The formation and failure of natural dams

Of the numerous kinds of dams that form by natural processes, dams formed from landslides, glacial ice, and late-neoglacial moraines present the greatest threat to people and property. Landslide dams form in a wide range of physiographic settings. The most common types of mass movements that form landslide dams are rock and debris avalanches; rock and soil slumps and slides; and mud, debris, and earth flows. The most common initiation mechanisms for dam-forming landslides are excessive rainfall and snowmelt and earthquakes.

Landslide dams can be classified into six categories based on their relation with the valley floor. Type I dams (11% of 184 landslide dams from around the world that we were able to classify) do not reach from one valley side to the other. Type II dams (44%) span the entire valley floor, in some cases depositing material high on opposite valley sides. Type III dams (41%) move considerable distances both upstream and downstream from the landslide failure. Type IV dams (