Stillaguamish River trickling through mudslide

Water that built up behind a massive mudslide near Oso began trickling downstream, reducing the threat of potential flash flooding.

Steve Thompson, public works director for Snohomish County, said the river found its way around the slide Sunday afternoon.

“It started cutting a new river channel through that slide, kind of on the north side of the valley,” Thompson said at a briefing Monday morning. “It’s doing what we expected it to do.”

Thompson said hydrologists are watching the river gauge upstream. Field crews reported the river appeared to be stabilizing and dropping upstream.

“That’s good news because it’s a semi-slow predicable release,” he said.

Officials said they don’t think the water would suddenly burst, but urged residents living in nearby communities to remain alert. The National Weather Service said a flash flood watch for Snohomish County was in effect until Monday afternoon.

Upstream, authorities said seven homes have been flooded.

“Homes are inundated up to the eaves in many cases,” said John Pennington, Snohomish County Department of Emergency Management Director.

The slow release is good news for rescue teams searching for survivors in areas where the soupy mud is over 15-feet deep.

“What that will do is start to de-water that valley so that Washington State Department of Transportation can get in and do more work on State Route 530,” said Thompson.

Crews have been stationed on five bridges downstream to look for large debris and any survivors, according to Thompson.

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 Associated Press contributed to this report.