Channel geometry, flood elevations, and flood maps, lower Toutle and Cowlitz rivers, Washington, June 1980 to May 1981

The volcanic eruption of Mount St. Helens on May 18, 1980, triggered mudflows that deposited upwards of 15 ft of sediment in the channels of the lower Toutle and Cowlitz Rivers. The major population areas along the lower Cowlitz River (Kelso, Longview,Lexington, and Castle Rock) were not flooded, but the channel capacity of the river was seriously reduced and the potential for unusually high flood elevations from fall and winter storms was an obvious concern. The U.S. Army Corps of Engineers began dredging operations in June 1980 to alleviate the flood hazard. Surveys to monitor the effect of changes to the channel and flood plains that resulted from dredging and additional sediment inflow from the upper Toutle River basin were started in June 1980 and continued until May 11, 1981, when dredging operations on the Cowlitz River had been completed. (USGS)