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Mount Rainier’s hidden danger
Mount Rainier is currently at normal background activity, but its most serious downstream hazard is a large lahar. This fast-moving mixture of water, mud, rock, and debris could rush through river valleys and reach populated communities with little warning.
The most immediate direct lahar threat is concentrated in river valleys downstream from Mount Rainier rather than central Seattle.
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Mount Rainier holds dangerous ice
Mount Rainier has approximately 26 glaciers, and its glacier ice cover exceeds that of any other mountain in the contiguous United States. That ice, combined with loose volcanic rock and weakened slopes, provides the ingredients for destructive lahars.
Large lahars generally follow river valleys, creating hazards along the Puyallup, Carbon, Nisqually, White, and Cowlitz drainage systems. The extent and severity differ among valleys and depend on where and how the flow begins.
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Mount Rainier may not erupt first
A large landslide from hydrothermally weakened rock could mix with water and ice, then rush downhill through established river channels…