California health officials have warned residents 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.
And even as the rain has relented, residents are bracing for landslides, with hundreds of homes evacuated.
During the storm, Los Angeles received half its annual rainfall in just six days. At least one person was killed.
Heavy rain was also reported in neighbouring Arizona, Nevada and Utah, with the storm heading east. Forecasters predict the storm will hit the US East Coast at the weekend, after the busiest of the Christmas travel days.
In California, meanwhile, the weather has cleared but public health officials warned residents to avoid the beaches for several days.
"It can be very nice the next day and everyone says: 'This is great! This is a beach day,"' Jonathan Fielding, director of the Los Angeles county public health department, told the Associated Press. "It could well be but we will be monitoring and testing water and we won't recommend people go back there until we're sure it's safe."
Now that the storm has moved east from California, residents have begun clearing flooded streets, while many remain wary of new mudslides.
Hundreds of people were evacuated in the suburbs of Los Angeles, with particular concerns for homes in steep-sided canyons previously ravaged by wildfires. Amtrak, the national passenger railway line, suspended service between Los Angeles and San Diego in southern California in reaction to the threat of mudslides.
"The ground is so saturated it could move at any time," said Bob Spencer, spokesman for the Los Angeles County Department of Public Works.
There were reports of mudslides in Laguna Beach, California, as intense rain hit the region, and 25 to 30 people were evacuated from their mountain homes in Silverado Canyon, Orange County, the Associated Press reported.
Heavy rains - estimated at up to 1in (2.5cm) per hour - brought down a hillside on a heavily used section of Interstate 10 early on Wednesday, covering three lanes near the city of Pomona.
In Highland, some 65 miles (104km) east of Los Angeles, two creeks overflowed, swamping as many as 20 homes in mud.
"This mud flow moved cars, picked them up, stood them up on their nose at 45-degree angles, buried them," Bill Peters, a spokesman for the California department of forestry and fire protection told AP.
In Riverside County, a woman was killed on Wednesday when her car was swept off a flooded road, the coroner's office reported. Other motorists were rescued from cars trapped in raging floods.
Homes in the mountains were blocked by boulders and mud as rescue workers helped residents seek shelter before the largest of the storms struck.
In far north-western Arizona, residents rushed to gather belongings from their homes along a flooded stretch where further structures risked being swept away.
"It is a mess," said Lois Rolfsmeyer, resident of Beaver Dam.
"The water is going to take our next-door neighbour's house and the one behind us, and it's eroding under our house."
On Tuesday, officials ordered the evacuation of 232 homes that sit beneath large hillsides in La Canada Flintridge and La Crescenta, in the suburbs of Los Angeles.
Flood warnings and emergency orders have also been put in place in Arizona, Nevada and Utah.