Authorities said Sunday that water that built up behind a massive deadly mudslide in Washington state is trickling downstream, but they're not alarmed by it.

The 2.6-square-kilometre field of mud and debris, which came down Saturday onto a small neighbourhood, raised fears about potential flooding downstream of Oso, Washington, about 90 kilometres north of Seattle.

Officials said they don't think the water would suddenly burst, but urged residents living in nearby communities to remain alert.

"There's a small amount coming around the north edge of the slide. It's not alarming, and is allowing the water building behind the dam to settle out a bit," said Bronlea Mishler, a spokeswoman for Snohomish County.

"It's flowing fairly slowly, and the on-scene folks have no major concerns," she said.

The National Weather Service extended a flash flood watch for Snohomish County into Monday afternoon.

Meanwhile, Gov. Jay Inslee urged residents in nearby communities to remain in a "heightened state of awareness" until things fully stabilize.

Saturday's slide killed at least four people, destroyed several dozen homes and blocked about a mile of Washington State Route 530. It also dammed the North Fork of the Stillaguamish River.

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An aerial photo taken Sunday shows a house partially submerged by the backup of the Stillaguamish River behind the mudslide debris dam. (Office of Jay Inslee/Flickr)

Inslee said there were no plans to move the debris blocking the river. "The river will find its way over the days and weeks to come," he said at a news briefing Sunday.

Brent Bower, a hydrologist with the National Weather Service in Seattle, said it's unlikely the water behind the slide will burst and come all at once because it's too wide an area. Even if it did burst, he said, the flow would still be below flood-level downstream.

John Pennington from the Snohomish County Department of Emergency Management said the area where the slide occurred has a history of unstable land. He said a slide also happened there in 2006.

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Photographs taken Sunday show a small breach in the landslide debris dam on the Stillaguamish River. (Washington State Department of Transportation)

Authorities believe the slide was caused by ground water saturation from recent heavy rainfall.

David Montgomery, an earth and space sciences professor at University of Washington in Seattle, said these deep-seated slides tend to occur from rainfall over months or seasons.

"It can raise the water table in a slope and that decrease its stability," he said. "This was a big deep one, a giant slump."

About 18 people are still unaccounted for and considered missing in the slide area. On Sunday, officials said searchers would continue scouring the debris field until nightfall.

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A relief map shows the floodplain downstream of the slide area on the Stillaguamish River near Oso, Wash. Snohomish County was advising those living in the floodplain area, which is the dark green area shown stretching west into Arlington, to leave the area overnight Sunday. (Snohomish County)

Map: Approximate slide area

With files from CBC News