Toutle River Sediment Retention Structure
{{Infobox dam
| name =Sediment Retention Structure
| image =USACE sediment retention structure Toutle River.jpg
| image_size = 300px
| image_caption =Aerial photograph looking upriver with Mount St. Helens in the background
| name_official =Sediment Retention Structure
| dam_crosses = North Fork Toutle River
| res_name =
| location = Cowlitz County, Washington, United States
| operator = US Army Corps of Engineers
| dam_length ={{convert|1888|ft|m|abbr=on}}
| dam_height ={{convert|184|ft|m|abbr=on}}
| dam_width_base =
| construction_began =December 1986
| opening =December 1989
| cost =$65 million
| res_capacity_total ={{convert|160000|acre feet|m3|abbr=on}} (now silted)
| res_catchment =
| res_surface ={{convert|3200|acre|km2}}
| coordinates = {{coord|46|21|41|N|122|33|06|W|type:landmark_region:US-WA|display=inline,title}}
| extra =
}}
The Sediment Retention Structure is an earthen dam, {{convert|1888|ft|m}} long and {{convert|184|ft|m}} high, on the North Fork Toutle River in the U.S. state of Washington.{{cite web|last=Denlinger|first=Roger P.|year=2012|title=Effects of Catastrophic Floods and Debris Flows on the Sediment Retention Structure, North Fork Toutle River, Washington|url=http://pubs.usgs.gov/of/2011/1317/of2011-1317.pdf|publisher=United States Geological Survey|pages=1–2|accessdate=February 9, 2013}} 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 {{convert|7|ft|m}}.{{cite web|title=Corps Awards Mount St. Helens SRS Spillway Contract|publisher=United States Army Corps of Engineers|url=http://www.nwp.usace.army.mil/Media/NewsReleases/tabid/1888/Article/2585/corps-awards-mount-st-helens-srs-spillway-contract.aspx|archive-url=https://web.archive.org/web/20120821013117/http://www.nwp.usace.army.mil/Media/NewsReleases/tabid/1888/Article/2585/corps-awards-mount-st-helens-srs-spillway-contract.aspx|url-status=dead|archive-date=August 21, 2012|date=July 20, 2012|accessdate=February 10, 2013}} In 2013, the contractor raised the dam's spillway by {{convert|7|ft|m}} to increase the ability of the SRS to trap sediment.{{Cite web|url=http://tdn.com/news/local/cowlitz-river-deepens-slightly-for-first-time-since-mount-st/article_400a5b8c-bdd0-11e3-b2c9-001a4bcf887a.html|title = Cowlitz River deepens slightly for first time since Mount St. Helens eruption| date=7 April 2014 }} 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.{{cite web|title=Final Environmental Assessment: Sediment Retention Structure (SRS) Spillway Raise Project|url=http://www.nwp.usace.army.mil/Portals/24/docs/locations/msh/Final_EA_SRS_Spillway_Raise.pdf|archive-url=https://web.archive.org/web/20130220185320/http://www.nwp.usace.army.mil/Portals/24/docs/locations/msh/Final_EA_SRS_Spillway_Raise.pdf|url-status=dead|archive-date=February 20, 2013|pages=1–5|publisher=United States Army Corps of Engineers|date=June 20, 2012|accessdate=February 10, 2013}}
The SRS is about {{convert|13|mi|km}} upstream from the confluence of the North Fork Toutle River with the Toutle River and {{convert|30.5|mi|km}} from the larger river's mouth on the Cowlitz River. The structure is about {{convert|45|mi|km}} north-northeast of Portland, Oregon. Large communities downstream of the SRS include Castle Rock, Kelso, and Longview.
