What is the lahar hazard at Mount Rainier?
Careful study of the deposits in the large valleys that drain Mount Rainier shows that, over the past 10,000 years, Mount Rainier has been the source of numerous lahars (volcanic debris flows) that buried now densely populated areas as far as 100 km from the volcano. Lahars are flowing mixtures of water and sediment that contain such a high concentration of rock debris that they look and behave like flowing wet concrete. They are capable of destroying buildings, bridges, and other man-made structures by battering, dislodgement, and burial. See effects of lahars for more information and photographs. Prehistoric lahars originated on the steep flanks of the volcano and were channeled into the big valleys that carry water and sediment westward to Puget Sound or the Columbia River. Evidence from their deposits combined with observations of modern debris flows suggest that they traveled at speeds as fast as 70-80 km/hr at depths of 30 m or more in the confined parts of the valleys but slowed
