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Geomorphological Analysis of North Fork, Toutle River, Washington: 1980- 1984
ARMY ENGINEER WATERWAYS EXPERIMENT STATION VICKSBURG MS GEOTECHNICAL LAB
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The 18 May 1980 eruption of Mount St. Helens, emplaced between 3-3.5 billion cubic yards bcy of poorly sorted material ranging from silt to boulders, extending from river mile RM 25 to RM 38 in the upper North Fork Toutle River drainage. Sediment yields from the debris avalanche were calculated for four water years, 1980-1983. During 1980, the North Fork Toutle River reestablished a main channel from RMs 20 to 27, yielding 15 million cubic yards mcy. In 1981 the channel network lengthened by head cutting, and the network was further integrated to pond and lake breaches on the debris avalanche. Channel length and width increased with winter storms. Within the channel area, braiding and incision were the major processes which yielded 31.5 mcy of sediment. Channel widening, associated with braiding, incision, and bank failure, was the dominant geomorphic processes. A total of 99.4 mcy of sediment was calculated to have been derived from the debris avalanche between 19 May 1980 and September 1983.
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