USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Peak flow responses to landscape disturbances caused by the cataclysmic 1980 eruption of Mount St. Helens, Washington
-- Major, J.J., and Mark, L.E., 2006,
Peak flow responses to landscape disturbances caused by the cataclysmic 1980 eruption of Mount St. Helens, Washington:
Geological Society of America Bulletin, July/August 2006,
v.118, no.7/8,
p.938-958.
Abstract
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 flows to abrupt,
widespread, devastating landscape disturbance.
Multiple basins surrounding Mount
St. Helens (300–1300 km2 drainage areas)
were variously disturbed by: (1) a debris avalanche
that buried 60 km2 of valley; (2) a lateral
volcanic blast and associated pyroclastic
flow that destroyed 550 km2 of mature forest
and blanketed the landscape with silt-capped
lithic tephra; (3) debris flows that reamed
riparian corridors and deposited tens to hundreds
of centimeters of gravelly sand on valley
floors; 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
influenced hydrological responses. Changes
to water transfer on hillslopes and to flow
storage and routing along channels both
enhanced and retarded runoff. Rapid posteruption
modifications 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 flows. In
general, peak flows were larger, and moderate
to large flows (>Q2 yr) were more substantively
affected than predicted by early
modeling efforts. Proportional increases in
the magnitudes of both small and large flows
in basins subject to severe channel disturbances,
but not in basins subject solely to
hillslope disturbances, suggest that eruption-induced
modifi cations to flow efficiency along
alluvial channels that have very mobile beds
differentially affected flows of various magnitudes
and likely played a prominent, and
additional, role affecting the nature of the
hydrological response.
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08/02/06, Lyn Topinka