Saturday’s fatal mudslide between Arlington and Darrington can likely be traced to the large amount of rain that Snohomish County has received this month, saturating the ground and making the terrain unstable, according to the sheriff’s office and weather experts.
The mudslide hit about four miles east of the tiny town of Oso. The National Weather Service doesn’t keep statistics for Oso, but it does for Arlington, which is about 12 miles southwest.
As of March 19 — the last day for which the weather service has a report — Arlington had recorded 7.14 inches of rain for the month, meteorologist Johnny Burg said. That’s just two inches shy of the wettest March on record for Arlington, and that total — 9.23 inches — was for the entire month.
Darrington, another city not far from Oso, received close to an inch of rain on Wednesday alone, Burg said.
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Slides and floods have long threatened Oso and other nearby communities. In January 2006, Snohomish County declared a state of emergency when a mudslide blocked the North Fork of the Stillaguamish River, diverting water and threatening about a dozen homes in the Steelhead Drive area.
At that time, about 300 yards of a hillside slid into the river.
The Army Corps of Engineers, assisted by others, fortified the new channel with logs and rocks along the river’s edge, hoping to keep the Stillaguamish from nearby homes and yards.
Flooding on the Stillaguamish also threatened homes in 2010 and 1995, among other years.
Because of the danger of a landslide, the Washington State Department of Transportation (WSDOT) began repairs in 2006 to protect drivers along a stretch of State Road 530, building a wall and installing drainage to stabilize the hillside. Those repairs, completed in 2012, were “not far” from Saturday’s slide, but were a little to the west, said Bart Treece, a WSDOT spokesman.
As with the 2006 slide, Saturday’s slide sent debris crashing into the Stillaguamish’s North Fork, creating a natural dam.
The National Weather Service declared a flood warning for the North Fork at 2:18 p.m., and later posted a flash-flood watch into Sunday to warn of a torrent should the debris give out.