PBS Nova: Killer Landslides (video) — The March 2014 landslide engulfed 49 homes and other structures in an unincorporated neighborhood known as “Steelhead Haven” 4 miles east of Oso, Washington. It also dammed the river, causing extensive flooding upstream as well as blocking SR530, the main route to the town of Darrington (population 1,347), approximately 15 miles east of Oso.
The natural rock and mineral formation (referred to by geologists as a “geological feature”) with the most recent activity in the area of Oso is known as the Hazel Landslide; the most recent landslide event is being referred to in the media as “the Oso mudslide”. Excluding landslides caused by volcanic eruptions, earthquakes or dam collapses, the Oso slide is the deadliest single landslide event in United States history.
The Hazel Landslide has a history of instability dating back to 1937. Prior to the March 2014 mudslide, the Oso area experienced up to 200 percent normal rainfall over the previous 45 days. Described by witnesses as a “fast-moving wall of mud”, the slide, which contained trees and other debris, cut through homes directly beneath the hill on the south side of the Stillaguamish River. A firefighter at the scene stated, “When the slide hit the river, it was like a tsunami”. A Washington state geologist stated the slide was one of the largest landslides he had personally seen. The mud, soil and rock debris left from the mudslide covered an area 1,500 ft long, 4,400 ft wide and deposited debris 30 to 70 ft deep. A national geologist stated the flow of the landslide was extreme because of the extraordinary run-out of mud and debris. While the landslide is now very well documented, a research team from the Geotechnical Extreme Events Reconnaissance (GEER) plans to investigate the factors contributing to the slide.