PORTLAND, Ore. -- PORTLAND, Ore. — In an effort to manage flood risk for downstream communities, the U.S. Army Corps of Engineers will raise the dam that captures some of the sediment that continues to flow from Mount St. Helen after its 1980 eruption.
USACE awarded a $35.9 million contract to Anvil Builders, Inc., to raise the 184-foot-tall Mount St. Helens Sediment Retention Structure (SRS) by 10 feet.
The contractor is scheduled to begin construction on the earthen dam this summer and is expected to complete the work by October 2027.
USACE completed the sediment retention structure on the North Fork of the Toutle River in 1989 to retain some of the sediment flowing from Mount St. Helens into the Toutle and Cowlitz rivers. This retention process also prevents the sediment from continuing downstream where it would reduce river channel capacity.
"This crest raise is a critical step in our long-term commitment to public safety and the region's economic vitality," said Jeff Hicks, Portland District Deputy Chief of Planning, Programs, and Project Management Division. "By increasing the capacity of the Sediment Retention Structure, the U.S. Army Corps of Engineers is ensuring it can effectively trap sediment for years to come.”
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BACKGROUND
The 1980 eruption of Mount St. Helens caused a massive debris avalanche, sending billions of cubic yards of mud, rock, and volcanic ash into the North Fork Toutle River. That’s enough debris to fill Lumen Field, the Seattle Seahawks’ home field, nearly 500 times. The sediment continues to erode and travel downstream, raising the riverbed of the Cowlitz River and increasing flood risk for over 50,000 people in the communities of Longview, Kelso, Castle Rock, and Lexington.
The SRS is an earthen dam that acts as a barrier, slowing the river's flow and causing the sediment to settle and collect behind the structure.
Since the eruption, USACE and Washington state have continued to work together to maintain the flood levels the project was designed to handle.
In 1985, Congress authorized the Mount St. Helens Sediment Control Project as a long-term strategy that included the SRS construction, levee improvements, dredging, and a fish collection facility. The plan also allows for additional construction, like the upcoming crest raise, as conditions warrant to manage the ongoing sediment flow.
For more information on the Mount St. Helens SRS, visit: https://www.nwp.usace.army.mil/Locations/Mount-St-Helens/