Graduate Thesis Or Dissertation
 

Geomorphological analysis of North Fork Toutle River, Washington, 1980-1984

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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 RM's 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 million cubic yards of sediment. Channel widening, associated with braiding, incision, and bank failure were the dominant geomorphic processes during 1981 and 1982. By 1982, the North Fork Toutle headwaters had extended to the topographic divide at Spirit Lake. Related to this headward growth, sediment yields from the upper 7 river miles of the debris avalanche increased. This accelerated erosion induced a cascading hydrologic effect, increasing sediment yields to 34.1 mcy during 1982. By 1983, channel widening slowed and aggredation increased in the lower 8 river miles, facilitating an overall increase in sediment storage. Establishment of a stable ground water table within the debris avalanche and reduced channel incision induced a decreased bank failure and sediment transport promoting channel stabilization. This increased stability resulted in a 15.1 million cubic yard reduction in sediment yield, producing a net yield of 19.0 mcy. The North Fork Toutle River long profile showed rapid incision from 1980-1982. While 1982-1983 showed definite smoothing and concave profile development. Profile plots for 1982 and 1983 show that the system adjusted toward the sediment load and hydrologic conditions in the study area. A total of 99.4 million cubic yards of sediment was calculated to have been derived from the debris avalanche between 19 May 1980 - September 1983.
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