HR: 14:55h
AN: H42J-06    [PDF]
TI: Extreme events and geomorphic resilience-insight from response to the cataclysmic 1980 Mount St. Helens eruption
AU: * Major, J J
EM: jjmajor@usgs.gov
AF: USGS, 1300 SE Cardinal Court, Vancouver, WA 98683 United States
AB: Extreme geomorphic events affect sediment transfer, landform development, and landscape evolution in mountain environments. Less understood are the magnitude and persistence of geomorphic responses to extreme events and the impacts those events have on various geomorphic regimes. Generally, states of geomorphic disequilibrium caused by extreme as well as by lesser events are transient. Periods of geomorphic response to extreme events, however, can range from years to millennia; defining recovery and resilience of geomorphic systems, and thus the long-term impact of an extreme event, depends upon spatial scale and the geomorphic regime (e.g., sediment transport, water discharge, or their associated relationship) examined. Varying geomorphic-regime responses are highlighted by post-eruption responses to the 1980 Mount St. Helens (MSH) eruption. The cataclysmic 1980 MSH eruption altered runoff and abruptly increased sediment supply in several watersheds. In basins ranging in area from 300-1300 km$^{2}$, post-eruption peakflow discharges increased by tens of percent for at least 5, and perhaps as long as 20, years. Post-eruption infiltration capacity quickly recovered a substantial proportion of pre-eruption capacity, predominant overland flow diminished rapidly, and runoff conditions approached a pre-eruption state. In comparison, post-eruption suspended-sediment yields increased by a much greater proportion, and have persisted much longer, than increased water discharges. Annual yields as much as 500 times greater than pre-eruption values were measured shortly after the eruption, and yields 10-100 times greater than pre-eruption values persist. Magnitudes and durations of post-eruption sediment delivery have varied chiefly with the nature of volcanic impact. Extraordinary suspended-sediment delivery has been greater and more persistent from zones of channel disturbance than from zones of hillslope disturbance. A classical observation in many "undisturbed" watersheds that moderate discharges do the most work moving sediment also holds true for the MSH basins so dramatically affected by volcanism. Moderate-magnitude discharges (those greater than the mean annual flows but less than 2-year floods) have transported 60% to about 90% of the suspended sediment redistributed during 20 years of post-eruption landscape adjustment. Post-eruption geomorphic responses at MSH suggest that lasting impacts of extreme events are reflected characteristically by long-term sediment delivery from watersheds. Extreme events do not fundamentally alter geomorphic processes or effective-discharge relations; perturbations in runoff and water discharge typically dissipate within several years. Recovery from and resilience to extreme events are therefore defined best by examining locations and magnitudes of long-term sediment transport.
DE: 1824 Geomorphology (1625)
DE: 1860 Runoff and streamflow
DE: 8404 Ash deposits
SC: Hydrology [H]
MN: 2003 Fall Meeting