When we think of volcanos, most of us picture spewing lava or, as in the case of the great Mount St. Helens eruption of 1980, a raining cloud of rock and ash after the volcano blows its top off. But the most devastating result of a volcanic eruption can actually be the lahar: a flood of mud, debris, and water that flows from a volcano when the water stored in snowpack or glaciers (Mount Rainier has plenty of both) is suddenly released. Mount Rainier feeds 11 different river valleys, including the Puyallup River valley where many cities and towns are built on top of lahar deposits that are only about 500 years old.
DNR’s newly released Loss Estimation Pilot Project for Lahar Hazards from Mount Rainier puts some numbers on the potential damage to building, bridges and other structures at today’s values if Mount Rainier were to produce a lahar that flowed down the most populous and highest-risk route: the Puyallup River valley.
The report predicts that an eruption from Mount Rainier would devastate the Puyallup Valley to the tune of $6 billion in damages. This does not include the damages caused to possessions and items inside of the buildings affected. A major lahar could cover schools, farms, businesses, and homes with about thirty feet of concrete-like mud. Those and other areas could also be affected by flooding after a major lahar.
In the past 10,000 years more than 60 of these devastating lahars have traveled at least 70 miles downstream of Mount Rainier. These lahars have reached not only the Puyallup River valley but also reached Puget Sound through the Nisqually River Basin, Commencement Bay and Elliott Bay, including the ports of Seattle and Tacoma.
The report concludes that:
“Due to the large and increasing population living and working along its lowland drainages and its multiple eruptions in recent geologic history, many geologists agree that Mount Rainier is the most dangerous volcano in the Cascade Range.”
Volcanos typically will produce warnings before an eruption. However, a lahar can also be caused by an avalanche unrelated to an eruption. Here in Washington we live with the risk of earthquakes, landslides, tsunamis, forest fires, volcanos, and lahars, which is why DNR works to assess and estimate these risks. We do so by producing maps, analysis and other information that landowners, residents, community leaders and emergency personnel can use.
Whatever the disaster, make sure you know your evacuation route and have your family’s emergency preparedness plan in place.
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