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.
“This is a setup for landslides: a lot of inherently unstable and impermeable material in a climate that gets a lot of rain,” says Sean La Husen, a geomorphologist at the University of Washington and lead author of the new study published in Geology.
Roughness curves are commonly used to identify landslides in a landscape.
But they may also be used to date adjacent slides, La Husen says, as newer landslides tend to have rougher surfaces, which then smooth out over time due to erosion.
The technique allowed the team to determine approximate dates for the remaining slides in the valley without having to radiocarbon date each one.
“But nobody knew how old any of the old slides were,” La Husen says.
To get an age for the valley’s landslides, La Husen and colleagues, including Alison Duvall, also at the University of Washington, started by dating plant material in the Rowan and Headache Creek landslides, just downstream and upstream of the 2014 event, respectively.
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.
On March 22, 2014, after a period of heavy rain, a hillside near the town of Oso, Wash., collapsed, sending 7.6 million cubic meters of mud and debris across the North Fork of the Stillaguamish River, destroying a rural neighborhood and killing 43 people.
The slide took Oso residents by surprise, but scientists say the event was not altogether unexpected, as evidence for dozens of past landslides can be found throughout the Stillaguamish River Valley.