Toutle/Cowlitz River Sediment Budget
BIEDENHARN GROUP LLC VICKSBURG MS
Pagination or Media Count:
Following the dramatic eruption of Mount St. Helens on 18 May 1980 and the deposition of approximately 3 billion cubic yards of primarily sand and gravel material in the upper 17 miles of the North Fork of the Toutle River, significant urban and industrial flooding occurred along the lower 20 miles of the Cowlitz River and the Columbia River s navigation channel was blocked between river miles RM 60 and 72. Subsequent mudflows and sedimentation problems along the lower Toutle and Cowlitz Rivers from 1981 to 1986 required the investigation and implementation of permanent measures by the U.S. Army Corps of Engineers USACE to address the long term impacts of the Mount St. Helens eruption. The Mount St. Helens MSH Project was formulated to control the movement of large amounts of sediment downstream from the debris avalanche resulting from the May 18, 1980 eruption and maintain a congressionally authorized level of flood protection along the lower Cowlitz River. Other significant sources of sediment in the Toutle watershed have also been identified as contributing to the overall supply to the Cowlitz River. The increase in sediment available for transport downstream to the Cowlitz River has contributed to decreasing levels of flood protection on the lower twenty miles of the Cowlitz River due to loss of channel conveyance and hydrologic trends in the basin. Figure 1.1 is a vicinity map of the Toutle and Cowlitz Rivers.
- Hydrology, Limnology and Potamology