The sediment retention structure and upstream sediment plain on the North Fork Toutle River.
The eruption of Mount St. Helens in the spring of 1980 caused an enormous debris avalanche that deposited more than 3 billion cubic yards of sediment into the Toutle River basin. The avalanche deposit covered 32 square miles, with an average depth of approximately 145 feet. Mudflows from the avalanche filled the Cowlitz River channel and ran downvalley into the Columbia River. At the time, the U.S. Army Corps of Engineers initiated emergency actions to reduce the flood risk faced by communities along the Cowlitz River and restore the Columbia River navigation channel.
Recognizing that erosion from the debris avalanche would result in elevated sediment loads for several decades, in 1985 the Corps completed a long-term plan to manage sediment and mitigate the flood risk to downstream communities. Based on the 1985 plan, Congress authorized the Corps to construct, operate and maintain a sediment retention structure (SRS) and associated downstream actions necessary to provide flood risk reduction for the communities of Longview, Kelso, Castle Rock and Lexington. Subsequently, the Corps constructed the SRS on the North Fork Toutle River and improved levees along the lower 20 miles of the Cowlitz River. The Corps has also performed as-needed dredging within the lower Cowlitz River.
Because the SRS blocks upstream passage of salmon and steelhead, the Corps also constructed a Fish Collection Facility just downstream from the SRS. Fish are collected at the FCF and then transported via tank truck and released at one of two release sites on tributaries located upstream from the SRS. The State of Washington, via the Washington Department of Fish and Wildlife (WDFW), assumed ownership and responsibility for operation and maintenance of the FCF and release locations in 1993 and continues to operate the FCF.