Seven debris flows large enough to reach Puget Sound have occurred in the past 6,000 years. Valleys that head on Mount Rainier are at great risk from debris flows. Dams and reservoirs would at least partly block giant debris flows moving down all river valleys that originate at Mount Rainier, except for the Puyallup River valley. However, the largest debris flows would not be contained by reservoirs. Instead, enough water could be displaced from reservoirs to cause devastating floods far downstream.
Volcanic ash can be hazardous over great distances.
Volcanic ash is erupted into the air above a volcano, carried downwind, and eventually falls to the ground. Most volcanic ash consists of fine, dusty particles. Because the wind usually blows from the west, ashfall will usually be heaviest to the east of Mount Rainier. However, different wind conditions could produce ashfall in communities to the west, along Puget Sound. In small quantities, volcanic ash is more a nuisance than a threat.