Northern California's Russian River was threatening to crest Wednesday evening at a near-record height, as floodwaters washed out roads, leaving at least one community isolated.
Travel to Guerneville, home to about 4,500 people, is nearly impossible by vehicle.
"The roadways leading into the town along the river and across the river have been cut off by the rising floodwaters," Sonoma County emergency manager Chris Godley said.
Authorities have put an emergency declaration in place, said David Rabbitt with the county board of supervisors.
The river reached 44.3 feet Wednesday afternoon, he said, more than 12 feet above flood stage.
Aerial video from KGO in San Francisco showed people paddling through Guerneville in canoes and kayaks. On some streets, the water reached the windows of cars. On one street, the water reached the rooflines of a few buildings.
"It is incredibly flooded. It is really nuts. You see just the rooftops popping over on the side of the road," Joe Pease, a county employee who delivered sandbags to firefighters but had to stop because of deep water, told KGO.
KGO reported that one wine country hotel was evacuated.
"I have lived here in this area for the last 10 years and this is the highest I have ever seen it," said Zak Wood, who lives in Forestville, which is about a mile from the river.
The river could crest at 46.1 feet, sheriff's Sgt. Spencer Crum said. The record is 49.5 feet, set in 1986, the National Weather Service says.
The sheriff tweeted a photo of part of a road in Monte Rio, about 5 miles away, deluged by water.
"Please heed evacuation orders," the sheriff tweeted.
People living near the river were ordered to evacuate Tuesday ahead of the flooding.
Thousands of people could get stranded if roads flood, Crum said.
About 20 miles upriver, musician Garrett Pierce posted a video of the Healdsburg Memorial Bridge on Wednesday. Water rushes under the truss bridge, close to the spans. The video shows some breaks in the clouds.
More rain is forecast through Sunday -- except on Thursday. A flood warning is in effect for parts of the area, the weather service said. A flood warning means flooding is imminent or occurring; a watch indicates potential for flooding based on forecasts.