Debris flows, sometimes referred to as mudslides, mudflows, lahars, or debris avalanches, are common types of fast-moving landslides. These flows generally occur during periods of intense rainfall or rapid snow melt. They usually start on steep hillsides as shallow landslides that liquefy and accelerate to speeds that are typically about 10 miles per hour, but can exceed 35 miles per hour. The consistency of debris flows ranges from watery mud to thick, rocky mud that can carry large items such as boulders, trees, and cars. Debris flows from many different sources can combine in channels, and their destructive power may be greatly increased. They continue flowing down hills and through channels, growing in volume with the addition of water, sand, mud, boulders, trees, and other materials. When the flows reach flatter ground, the debris spreads over a broad area, sometimes accumulating in thick deposits that can wreak havoc in developed areas.
Wildfires can also lead to destructive debris-flow activity. In areas burned by forest and brush fires, a lower threshold of precipitation may initiate debris flows. In the summer of 2000, a moderate wildfire, the Cave Gulch Fire, swept the area near Canyon Ferry Lake, northeast of Helena, denuding the slopes of vegetation. During the summer of 2001, a near stationary thunderstorm dropped heavy rain on the area resulting in a debris flow through Kim's Marina.
More information may be obtained from your National Weather Service offices in Montana...