| Sediment Retention Structure | |
| Name Official: | Sediment Retention Structure |
| Dam Crosses: | North Fork Toutle River |
| Location: | Cowlitz County, Washington, United States |
| Operator: | US Army Corps of Engineers |
| Dam Length: | 1888feet |
| Dam Height: | 184feet |
| Construction Began: | December 1986 |
| Opening: | December 1989 |
| Cost: | $65 million |
| Res Capacity Total: | 160000acre feet (now silted) |
| Res Surface: | 3200acres |
| Coordinates: | 46.3614°N -122.5517°W |
The Sediment Retention Structure is an earthen dam, 1888feet long and 184feet high, on the North Fork Toutle River in the U.S. state of Washington.[1] 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.[1]
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.[1]
In July 2012, the Corps awarded a $4.5 million contract to LKE Corporation of Washougal, to raise the spillway by 7feet.[2] In 2013, the contractor raised the dam's spillway by 7feet to increase the ability of the SRS to trap sediment.[3] 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.[4]
The SRS is about 13miles upstream from the confluence of the North Fork Toutle River with the Toutle River and 30.5miles from the larger river's mouth on the Cowlitz River.[4] The structure is about 45miles north-northeast of Portland, Oregon. Large communities downstream of the SRS include Castle Rock, Kelso, and Longview.[4]