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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.
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That’s about 15 years, or roughly a fifth of their lives.
For an activity undertaken over such a long period of time, dating is remarkably difficult to characterize.
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 researchers found that many of the Stillaguamish slides were younger than expected, less than 2,000 years old, meaning that landslides have been occurring more frequently in recent times, perhaps as often as every 140 years on average instead of every 500 years.
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.
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.
And now, thanks to mobile apps, dating can involve a succession of rendezvous over drinks to check out a dizzying parade of “matches” made with the swipe of a finger.