Major disruptions to the channels of the Toutle River system were caused predominantly by (1) the massive debris avalanche that virtually obliterated the drainage network of the upper North Fork Toutle River, reduced its contributing drainage area to about 80 square kilometers, and raised its valley-bottom elevations by as much as 140 meters; (2) lahars that swept down the lower North Fork Toutle River, South Fork Toutle River, and Toutle River main stem; (3) airfall deposition of a fine-grained tephra layer on hillslope surfaces in the upper North Fork Toutle River and upper Green River Basins; and (4) blowdown by the lateral blast of large tracts of forests on hillslopes and into stream channels in the upper Green River Basin. With the exception of the Green River, channels were transformed from low sinuosity gravel-cobble streams to sand-bed streams with smoothed boundaries and straighter alignments. The net result of these changes was an increase in the magnitude and frequency of flood flows through 1984. Hillslope erosion by sheet wash, tilling, and gullying attained peak rates in the upper Green and North Fork Toutle River Basins during the first wet season after the eruption and then decreased substantially and became negligible by 1983.
The most severely affected subbasin was the upper North Fork Toutle River. Depressions formed on the surface of the debris avalanche by phreatic explosions of trapped super-heated water and by subsidence of the deposit. Water from runoff and subsurface seepage filled and spilled out of these closed depressions to initiate the development of a new drainage network. By the end of 1982, the contributing drainage area of this subbasin had increased to the pre-eruption level of 282 square kilometers. Channel evolution on the debris avalanche deposit was dominated by channel widening (hundreds of meters), although depths of degradation were 30 meters in some reaches. Changes in channel widths relative to changes in channel depths were generally similar to pre-disturbed width-to-depth ratios (60 to 100). Channel widening resulted in reduced flow depths, consequent increases in hydraulic roughness and, therefore, decreased flow velocities. In combination with bed-material coarsening, these morphologic changes resulted in rapid reductions in the rate of energy dissipation.
A dimensionless exponential decay function was used to describe the temporal variation in bed elevations for sites along all of the major drainages of the Toutle River system. The total dimensionless change in bed elevation was plotted against river kilometer and served as an empirical model of initial and secondary bed-level responses. Channel adjustments along the lahar-affected South Fork Toutle River were considerably less dramatic than along the North Fork because of dissimilar impacts which resulted in smaller changes to available stream energy. Annual sediment yields for the South Fork Toutle River were generally an order of magnitude lower than for the North Fork, but were still dominated by channel widening. The greatest adjustments along the Green River occurred along the lower 2 kilometers of the stream that had been inundated by the North Fork lahar.
Channel evolution consisted of a five-step sequence that was characterized by the shifting dominance of fluvial and mass-wasting processes: (1) channel formation, development, or disruption; (2) degradation if upstream from the area of maximum disturbance, aggradation if downstream from the area of maximum disturbance; (3) channel widening; (4) channel widening with aggradation if upstream from the area of maximum disturbance, degradation if downstream from the area of maximum disturbance; and (5) channel widening with scour and fill, and the initiation of floodplain development by valley sidewall collapse and retreat.
The dominance of various adjustment processes was described in terms of flow-energy principles and the minimization of the rate of energy dissipation. Degradation in combination with channel widening was a common response in this disturbed system and was determined to be the most effective means of minimizing the rate of energy dissipation, because all components of total-mechanical energy (datum head, pressure head, and velocity head) decreased simultaneously. Reductions in hydraulic depth and increases in bed-material particle size confirmed the temporal increase in hydraulic roughness. In combination with nonlinear reductions in boundary shear stress, increases in critical shear stress resulted in nonlinear reductions in excess shear stress and the capacity of streams to transport bed-material sediment. Thus, available stream energy and shear stress acted in concert with critical shear stress to attain a new equilibrium condition.
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