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.
On March 27, 1980, Mount St. Helens, Washington erupted for the first time in more than 100 years. On May 18, following 7 weeks of earthquakes and minor eruptions, the volcano exploded violently, setting off a chain of devastating geologic and hydrologic events. The major components of the eruption included: (1) a directed blast of hot gases and ash which leveled the forest to the north for a distance of up to 15 miles, (2) a massive debris avalanche that deposited the remnants of the detached north flank of the volcano (approximately 3 billion cubic yards of rock, ash, pumice, snow, and ice) in the upper 17 miles of the North Fork Toutle River valley (Youd and Wilson, 1980), (3) a northward flow of fragmented volcanic materials that dammed the outlet of Spirit Lake, and (4) an ash column 10 miles high that traveled eastward across the United States as a giant ash cloud.
Previous geologic studies had determined that mudflows were common in past volcanic eruptions at Mount St. Helens (Crandell and Mullineaux, 1978). Thus, following the initial activity on March 27, mudflows were anticipated as part of a large eruption, although the streams most likely to be affected and the general magnitudes of flows were matters of conjecture. In preparation for potential flooding hazards, a flash flood warning system was devised by Cowlitz and Skamania county officials (see fig. 1).
Within minutes of the May 18 eruption, county personnel were ordered to previously-designated observation posts as part of the flood warning system. Simultaneously, residents along the major rivers that drain the volcano were instructed to prepare for hasty evacuation. The eruption quickly led to mudflows in the South Fork Toutle River and in the Lewis River tributaries of Smith Creek, Muddy River, and Pine Creek. Several hours later, a large mudflow came down the North Fork Toutle River after originating from the massive debris avalanche deposit.
This report describes the chronology and magnitude of mudflows on May 18 in tributaries of the Lewis River, and in the South and North Forks of the Toutle River. Particular attention is given to the chronology of events in the Toutle River system, with the North Fork mudflow given the greatest attention because of its relatively large magnitude and the consequent destruction.
Mudlfows are masses of water-saturated debris of various sizes that flow downslope as a result of gravity. Mudflows commonly resemble masses of wet concrete or mortar and, indeed, these terms were used by several observers to characterize the mudflows in the Toutle River system.
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