USGS/Cascades Volcano Observatory, Vancouver, Washington
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Posteruption suspended sediment transport at Mount St. Helens:
Decadal-scale relationships with landscape adjustments and river
discharges
-- J.J. Major, 2004,
Posteruption suspended sediment transport at Mount St. Helens:
Decadal-scale relationships with landscape adjustments and river
discharges:
Journal of Geophysical Research (Earth Surface), v. 109, F01002, doi:10.1029/2002JF000010.
Abstract
Widespread landscape disturbance by the cataclysmic 1980 eruption of Mount St.
Helens abruptly increased sediment supply in surrounding watersheds. The magnitude and
duration of the redistribution of sediment deposited by the eruption as well as decades- to
centuries-old sediment remobilized from storage have varied chiefly with the style of
disturbance. Posteruption suspended sediment transport has been greater and more
persistent from zones of channel disturbance than from zones of hillslope disturbance.
Despite the severe landscape disturbances caused by the eruption, relationships between
discharge magnitudes and frequencies and suspended sediment transport have been
remarkably consistent. Discharges smaller than mean annual flows generally have
transported <5%, but locally 15%, of the annual suspended sediment loads, and
infrequent (p < 0.01), large floods have transported as much as 50% of the annual
suspended sediment loads in a single day. However, moderate-magnitude discharges
(those greater than mean annual flows but less than 2-year floods) have transported the
greatest amounts of sediment from all disturbance zones. Such discharges have
transported, on average, 60% to 95% of the annual suspended sediment loads, usually
within cumulative periods of 1–3 weeks each year. Although small-magnitude and largemagnitude
discharges have locally and episodically transported considerable amounts of
suspended sediment, there has not been any notable change in the overall nature of the
effective discharges; moderate-magnitude flows have been the predominant discharges
responsible for transporting the majority of suspended sediment during 20 years of
posteruption landscape adjustment.
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02/25/04, Lyn Topinka