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Lahars and Debris Flows at Mount Rainier

Lahar pathways from events heading on Mount Rainier - map showing three major events from last 10,000 years. (Public domain.)

The greatest hazard from Mount Rainier is from lahars, also known as volcanic mudflows or debris flows. Areas inundated by past lahars are now densely populated and contain important infrastructure such as highways, bridges, ports, and pipelines.

At Mount Rainier, scientists use the word lahar for large flows of eruption or landslide origin with potential to travel to densely populated valleys, and use the term debris flow for much smaller, more common events caused by glacier floods and precipitation, which stay generally within park boundaries.

Mount Rainier is particularly susceptible to lahars and debris flows because ice, loose volcanic rock and surface water are abundant, and because some slopes have been weakened by hydrothermal alteration of rocks, which now contain abundant water and slippery clay. Lahars and debris flows look and behave like flowing concrete, and they destroy or bury most manmade structures in their paths.

Past lahars at Mount Rainier traveled as fast as 70-80 km per hour (45-50 mi per hour) and were as much as 150 m (490 ft) deep where confined in valleys near the volcano. They thinned, slowed, and spread out in the wide valleys and lowlands downstream. Deposits of past lahars crop out in all of the valleys that head on Mount Rainier's summit edifice. To read more detail about these events at Mount Rainier, visit the significant lahars page.

Almost annually, water released from saturated sediments, glaciers, or runoff from intense rainfall incorporate loose sediment to form debris flows, which sweep down valleys on the flanks of the volcano. Such debris flows seldom travel beyond the base of the volcano and generally only affect valley bottoms areas within the boundaries of Mount Rainier National Park. Summer and autumn are the seasons during which debris flows are most common, because glaciers produce large amounts of meltwater and intense rains can fall on unconsolidated ground with little to no snow cover.

General information about lahars, their triggering mechanisms, and their related hazards can be found in the lahar hazards section of the Volcano Hazards Program website.