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 ...

, long and high, on the North Fork Toutle River in the U.S. state of

Washington Washington most commonly refers to: * George Washington (1732–1799), the first president of the United States * Washington (state), a state in the Pacific Northwest of the United States * Washington, D.C., the capital of the United States ** A ...

. Completed by the

United States Army Corps of Engineers The United States Army Corps of Engineers (USACE) is the military engineering branch of the United States Army. A direct reporting unit (DRU), it has three primary mission areas: Engineer Regiment, military construction, and civil wo ...

in 1989, it is meant to prevent

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

from the

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

from increasing flood risks along the

Toutle Toutle is an unincorporated community in Cowlitz County, Washington. Etymology Both the town and Toutle River, derive their name from a band of the Skillot people of the Chinookan tribe, the ''Hullooetell''. Geography Toutle is located east of ...

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 Portland ( ) is the List of cities in Oregon, most populous city in the U.S. state of Oregon, located in the Pacific Northwest region. Situated close to northwest Oregon at the confluence of the Willamette River, Willamette and Columbia River, ...

. Large communities downstream of the SRS include

Castle Rock Castle Rock may refer to: Geography Islands * Castle Rock (Alaska), an island off the coast of the U.S. state of Alaska * Castle Rock, Hong Kong, an island of Hong Kong, part of the Po Toi Islands * Castle Rock (Massachusetts), an island in th ...

, Kelso, and Longview. MSHtoutleareaedit

References

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