Deadly mudslide in Washington

This image, taken from an airplane, shows the best view I’ve seen of a mudslide just east of the town of Oso in rural, northwestern Washington State.

The mudslide occurred at about 10:45 a.m. on the morning of Saturday, March 22nd along the Stillaguamish River. As of the time of this writing, there have been 8 people confirmed dead but today searchers stunned the press by reporting that the list of potential missing includes up to 108 names.

The searchers stress that the list includes vague descriptions like “John who once lived in this neighborhood”, so the total of missing should not be immediately interpreted as a possible death toll. The mudslide hit 49 different lots containing property, at least ½ of which were believed to be occupied full time, and buried Highway 530 – the only route to a nearby town of 1,359 called Darrington.

The slide also completely blocked the Stillaguamish River, reducing its output to a trickle. When landslides occur, they can completely block rivers, creating lakes behind them that eventually overtop the dam leading to a major flood. Flood warnings have been issued on the river, but most likely some degree of digging/dredging will open up a flow path to let the water slowly pass through.

The rocks that collapsed appear to be loosely-consolidated, possibly-glacial sediments. Similar events are common in this area; the state of Washington recently completed a $13 million project to stabilize a portion of hillslope on the opposite side of the river from this slide. Additionally, this area has endured significant rainfalls over the past several weeks, although there was no specific storm right before this slide. Instead, the amount of rain caused buildup of groundwater pressures in the area, and eventually this groundwater pressure allowed the hillslope to give way. Rainfall totals during the month of March in this area are about 300% above the average totals for the month.

Layering in the disrupted central block appears somewhat intact and can be seen in this photo; the slide broke away along an arcuate scarp and left debris piles that are reportedly up to 4.5 meters (15 feet) thick.

From here, search teams will have to simply dig through significant portions of that sediment to try to locate and identify those missing, in addition to digging through that big pile of dirt to prevent flooding on the river.


Image credit: Washington State DOT via Reuters/NBC

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