The Osceola Mudflow and the Cities Downstream
Near Ashford, Washington
Mount Rainier's greatest danger is not fire. It is mud. Because the volcano is wrapped in glacial ice and its upper rock has been weakened by centuries of acidic steam, a collapse can turn a whole flank of the mountain into a fast moving river of mud, rock, and water called a lahar.
About five thousand six hundred years ago, one such flow, the Osceola Mudflow, roared off the summit and down the White River valley. It swept past the site of present day Enumclaw and buried the ground where Auburn now stands, filling the lowland with mud that in places lies more than two hundred feet deep. Today more than one hundred thousand people live on top of those old deposits.
Geologists warn that another lahar will someday reach the Puget Sound lowland, though no one can say when. Most alarming, a lahar does not need an eruption to begin, which can leave very little warning. That is why sensors now line the valleys, listening for the ground to shake, ready to sound an alarm that could give people below just enough minutes to climb to higher ground.
