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).