DESCRIPTION:
Mount St. Helens River Drainages, Washington

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

Streams that head on the volcano enter three main river systems -- the Toutle River on the north and north-west, the Kalama River on the west, and the Lewis River on the south and east. The streams are fed by abundant rain and snow that dump an average of about 140 inches of water on Mount St. Helens a year, according to National Weather Service data. The Lewis River is impounded by three dams for hydropower generation. The southern and eastern sides of the volcano drain into an upstream impoundment, the Swift Reservoir, which is directly south of the volcano.

Water-related recreation has been one of the major activities in the area. All three reservoirs on the Lewis River have been used extensively for recreation, as was Spirit Lake before 1980. Before the eruption, Spirit Lake was impounded in the North Fork Toutle River valley by a natural dam formed chiefly of deposits from one or more ancient mudflows. The principle resource of the region is timber, and many areas near the volcano had been logged recently and were still being logged at the beginning of the 1980 eruptive activity.

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 Cowlitz River drainage basin has an area of 2,480 square miles and includes the Toutle River basin, which was severely altered by the eruption and was inundated by mudflows. Prior to the eruption, the Toutle River was a typical Cascades Range stream, having a cobble bed, forested watershed, and headwaters at several glaciers. Devastation of the upper basin by the volcanic blast, the massive collapse of the volcano's north face into the North Fork Toutle River valley, and deposits from the resulting debris flows and mudflows provided enormous supplies of sediment for transport. Storm flows immediately eroded large volumes from the debris avalanche and mudflow deposits, and induced widespread collapse of unprotected bank material along the Toutle River. Gullying and channel extension on the North Fork Toutle River debris avalanche made additional volumes of sediment available for transport in subsequent years. Intensive and periodic sediment sampling began in the Toutle River basin immediately following the eruption.

On May 18, 1980, Mount St. Helens violently erupted. During the eruption, a massive debris avalanche, moving down the north side of Mount St. Helens, was blasted into the North Fork Toutle River valley, depositing approximately 3 billion cubic yards of material in the upper 17 miles of the valley. Mudflows quickly developed in the South Fork Toutle River and in the Lewis River tributaries of Smith Creek, Muddy River, and Pine Creek. At least 11,000 acre-ft of water, mud, and debris were deposited in Swift Reservoir between 9 a.m. and noon on May 18. A massive mudflow originated on the debris pile in the North Fork Toutle River valley and caused widespread destruction as it moved downstream through the Toutle and Cowlitz Rivers. Considerable deposition occurred in these river channels and in the channel of the Columbia River, which was closed to shipping for about 1 week. Channel capacity of the Cowlitz River was reduced from 76,000 to 7,300 cubic feet per second (at the flood stage of 23.0 feet). Dredging of the shipping channel in Columbia River commenced shortly after May 18, and dredging was started in July in the lower reach of Toutle River and Cowlitz River.

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