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Mudslide and Prevention
   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.
   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).
   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.