Floodwaters that turned two Northern California wine country communities into islands reachable only by boat began receding Thursday as a rain-engorged river finally peaked after swamping thousands of homes and businesses.

The Russian River in wine country north of San Francisco crested at more than 46 feet Wednesday night, Sonoma County officials said. The water was not expected to return to the river's banks until late Thursday.

The Sonoma County Sheriff's Office said in a statement that officials received no calls for help overnight from hundreds of people who stayed in their homes instead of heeding evacuation orders.

The river frequently floods in rainy weather but it had not reached that level in 25 years. The estimated 2,000 inundated buildings were mainly in and around the community of Guerneville, said Briana Khan, a Sonoma County spokeswoman.

Gov. Gavin Newsom declared a state of emergency in Sonoma County Thursday, as well as in counties of Amador, Glenn, Lake and Mendocino, in response to flooding, mudslides and storm damages.

Flood Photos: Water Overtakes Roadways, Homes in North Bay

"Guerneville has essentially become an island," Khan said. The nearby town of Monte Rio also was isolated when roads leading to it were swamped.

In addition, two sewage treatment plants were not working, leading to concerns about potential sewage spills, she added.

The river was one of several in Northern California that was engorged by days of rain from western U.S. storms that also dumped heavy snow in the Sierra Nevada, throughout the Pacific Northwest and into Montana, where Gov. Steve Bullock issued an emergency order to help keep up the supply of heating fuel amid frigid temperatures.

By Wednesday night, the rain had eased but about 3,500 people in two dozen river communities remained under evacuation orders. No injuries were reported in the Guerneville area but in Humboldt County, the sheriff's office said a man presumably drowned in floodwaters trying to get to his home, where three children were trapped.

The man was walking to his home from a barn through up to 5 feet of water when he was carried away by the fast-moving current, Humboldt Sheriff's Office said in a statement Thursday. Deputies were sent Wednesday night to the house in Ferndale, about 200 miles north of Guerneville, and rescued the three people from the tractor and the children from the home.

NBC Bay Area’s Janelle Wang provides more perspective of how different things look after flooding in Sonoma County.

Elsewhere in the western U.S., two Amtrak trains together carrying nearly 300 passengers stopped and reversed directions because of an avalanche that closed railroad tracks in the Sierra Nevada and shut down service between Sacramento and Reno, Nevada.

In Idaho, the mountain town of Stanley became marooned Wednesday after all three highways leading to the town were closed because of drifting snow, avalanches and the risk of more slides.

Several areas in California set record-high rainfall totals, including nearby Santa Rosa, which had nearly 8 inches of rain in one day. The often-waterlogged Venado weather station 5 miles from Guerneville recorded more than 20 inches of rain in 48 hours.

An atmospheric river, which is defined as a slender band of water vapor that pulls moisture from the tropics, is known for dumping "significant levels of rain and snow," especially in locations across the West Coast, according to the National Weather Service.

"Everything is under water, restaurants, all the rooms, the bar, the office," Hotel R3 employee in Guerneville Luis Lowenbery said. "Everybody was evacuated yesterday afternoon because we knew it was going to come in but we weren't expecting to come in last night around 11."

Dozens of people were rescued from cars that became stranded after drivers tried to cross 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.

"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.

Rhondell Rasmus had pulled to the side of a road too flooded to cross in Sebastopol in Sonoma County late Tuesday night and emergency dispatchers told her to wait for help. But just before dawn, she awoke to find the car was nearly submerged and she was out of gas.

"The water just came up so fast next to my car, it was crazy," she told the Press-Democrat newspaper of Santa Rosa.

She wound up in an emergency shelter, bringing with her a handbag, a backpack and a pair of rain boots.

California is no stranger to atmospheric rivers. In fact, one nearly wiped out Sacramento in Gold Rush times. Scientists say climate change will mean bigger and more frequent atmospheric river events that dump more water on the West Coast.

In Guerneville, streets became seas of muddy brown water. Jeff Bridges, a hotel co-owner who is president of the Russian River Chamber of Commerce, spent the day canoeing through town and gave a ride to a couple and their dog who were stranded in a low-lying apartment.

Five people whose homes were flooded were bunking down at his two-bedroom home.

"We saw quite a few fish swimming by my front porch," he said.

[NATL] Extreme Weather Photos: Record Heat Threatens Europe

Bridges said this flood was the fourth he's experienced in 33 years and the locals took the disaster calmly.

"It's the price you pay to live in paradise," he said. "Buffalo, New York puts up with blizzards. Miami and Houston put up with hurricanes...we have floods."

However, Bridges said it will take weeks to clean up his R3 Hotel, as he has done in past floods. More than 8 feet of water inundated the 23-room business.

"Anything that's been flooded you've got to rip it out, sanitize everything...and rebuild," he said, but added nonchalantly: "Everything's fixable."