Oso landslide poses new flood threat as winter approaches

The massive Oso landslide last March wiped out an entire neighborhood, killing 43 people. Now, as another winter approaches, a new threat is emerging in the Stillaguamish Valley.

The devastating slide temporarily blocked the flow of the North Fork of the Stillaguamish River and forced the river to establish a new channel, doing what takes decades, even centuries in nature. After the slide, the constriction of mud and debris created a dangerous pooling upstream.

“There’s still a constriction so there’s still – at high flows – a risk that flooding will occur right upstream if it pools up behind that constriction, trying to get through that narrow area,” said National Weather Service hydrologist Brent Bower.

New sediment on the river bottom could also increase the danger of flooding downstream of Oso.

“And if the stream bed starts getting higher and higher, that can push the water level up out of banks and flooding at levels that wouldn’t usually do that,” Bower explained.

Flood watchers are keeping one eye on the river and another on the hillside that tore away.

“Watching any movement on the slide,” said Bower. “There’s no expectation that it will be anything large.”

But after the slide last March, state geologist Dave Norman warned of that possibility.

“It is not unusual, after a major landslide like this, to see additional movement because you’ve relieved that restraining force that’s against the hillside, material wants to expand off of that so there can be additional movement.”

The North Fork of the Stillaguamish continues to cut a new channel. As flood season approaches, new river gauges have been added to monitor a dynamic situation. Bowers said it will be a challenge to predict the level of danger posed this winter by rising waters on the Stillaguamish River.