Blocked river trickling through massive mud wall in deadly Washington landslide

SEATTLE – Water that built up behind a massive deadly mudslide in Washington state began trickling downstream, reducing the threat of potential flooding along the North Fork of the Stillaguamish River, authorities said Sunday.

The square-mile of mud and debris that killed four people had raised concerns about a possible flash flood downstream of Oso, about 55 miles north of Seattle.

“The danger is much less than it was last night,” Steve Thompson, public works director for Snohomish County said at a news briefing Sunday evening.

The river was slowly starting to carve out a new channel, allowing enough water to pass through to relieve pressure behind the debris field, he and others said.

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.”

The National Weather Service said a flash flood watch for Snohomish County was in effect until 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 destroyed several dozen homes and blocked about a mile of State Route 530. It also dammed the North Fork of the Stillaguamish River.

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.

The National Weather Service in Seattle said Sunday afternoon that water is clear as it flows through the new channel, indicating that “we can expect a steady release of water through the blockage rather than any sudden burst or rapid rise.”

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.

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.”

There may be many factors, but “the very wet month of March that we had is clearly a factor,” he added. All that rain can raise the groundwater table in a slope and undermine its stability, he said.

The weather service’s Brent Bower said Sunday about a quarter- or half-inch of rain is expected in the area Monday night and Tuesday. “It’ll make things messy but it’s not going to affect the river much,” he said.