Debris Flow in Snohomish County, Washington

On a warm and dry 22nd of March, 2014, a large landslide occurred four miles east of the town of Oso, Washington. The slide debris, traveling at over 60 miles per hour, crossed the entire river valley, covering an area of over a square mile with an approximate deposit thickness of 15 to 75 feet in some areas. The force of the slide flow caused 43 fatalities, 12 injuries, destroyed 37 homes, and wiped out SR 530 for over a mile. In addition, the slide debris completely blocked the North Fork Stillaguamish River for over 24 hours, backing up a pool of water that flooded the valley about 2 miles upstream and reached approximately 20 feet deep, inundating an additional 6 homes. The cause of the slide is not fully known, though precipitation in the previous months to years contributed to the ground failure. A rain gage located 2 miles west of the slide had 19.93 inches of rain in the 40 days leading up to the slide.

As of March 19th the nearest reporting station Arlington, had reported 7.14 inches of rain, which is about 2 inches shy of the record 9.23 for the entire month of March. Three days before the slide, the area received close to an inch of rain in one day. Most all of the fatalities occurred at the permanent homes impacted by the slide.

A relatively dry October 2013 through early February 2014 period with precipitation running around only 50 percent of normal turned unusually wet one week into February and continued through March. Precipitation during February and March ranked in the top 10 all-time for this time period, running between 150 and 200 percent of average.

On a warm and dry 22nd of March, 2014, a large landslide occurred four miles east of the town of Oso, Washington. The slide debris, traveling at over 60 miles per hour, crossed the entire river valley, covering an area of over a square mile with an approximate deposit thickness of 15 to 75 feet in some areas. The force of the slide flow caused 43 fatalities, 12 injuries, destroyed 37 homes, and wiped out SR 530 for over a mile. In addition, the slide debris completely blocked the North Fork Stillaguamish River for over 24 hours, backing up a pool of water that flooded the valley about 2 miles upstream and reached approximately 20 feet deep, inundating an additional 6 homes. The cause of the slide is not fully known, though precipitation in the previous months to years contributed to the ground failure. A rain gage located 2 miles west of the slide had 19.93 inches of rain in the 40 days leading up to the slide.

As of March 19th the nearest reporting station Arlington, had reported 7.14 inches of rain, which is about 2 inches shy of the record 9.23 for the entire month of March. Three days before the slide, the area received close to an inch of rain in one day.