Toutle River Sediment Retention Structure

The Sediment Retention Structure is an earthen dam, 1,888 feet (575 m) long and 184 feet (56 m) 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]

Sediment Retention Structure
Aerial photograph looking upriver with Mount St. Helens in the background
Official name	Sediment Retention Structure
Location	Cowlitz County, Washington, United States
Coordinates	46°21′41″N 122°33′06″W
Construction began	December 1986
Opening date	December 1989
Construction cost	$65 million
Operator(s)	US Army Corps of Engineers
Dam and spillways
Impounds	North Fork Toutle River
Height	184 ft (56 m)
Length	1,888 ft (575 m)
Reservoir
Total capacity	160,000 acre⋅ft (200,000,000 m3) (now silted)
Surface area	3,200 acres (13 km2)

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 7 feet (2.1 m).^[2] In 2013, the contractor raised the dam's spillway by 7 feet (2.1 m) 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.^[4] 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 13 miles (21 km) upstream from the confluence of the North Fork Toutle River with the Toutle River and 30.5 miles (49.1 km) from the larger river's mouth on the Cowlitz River.^[4] The structure is about 45 miles (72 km) north-northeast of Portland, Oregon. Large communities downstream of the SRS include Castle Rock, Kelso, and Longview.^[4]

Major changes in hydrology of Mt. Saint Helens after its 1980 eruption, including artificial modifications designed to mitigate sediment and water flow

References

1. Denlinger, Roger P. (2012). "Effects of Catastrophic Floods and Debris Flows on the Sediment Retention Structure, North Fork Toutle River, Washington" (PDF). United States Geological Survey. pp. 1–2. Retrieved February 9, 2013.
2. "Corps Awards Mount St. Helens SRS Spillway Contract". United States Army Corps of Engineers. July 20, 2012. Retrieved February 10, 2013.
3. "Cowlitz River deepens slightly for first time since Mount St. Helens eruption". 7 April 2014.
4. "Final Environmental Assessment: Sediment Retention Structure (SRS) Spillway Raise Project" (PDF). United States Army Corps of Engineers. June 20, 2012. pp. 1–5. Retrieved February 10, 2013.