Seattle Weekly: News: The Super Flood by Frank Parchman:
Some 5,600 years ago, the body of water we call Puget Sound had an arm that extended 30 miles inland from present-day Elliott Bay in Seattle to a point halfway between Auburn and Sumner. Today, of course, that is the Green River Valley—the narrow, flat suburban land of Kent and Renton and the industrial lowlands of South Seattle. It would be reasonable to think that this change happened gradually, but scientists have determined that most of the long-gone stretch of inland sea was transformed by a single event that created 200 square miles of land in a matter of hours, with waves of mud 20 feet to 600 feet high. Imagine a wall the consistency of wet concrete traveling up to 60 mph. This mudflow destroyed everything in its path, uprooting entire old-growth forests. It hit Puget Sound with such force and with so much material that it flowed underwater for 15 miles, maybe farther. An area of hundreds of square miles was covered with mud and debris up to 350 feet deep.