Lahars as the greatest of threats that Mount Rainier poses to people and property downstream.
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.
Lahars that begin as large landslides high on Mount Rainier’s flanks can occur without warning—that is, without any precursory activity detected by seismometers or other instruments. To help downstream communities prepare for this hazard, a USGS computational model called D-Claw is used to simulate the extents, depths and rates of lahar inundation in the Puyallup and Nisqually River drainages downstream of Mount Rainier. Model results show that if the landslide consists of weak rock that is rich in clay, a landslide similar in size to the Electron Mudflow that occurred about 500 years ago at Mount Rainier could transform into a highly mobile, swift-moving lahar that reaches populated areas in under an hour.
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.