DESCRIPTION:
Lewis River Drainage, Mount St. Helens, Washington

[Map,18K,InlineGIF)
Major Hydrologic Features in the Mount St. Helens Region
-- Modified from: Crandell and Mullineaux, 1978, USGS Bulletin 1383-C

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. ...

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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