| 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. -- Lombard, et.al., 1981 |
