At about the same time as the Paradise avalanche and mudflow occurred, a tremendous rock mass also slid off the east side of the volcano in the area between Steamboat Prow and Little Tahoma Peak. This slide formed a mudflow on the floor of the White River valley that was several hundred feet deep at the north boundary of the park and that extended at least 30 miles beyond the base of the volcano. The most remarkable feature of the deposit left by this mudflow is its surface, which is dotted with scores of mounds 5-35 feet high and as much as several hundred feet in diameter. These mounds have cores of huge rocks which are similar in size to those scattered on the surface of the avalanche deposits from Little Tahoma Peak. You can see the mounds best in an area which is a few miles north of the park boundary, west of the White River, and which can be reached by the Huckleberry Creek Forest Road. The total volume of this mudflow deposit may be as much as one-fifth of a cubic mile.
These great landslides and mudflows were followed shortly by another whose size surpassed that of anything before or since. This was the remarkable Osceola Mudflow, which streamed down the valleys of the White River and West Fork about 5,800 years ago. When these great rivers of mud joined in the White River valley, they formed an even larger mudflow which swiftly flowed downvalley for a distance of 15 miles and then spread beyond the Cascade mountain front into the Puget Sound lowland. There the mudflow submerged a total area of more than 100 square miles to depths as great as 70 feet and buried the sites of the present towns of Enumclaw and Buckley. One tongue of it even flowed into an arm of Puget Sound, south of Seattle, that has since been filled with river deposits to form the fertile valley occupied by the towns of Kent, Auburn, Sumner, and Puyallup.
The Osceola Mudflow is remarkable in that it affected areas so far from its place of origin. This long distance of travel was due to its great volume, which we estimate to have been more than half a cubic mile, and to the abundance of slippery clay in it. The clay had been formed by the alteration of rocks in the volcano by hot gases and solutions over many centuries.