Coastal residents in California are being warned to stay away from beaches contaminated by run-off after a week of torrential rain.
Miles of beaches remain closed after the deluge washed rubbish, pesticides and harmful bacteria into the sea.
Residents are bracing for landslides, with hundreds of homes evacuated.
Los Angeles received half its annual rainfall in six days. At least one person was killed.
Heavy rain was also reported in Arizona, Nevada and Utah, with the storm heading east.
The BBC reports hundreds of people were evacuated in the suburbs of Los Angeles, with particularly in steep canyons previously ravaged by wildfires.
There were reports of mudslides in Laguna Beach, California.
Two creeks overflowed in Highland, 104km east of Los Angeles, swamping as many as 20 homes in mud.
On Tuesday, officials ordered the evacuation of 232 homes located beneath large hillsides in La Canada Flintridge and La Crescenta, in the suburbs of Los Angeles.
Flood warnings and emergency orders are also in place in Arizona, Nevada and Utah.