A massive storm system stretching from California into Nevada lifted rivers climbing out of their banks, flooded vineyards and forced people to evacuate after warnings that hillsides parched by wildfires could give way to mudslides.
Northern California's Russian River rose to its highest level since 2006, and schools and roads were closed across the wine-making region of Sonoma County, where thousands of people were without power.
Avalanche concerns kept some California ski areas closed for a second day Monday in the Sierra Nevada. Forecasters said more snow and rain was on the way.
In Nevada near Reno, Nevada National Guard high-water vehicles were deployed to help people evacuate from a town.
The Russian River is prone to flooding, but this year's flood has been particularly worrisome because it threatened to topple trees weakened by six years of drought.
Jeff Watts, an artist, spent an anxious night 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. Emergency crews were working to extract the vehicle.
"I couldn't get past the tree, so I turned around and I'm doing this," said Watts, who had pulled over to photograph oak trees and their reflections in the floodwater.
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 highway in Hillsborough to the south of San Francisco, injuring a driver who couldn't brake in time and drove into the tree. And a woman was killed Saturday by a falling tree while she took a walk on a golf course.
To the south near Los Angeles, commuters were warned of possible highway flooding and mudslides in hilly areas.
Emergency workers in Nevada voluntarily evacuated about 1,300 people from 400 homes in a Reno neighbourhood as the Truckee River overflowed and drainage ditches backed up.
Schools were also cancelled in Reno and Sparks, and Gov. Brian Sandoval told all nonessential state government workers to stay home Monday after he declared a state of emergency.
The back-to-back storms that have hit California and Nevada since last week are part of an "atmospheric river" weather system that draws precipitation from the Pacific Ocean as far west as Hawaii. Such systems pose catastrophic risks 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, The San Francisco Chronicle reported.
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 told the newspaper.
Meanwhile, rescue teams mobilized in and around Forestville on Monday. A helicopter circled overhead after emergency crews spotted a vehicle submerged on a flooded road.
Rescuers also removed about 15 people from a youth probation camp near the river, Fire Chief Max Ming said, adding that authorities rescued a homeless man trapped by the river overnight.
Michelle Wolfe, a substitute teacher who lives in Forestville, evacuated from her mobile home park Sunday along with others, taking her RV to a parking lot on higher ground.
On Monday, she returned to see grey-brown floodwater covering where her home had been.
"It was odd, seeing what it looked like," Wolfe said. "We're all safe, and that's the most important thing."