USGS/Cascades Volcano Observatory, Vancouver, Washington
DESCRIPTION:
Lewis River Drainage, Mount St. Helens, Washington
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[Map,18K,InlineGIF)
Major Hydrologic Features in the Mount St. Helens Region
-- Modified from: Crandell and Mullineaux, 1978, USGS Bulletin
1383-C
From:
Dinehart, 1992, Sediment Data for Streams near Mount St. Helens,
Washington: U.S.Geological Survey Open-File Report 91-219, p.2.
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Two major river basins in Washington State, The Cowlitz River and Lewis River
basins, were affected by excessive sediment loads following the May 18, 1980,
eruption of Mount St. Helens. ...
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The Lewis River basin has a drainage area of 1,046 square miles. The headwaters
of several tributaries to the Lewis River are on Mount St. Helens. Two
tributaries, the Muddy River and Pine Creek, were inundated by mudflows during
the May 18, 1980 eruption.
The Clearwater River basin,
while not affected my mudflows, received heavy tephra
deposits with the eruption blast. Annual suspended-sediment yields for the
Muddy River, Pine Creek, Clearwater Creek, and other Lewis River tributaries
increased from pre-eruption levels. Swift Reservoir, on the Lewis River, is
downstream from these tributaries, and most of the mudflow sediment was
deposited there.
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02/18/99, Lyn Topinka