A massive storm system stretching from California into Nevada has swollen rivers and caused severe flooding, with hundreds of people evacuated amid the threat of mudslides.
Northern California's Russian River has risen to its highest level since 2006, and schools and roads are closed across the wine-making region of Sonoma County, where thousands of people are without power.
Avalanche concerns have kept some California ski areas closed for a second day in the Sierra Nevada, and forecasters said more snow and rain is on the way.
Near Reno, Nevada, National Guard high-water vehicles were deployed to help people evacuate.
The Russian River is prone to flooding, but this year's flood has been particularly concerning because it threatens to topple trees weakened by six years of drought.
Resident Jeff Watts said he had spent an anxious night on Sunday listening for the sound of falling trees on his land.
On Monday, he found his drive to work blocked by a tree that had fallen on a car, and emergency crews were working to extract the vehicle.
Over the weekend, toppled trees crashed against cars and homes and blocked roads in the San Francisco Bay area. Stranded motorists had to be rescued from cars stuck on flooded roads.
A giant tree fell across a road in Hillsborough to the south of San Francisco, injuring a driver who could not brake in time and drove into it.
A woman was killed on Saturday by a falling tree while she walked on a golf course.
To the south near Los Angeles, commuters were warned of possible flooding on the roads and mudslides in hilly areas.
Los Angeles County health officials are advising swimmers and surfers to stay out of the ocean for at least three days because of heavy storm run-off.
Seawater bacteria levels can increase significantly during and after rainstorms as contaminants enter the ocean via storm drains, creeks and rivers.
Emergency workers in Nevada voluntarily evacuated about 1,300 people from 400 homes in an area of Reno as the Truckee River overflowed and drainage ditches backed up.
In the city of Sparks, near Reno, resident Bob Elsen said he never expected so much rain in Nevada's high desert, where only 8in normally falls each year.
He moved to Sparks from perpetually-wet Bremerton, Washington.
"I don't think I've seen this much rain since I moved here six years ago," Mr Elsen said, watching the Truckee River rise. "It's why I moved out of Washington, to get away from this stuff."
The back-to-back storms that have hit California and Nevada since last week are part of a so-called "atmospheric river" weather system that draws precipitation from the Pacific Ocean as far west as Hawaii, with potentially catastrophic consequences for areas hit by the heaviest rain.
In California's Calaveras Big Trees State park, a well-known giant sequoia tree known for the huge tunnel carved through its trunk that cars once passed through came toppling down.
Park volunteer Joan Allday said the tree had been weakening and leaning severely to one side for several years. "It was barely alive. There was one branch alive at the top," she said.
Further north, two major roads across the Sierra Nevada were partially closed because of mudslides and roads leading to Yosemite National Park's valley floor remained shut amid fears that the Merced River could overflow and cause major flooding.