The volcanic eruption of Mount St. Helens on May 18, 1980,
triggered mudflows on the South and North Forks of the Toutle
River that deposited up to 15 ft of sediment in the channels of
the lower Toutle and Cowlitz Rivers. The thickness of the sediment deposited on the flood plain ranged from almost none just upstream from Kelso to about 10 ft near Castle Rock and the
mouth of the Toutle River. Existing levees in Castle Rock, Kelso, and Longview contained the mudflows and prevented flooding of those communities. The mudflow reduced the within-channel flow capacity, except at Kelso and Longview where the effect was minor. The potential exists for unusually high flood levels, up to 10-ft higher than normal, from autumn and winter precipitation. Planned flood-alleviation measures include dredging, additional levees, and sediment-retention structures. A flood-warning system has been established, and the current potential for flooding is being monitored through continued surveillance of the river system.
The explosive volcanic eruption of Mount St. Helens on May 18, 1980, triggered mudflows in several streams that head on the flanks of the volcano. Mudflows that originated in the North and South Forks of the Toutle River were especially significant because of their immense magnitudes and consequent destruction. These two mudflows deposited large volumes of sediment in the North and South Forks of the Toutle River; and in the Toutle, lower Cowlitz, and Columbia Rivers (see fig. 1). In the lower Toutle and Cowlitz River valleys as much as 15 ft of mud and debris were deposited in the river channels, with up to 10 ft on the adjacent flood plains. These deposits destroyed buildings, covered agriculturally productive bottomlands, killed extensive tracts of timber, and seriously reduced the flow carrying capacities of the channels of the Cowlitz and Toutle Rivers. In addition, the volcanic eruption provided immense volumes of erodible material in the upper Toutle River system that potentially could be transported into the lower reaches of the Toutle and Cowlitz Rivers to further impact the channels.
The potential for significant flooding was immediately evident. More than 40,000 people live in the areas affected by the mudflow deposits and Interstate Highway 5, the major north-south highway in western Washington, was at risk. Flood mitigation measures were begun immediately and included such activities as channel dredging, levee raising, reservoir construction, and flood warning systems.
This report describes: (1) changes in the channels and flood plains of the lower Toutle and Cowlitz Rivers as a result of the May 18-19, 1980, mudflows; (2) the flood hazards that existed in the Cowlitz River immediately after the mudflows; (3) the flood mitigation system created. Information on the morphologic changes resulting from the mudflows was obtained by field surveys and observations of May and early June 1980. Potential flood stages and inundated limits reported herein were defined by the Geological Survey from computer models for channel conditions observed following the mudflow deposition. Even though the flood potential obviously would be altered by the flood mitigation activities and possibly by subsequent mudflows, they are reported here to document the magnitude of the hazard that existed immediately after the eruption.
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