The Sediment Retention Structure is an

earthen dam An embankment dam is a large artificial dam. It is typically created by the placement and compaction of a complex semi-plastic mound of various compositions of soil or rock. It has a semi-pervious waterproof natural covering for its surface and ...

, long and high, on the

North Fork Toutle River The North Fork Toutle River is a tributary of the Toutle River in southwestern Washington in the United States. The river has its headwaters near Spirit Lake, on the north side of Mount St. Helens, and flows to the Toutle River, about upstream ...

in the U.S. state of

Washington Washington commonly refers to: * Washington (state), United States * Washington, D.C., the capital of the United States ** A metonym for the federal government of the United States ** Washington metropolitan area, the metropolitan area centered on ...

. Completed by the

United States Army Corps of Engineers , colors = , anniversaries = 16 June (Organization Day) , battles = , battles_label = Wars , website = , commander1 = ...

in 1989, it is meant to prevent

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...

from the

1980 eruption of Mount St. Helens On March 27, 1980, a series of volcanic explosions and pyroclastic flows began at Mount St. Helens in Skamania County, Washington, United States. A series of phreatic blasts occurred from the summit and escalated until a major explosive eru ...

from increasing flood risks along the Toutle and

Cowlitz Cowlitz may refer to: People * Cowlitz people, an indigenous people of the Pacific Northwest ** Cowlitz language, member of the Tsamosan branch of the Coast Salish family of Salishan languages * Cowlitz Indian Tribe, a federally recognized tribe of ...

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 Washougal ( ) is a city in Clark County, Washington, United States. The population was 17,039 as of the 2020 census. History Washougal was officially incorporated on December 4, 1908. Its Mount Pleasant Grange Hall is the oldest continually u ...

, 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 Portland (, ) is a port city in the Pacific Northwest and the largest city in the U.S. state of Oregon. Situated at the confluence of the Willamette and Columbia rivers, Portland is the county seat of Multnomah County, the most populous co ...

. Large communities downstream of the SRS include Castle Rock, Kelso, and Longview. MSHtoutleareaedit

References

{{reflist Dams in Washington (state) Buildings and structures in Cowlitz County, Washington Dams completed in 1989 Mount St. Helens