HR: 16:00h
AN: V14A-02 INVITED     [Abstracts]
TI: Response of peakflow discharges to the 1980 Mount St. Helens eruption: Seasonality and effects of channel flow resistance
AU: * Major, J J
EM: jjmajor@usgs.gov
AF: USGS, 1300 Cardinal Ct, Vancouver, WA 98683
AU: Mark, L E
AF: USGS, 1300 Cardinal Ct, Vancouver, WA 98683
AB: Decades of streamflow measurements that precede and follow the major 1980 eruption of Mount St. Helens (MSH) provide an exceptional opportunity to examine the response of peakflow discharges in large (300-1300 km² drainage area) basins to devastating landscape disturbance. About 100,000 ha of landscape across multiple basins surrounding MSH were reconfigured by volcanic processes that included a huge debris avalanche, a widespread lateral blast and associated pyroclastic flow, large (to 10⁸ m³) debris flows, and thick (>5 cm) plinian tephra fall. We determined the nature, longevity, and seasonality of discharge responses by linearly regressing logarithms of seasonal unit-area peakflow discharges compiled from pre- and post-eruption hydrographs from disturbed basins against paired discharges from a nearby control basin relatively unaffected by the major and smaller 1980 eruptions, and then compared regression models. We tested the null hypothesis that seasonal pre- and post-eruption regression models were coincident versus an alternative hypothesis that they were unequal. From 1980 to 1984, autumn discharges increased by a few to many tens of percent in basins heavily disturbed by the eruption. In contrast, pre- and post-eruption regression models for other seasons and later time periods do not differ significantly. Hydrologic responses to the eruption were thus strongly seasonal and short lived at the scale examined. Contrary to model predictions that smaller discharges would increase proportionately more than larger discharges, based chiefly on postulated changes in soil moisture and hillslope runoff, we found discharges across a range of magnitudes increased nearly proportionately relative to predisturbance regression models of unit-area discharges in several basins that sustained significant channel disturbance. Proportionate increases among small and large discharges are inconsistent with changes simply in hillslope hydrology. We attribute the proportionate increases in discharges to eruption-induced variations in channel geometry and flow resistance. We contend that transport of large amounts of easily erodible sediment created conditions that differentially affected flows of various magnitudes and that those conditions played as key a role in magnifying discharges as did perturbations to hillslope hydrology.
DE: 1824 Geomorphology (1625)
DE: 1860 Runoff and streamflow
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly