturated, it takes less and less rain to trigger a major slide.