OSO, Wash. — March 22 marks a solemn day in Washington history. It was 11 years ago when the Oso/SR 530 landslide occurred. Forty-three lives were lost in this tragic event, the deadliest landslide in American history.
It was a Saturday morning. The sun was peeking through the clouds, offering a pleasant early spring day. The North Fork of the Stillaguamish River was receding in the wake of three days of dry weather following a wet winter.
Then, just past 10:30 a.m. that morning, a large segment of a hillside gave way, surging across the valley just east of Oso in less than 60 seconds, sweeping away everything in its path — including the Steelhead neighborhood.
It remains unclear what triggered the massive slide. Was it the river nibbling at the foot of the hillside? Was it the weight of all the winter rainfall soaked into the hillside? Were other factors involved? Geologically, going back centuries, similar landslides had occurred in the valley.
The landslide temporarily blocked the river. Water in the river backed up, flooding homes just upstream of the slide area.
As the day wore on, concerns about the landslide-blocked river steadily rose, raising theories about the blockage suddenly giving way and producing a flash flood-like surge of water downstream.
There were some authorities and downstream communities who felt it was imperative to evacuate from the threat of flash flooding in Arlington and downstream into Stanwood, adversely impacting the Interstate-5 (I-5) bridge over the Stillaguamish River.
Another look at Oso landslide
At the time, I was the Warning Coordination Meteorologist with the National Weather Service (NWS) in Seattle, a key liaison position with the emergency management community. Joining our NWS Seattle team on duty that day, we collaborated with the NWS River Forecast Center (RFC) in Portland on what are called dam break scenarios.
The RFC conducted several scenarios throughout the day as the situation evolved and found that when water managed to create a gap in the landslide blockage, the threat of downstream flash flooding was quite low.
The emergency manager for Snohomish County at the time, John Pennington, called late that afternoon to ask whether he should authorize activating the Emergency Alert System (EAS) to evacuate downstream communities and close the I-5 bridge as a result of the potential for flash flooding. Given the NWS information collected throughout the day, he was advised of the dam break scenario results and chose not to activate EAS.
That decision turned out to be wise. Late in the day, water in the river did manage to create and flow smoothly through a gap in the landslide debris, easing fears of flash flooding downstream.
During the initial response and 6-week recovery efforts at the scene, spring showers and even a few thunderstorms hampered crews at work. Yet, response crews and their support teams did a monumental job with the recovery of those lost.
In September of that year, rebuilding State Route 530 through the devastated area was completed and fully reopened, and a roadside memorial was put in place near the landslide site.
Last year, on the 10th anniversary of the tragic event, a permanent memorial was dedicated, commemorating the 43 victims. The memorial has exhibits for visitors that honor the victims, survivors, and those who responded to the catastrophic event, serving to educate visitors about the disaster.
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