Sediment Retention Structure

The Sediment Retention Structure is an earthen dam, long and high, on the North Fork Toutle River in the U.S. state of Washington. Completed by the United States Army Corps of Engineers in 1989, it is meant to prevent sediment from the 1980 eruption of Mount St. Helens from increasing flood risks along the Toutle and Cowlitz rivers. The Corps initially expected the basin behind the dam to be filled by 2035 with 258 million cubic yards (197 million m3) of sediment. However, additional debris flows from Mount St. Helens plus sediment from storm-related riverbank failures quickly filled the basin. It was nearly full by 2012, and large amounts of sediment overflowed the SRS and continued downstream. In July 2012, the Corps awarded a $4.5 million contract to LKE Corporation of Washougal, to raise the spillway by . In 2013, the contractor raised the dam's spillway by to increase the ability of the SRS to trap sediment. The higher spillway reduces the slope of the sediment plain behind the dam, thus allowing more debris to accumulate on top of existing debris. The Corps estimated that the higher spillway would maintain the trapping efficiency of the SRS at or above 31 percent—the fraction being trapped without the higher spillway—for 5 to 10 years after completion of the project. The SRS is about upstream from the confluence of the North Fork Toutle River with the Toutle River and from the larger river's mouth on the Cowlitz River. The structure is about north-northeast of Portland, Oregon. Large communities downstream of the SRS include Castle Rock, Kelso, and Longview.