Mudslides Explained: Behind the Washington State Disaster

A fatal mudslide in rural northwestern Washington State over the weekend underscores the dangers of this fast-moving natural hazard.

On Saturday morning, a mudslide moved down the Stillaguamish River near the small former fishing village of Oso, Washington. Authorities have confirmed eight dead, eight injured, and as many as 108 people missing or unaccounted for as of Monday morning. The one-square-mile (2.6-square-kilometer) track of the mudslide also destroyed about 30 homes.

Jim O'Connor, a research hydrologist with the U.S. Geological Survey in Portland, Oregon, told National Geographic that the mudslide, which was up to 15 feet (4.5 meters) deep in some areas, was caused by ground made unstable by heavy rainfall.

"This area has had slides in small increments over the last several years, but this took a huge bite of the hillslope this time," says O'Connor.

Not only has there been a lot of precipitation in the area over the past few months, but the Stillaguamish River also has been eroding away the base of the hillside, or "undercutting it," making it less stable, says O'Connor.

"A whole section of a hillside, about 700 feet [213 meters] high above the river, collapsed all at once," says O'Connor. "It's amazing how much terrain it ended up covering."

This house on Highway 530 near Arlington, Washington, was destroyed by the mudslide on Saturday.

Photograph by Lindsey Wasson, The Seattle Times-Pool/Getty

What Is a Mudslide?

A mudslide, also called a debris flow, is a type of fast-moving landslide that follows a channel, such as a river. A landslide, in turn, is simply when rock, earth, or other debris moves down a slope. (See photos of a mudslide and a video on landslides.)

Mudslides occur after water rapidly saturates the ground on a slope, such as during a heavy rainfall. According to O'Connor, it doesn't take high relief in the topography to create a slide. Rather, it just takes a pull of gravity strong enough to bring down material that is made fluid enough by water.

Mudslides tend to happen during wet seasons, says O'Connor. For the Pacific Northwest, that's generally during winter or spring.

Mudslides are also often triggered by earthquakes or by disturbances in hillsides caused by fires or human activity.

In the United States, mudslides and landslides result in an average of 25 to 50 deaths a year, according to the Centers for Disease Control and Prevention (CDC).

The fast-moving mudslide razed this nearby house.

Photograph by Lindsey Wasson, The Seattle Times-Pool/Getty

How Are Mudslides Prevented?

Strategies to decrease the risk of mudslides include draining water off hillsides, armoring the bases of hills so they are not undercut by rivers, and "loading the toe," says O'Connor. In the case of "loading the toe," engineers put heavy mass, such as large rocks, at the base of a hill to try to anchor the slope and prevent it from coming loose.

O'Connor says the piles of rock that are often seen at the base of roadcuts are the most visible example of that strategy.

O'Connor adds that when it comes to the Stillaguamish River area, the valley is scalloped with the evidence of many past slides.

"This isn't a situation where [the authorities] should have done something [to prevent it] because there is so much terrain there that this could have happened to," he says.

The CDC recommends that people exercise caution around steep slopes during rainfall. Immediate signs of a pending slide include tilting trees and sudden increases or decreases in rivers.

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