Natural Debris Dams and Debris-Dam Lakes

This site contains information that will help students to realize that in addition to the direct risk from debris flows and debris avalanches, voluminous deposition of volcanic material in valleys commonly forms unstable natural dams by blocking preexisting drainages and that if the size and structural integrity of the blockage is insufficient to hold back the reservoir that will form, catastrophic failure of the dam will result. Such a hazardous situation may persist for months, years, or decades following an eruption. An example is given of Mount St. Helens, where the debris avalanche raised the level of Spirit Lake 64 meters and dammed its natural outlet even higher and lakes formed in the tributaries of Coldwater and Castle Creeks. In addition, the site has links to publications and reports documenting historical landslide dams from around the world. Educational levels: High school, Middle school