Osceola Mudflow

The Osceola Mudflow, also known as the Osceola Lahar, was a debris flow and lahar in the U.S. state of Washington that descended from the summit and northeast slope of Mount Rainier, a volcano in the Cascade Range during a period of eruptions about 5,600 years ago. It traveled down the west and main forks of the White River, passed the location of present-day Enumclaw then reached Puget Sound in several areas, including near the present day sites of Tacoma and Auburn.^[1]

The Osceola flow began either as an avalanche or series of avalanches near the summit of Mount Rainier but had transformed to a lahar within 2 km (1.2 mi) of where it was initiated as it incorporated significant amounts of water from within the volcano's hydrothermal system.^[1] The sector collapse formed a 1.8 km (1.1 mi) wide horseshoe-shaped crater, open to the northeast, almost the same size as the crater produced by the 1980 eruption of Mount St. Helens. Most of the Osceola crater has been filled in by subsequent lava eruptions, most recently about 2,200 years ago.^[2]

With a volume of 3.8 km³ (0.91 cu mi) and an areal extent of about 550 km² (210 sq mi), the Osceola Mudflow buried a large portion of the Puget Sound lowland with hydrothermally altered volcanic material that is estimated to have been traveling at 70 km/h (43 mph) up to 50 km (31 mi) downstream from the source region. Many communities in King and Pierce counties, notably Kent, Enumclaw, Orting, Buckley, Sumner, Puyallup and Auburn, are wholly or partly located on top of Osceola Mudflow deposits which reach a depth of up to 100 m (330 ft).^[2] The flow may have buried embayments of Puget Sound.^[1]

The Osceola Mudflow was Mount Rainier's signature event during the Holocene epoch.^[2] It was named after the unincorporated community of Osceola.