| On May 18, 1980, Mount St. Helens, in southwestern Washington, erupted violently, setting off a chain of devastating hydrologic event. During the eruption, a massive debris avalanche moving down the north side of Mount St. Helens was deposited in the North Fork Toutle River valley. Approximately 3 billion cubic yards of material, including rock, ash, pumice, snow, and ice was deposited directly north of the mountain in the upper 17 miles of the valley. Following the eruption, mudflows quickly developed in the South Fork Toutle River, and in the Lewis River tributaries of Smith Creek, Muddy River, and Pine Creek. Several hours laer a massive mudflow originated from the debris avalanche deposit in the North Fork Toutle River valley. This North Fork mudflow caused widespread destruction as it moved downstream through the lower Toutle and Cowlitz Rivers. This report describes the location and chronology of the mudflows which followed the May 18 eruption. Average velocities are presented for the mudflows in the South and North Fork Toutle Rivers, and photographs illustrate the character of the debris and mud deposits. -- Cummans, 1981 |
