The most common types of mass movements that form landslide dams are rock and soil slumps and slides; mud, debris, and earth flows: and rock and debris avalanches. The most common initiation mechanisms for dam-forming landslides are excessive rainfall and snow melt, and earthquakes. Most landslide dams are remarkable short-lived. In a sample of 63 documented cases, 22 percent of the landslide dams failed in less than 1 day after formation, and half failed within 10 days. Overtopping was by far the most frequent cause of landslide-dam failure. Backwater flooding behind landslide dams can inundate communities and valuable agricultural land. Floods from the failure of landslide dams are smaller than floods from constructed dams impounding bodies of water with the same potential energy, but larger than floods from failure of ice dams. Secondary effects of landslide-dam failures include additional landslides as reservoir levels drop rapidly, aggradation of valleys upstream and downstream of the dams, and avulsive channel changes downstream.
