The small city of Guerneville north of San Francisco "is officially an island," with the overflowing Russian River forecast to hit its highest level in about 25 years, the Sonoma County Sheriff's Office said in a statement.

A man uses a paddle board to make his way through the flooded Barlow Market District in Sebastopol, California, US, Wednesday, February 27, 2019.
A man uses a paddle board to make his way through the flooded Barlow Market District in Sebastopol, California, US, Wednesday, February 27, 2019. (AP)

Two communities in Northern California's wine country were accessible only by boat on Wednesday after a rain-swollen river overflowed its banks following a relentless downpour across an already waterlogged region.

"Nobody is coming or going from the Guerneville area at this time," said sheriff's Sergeant Spencer Crum. The nearby town of Monte Rio was also isolated by floodwaters and all roads leading to it were swamped.

Flooding in the Barlow Market District, in Sebastopol, California, US, Wednesday, February 27, 2019.
Flooding in the Barlow Market District, in Sebastopol, California, US, Wednesday, February 27, 2019. (AP)

The still rising Russian River was engorged by days of rain from western US storms that have also dumped heavy snow in the Sierra Nevada, throughout the Pacific Northwest and into Montana, where Governor Steve Bullock signed an emergency order to help keep up the supply of heating fuel amid frigid temperatures.

Snow from the storms closed roads and schools and toppled trucks and trees from Oregon to Montana and an avalanche in the Sierra prompted Amtrak to suspend rail service between Sacramento and Reno, Nevada.

The Russian River topped 13 metres Wednesday afternoon, when television helicopter footage showed homes underwater and cars submerged. It could crest at more than 14 metres by Wednesday night, officials said. 

About 4,000 residents in two dozen river communities were ordered to evacuate Tuesday evening but officials estimate only about half heeded the orders, Crum said.

Jeff Bridges, co-owner of the R3 Hotel in Guerneville, said he and others who stayed behind were well prepared to ride out the storm. He and employees spent most of the night moving computers, business records and furniture to second-floor room.

A pickup truck in flood waters from the Russian River in Forestville, California, Wednesday, February 27, 2019.
A pickup truck in flood waters from the Russian River in Forestville, California, Wednesday, February 27, 2019. (AP)

Reached by telephone, Bridges said there was about 2 metres of water at his two-storey home in Guerneville Wednesday but was not worried.

"As long as everybody is safe, dry and warm, it's all fine. You just ride it out," said Bridges, noting that this flood was the fourth he's experienced in 33 years.

He added: "People in Florida have hurricanes, people in Maine have blizzards; we have floods," he said. "It's the price we have to pay to live in paradise."

Several areas in California set record-high rainfall totals, including nearby Santa Rosa, which had nearly 20 centimetres of rain in one day.

The often-waterlogged Venado weather station 8 kilometres from Guerneville recorded more than 50 centimetres of rain in 48 hours.

Paddling through the flooded Barlow Market District in Sebastopol, California, US, Wednesday, February 27, 2019.
Paddling through the flooded Barlow Market District in Sebastopol, California, US, Wednesday, February 27, 2019. (AP)

In the Sierra Nevada, which has already seen a month of heavy snow, two Amtrak trains together carrying nearly 300 passengers stopped and reversed direction because of an avalanche that closed railroad tracks. 

Service on Amtrak's California Zephyr between Reno and Sacramento, California, has been suspended until weather conditions improve, Amtrak spokeswoman Kimberly Woods said.

California officials were also concerned about potential mudslides in saturated wine country hillsides and in areas scarred by wildfires in 2017.

A mudslide Tuesday near Monte Rio trapped a man and a woman before they were rescued, messy but unharmed.

"I fell into the mud when the tree fell over the top of me. It happened so fast you don't even know," Kear Koch told KGO-TV.

Elsewhere in the area, several people had to be rescued from cars stranded while motorists tried to drive through flooded roads. Nina Sheehan, who is visiting from North Carolina, had to abandon her rental SUV after it got stuck in a flooded hotel parking lot.

Cheryl Hughes looks out of her window at her van in the flood waters of the Russian River in Forestville, California, Wednesday, February 27, 2019.
Cheryl Hughes looks out of her window at her van in the flood waters of the Russian River in Forestville, California, Wednesday, February 27, 2019. (AP)

"We made a decision to take the rental car through the waist-high water and we got two thirds of the way and then the car stalled," she said. "Do not try to go through any water over a foot high because you never know what you're going to find."

Firefighters in Monte Rio worked through the night pulling people out of cars stuck in flooded roadways and getting people out of their homes as water approached, Fire Chief Steve Baxman told the Press-Democrat newspaper of Santa Rosa.

"We took 17 people out of cars and houses during the night. Too many people are driving into water," he said.

Other waterways, including the Napa River, also were expected to overflow their banks as an ocean-spanning plume of moisture continued tracking through the West.