“But nobody knew how old any of the old slides were,” La Husen says.
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Lidar studies conducted before and after the 2014 Oso slide revealed 25 landslides up and down the Stillaguamish River Valley.
The river is known to have begun cutting the valley through the glacial sediments roughly 12,000 years ago, suggesting an average recurrence rate of about 500 years based on the 25 landslides.
As the river erodes laterally into the glacial material, it destabilizes [valley] walls, leading to slides.
Landslides of sufficient volume in turn block the course of the river, forcing it to the other side of the valley.
They found that the Rowan Landslide — approximately five times the size of the Oso slide — occurred between 300 and 700 years ago, and the Headache Creek Landslide is about 6,000 years old.
Then, with dates for three area slides, the team used lidar data to create what’s called a roughness curve for the 22 other slides along the 6-kilometer section of the valley studied.
The ice sheet dammed many of the rivers flowing west out of the Cascade Mountains, building up massive deposits of fine clay and silt particles more than 200 meters thick in what is now the Stillaguamish River Valley.