Peak flow responses to landscape disturbances caused by the cataclysmic 1980 eruption of Mount St. Helens, Washington
Years of discharge measurements that precede and follow the cataclysmic 1980 eruption of Mount St. Helens, Washington, provide an exceptional opportunity to examine the responses of peak fl ows to abrupt, widespread, devastating landscape disturbance. Multiple basins surrounding Mount St. Helens (300‐1300 km 2 drainage areas) were variously disturbed by: (1) a debris avalanche that buried 60 km 2 of valley; (2) a lateral volcanic blast and associated pyroclastic fl ow that destroyed 550 km 2 of mature forest and blanketed the landscape with silt-capped lithic tephra; (3) debris fl ows that reamed riparian corridors and deposited tens to hundreds of centimeters of gravelly sand on valley fl oors; and (4) a Plinian tephra fall that blanketed areas proximal to the volcano with up to tens of centimeters of pumiceous silt, sand, and gravel. The spatially complex disturbances produced a variety of potentially compensating effects that interacted with and infl uenced hydrological responses. Changes to water transfer on hillslopes and to fl storage and routing along channels both enhanced and retarded runoff. Rapid posteruption modifi cations of hillslope surface textures, adjustments of channel networks, and vegetation recovery, in conjunction with the complex nature of the eruptive impacts and strong seasonal variability in regional climate hindered a consistent or persistent shift in peak discharges. Overall, we detected a short-lived (5‐10 yr) increase in the magnitudes of autumn and winter peak fl ows. In general, peak fl ows were larger, and moderate to large fl ows (>Q 2 yr ) were more substantively affected than predicted by early modeling efforts. Proportional increases in the magnitudes of both small and large fl ows in basins subject to severe channel disturbances, but not in basins subject solely to hillslope disturbances, suggest that eruptioninduced modifi cations to fleffi ciency along alluvial channels that have very mobile beds differentially affected fl ows of various magnitudes and likely played a prominent, and additional, role affecting the nature of the hydrological response.