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Mount St. Helens Long Term Sediment Management Plan Supplemental Environmental Impact Statement

The sediment retention structure and upstream sediment plain on the North Fork Toutle River.

The eruption of Mount St. Helens in the spring of 1980 caused an enormous debris avalanche that deposited more than 3 billion cubic yards of sediment into the Toutle River basin. The avalanche deposit covered 32 square miles, with an average depth of approximately 145 feet. Mudflows from the avalanche filled the Cowlitz River channel and ran downvalley into the Columbia River. At the time, the U.S. Army Corps of Engineers initiated emergency actions to reduce the flood risk faced by communities along the Cowlitz River and restore the Columbia River navigation channel.

Recognizing that erosion from the debris avalanche would result in elevated sediment loads for several decades, in 1985 the Corps completed a long-term plan to manage sediment and mitigate the flood risk to downstream communities. Based on the 1985 plan, Congress authorized the Corps to construct, operate and maintain a sediment retention structure (SRS) and associated downstream actions necessary to provide flood risk reduction for the communities of Longview, Kelso, Castle Rock and Lexington. Subsequently, the Corps constructed the SRS on the North Fork Toutle River and improved levees along the lower 20 miles of the Cowlitz River. The Corps has also performed as-needed dredging within the lower Cowlitz River.

Because the SRS blocks upstream passage of salmon and steelhead, the Corps also constructed a Fish Collection Facility just downstream from the SRS. Fish are collected at the FCF and then transported via tank truck and released at one of two release sites on tributaries located upstream from the SRS. The State of Washington, via the Washington Department of Fish and Wildlife (WDFW), assumed ownership and responsibility for operation and maintenance of the FCF and release locations in 1993 and continues to operate the FCF.

Mount St. Helens Environmental Impact Statement timeline

Mount St. Helens SEIS Scoping Process

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The Corps developed SEIS alternatives through a multi-step process to identify, screen and refine a broad range of potential measures capable of addressing identified sediment issues. Based on data analysis and research, the Corps developed four alternatives. A brief summary is included here. For more information about the alternatives and how they were developed, please review the Alternatives section in the SEIS.

"No Action" alternative

Under the No Action alternative, the Corps would take no further action to manage sediment in the Toutle/Cowlitz River system. No changes to the SRS would be made and no dredging in the lower Cowlitz River would be undertaken to manage levels of protection for the lower Cowlitz River communities.


"Dredging Only" alternative

The Dredging Only alternative would rely solely on dredging to address sediment accumulation in the lower Cowlitz River and manage levels of protection to maintain authorized levels. Components of this alternative would be: dredging, dredged material placement and storage, and monitoring.


"SRS Raise" alternative

This alternative involves raising the SRS as the primary sediment management measure. It would raise the SRS spillway by 43 feet and would raise the top of the SRS dam by 30 feet. It would also involve constructing new outlet works consisting of four rows of eight 4-foot diameter pipes in each row (32 pipes total), allowing the modified SRS to function as it did when originally constructed. Construction of this alternative would take about 2 years.


"Phased Construction" alternative (preferred alternative)

The Phased Construction alternative involves up to two incremental raises, not to exceed 23 feet, of the SRS spillway crest elevation without raising the elevation of the top of dam; constructing grade-building structures in the sediment plain upstream of the SRS; and as-needed dredging in the lower Cowlitz River. Each phase of this alternative would be implemented only if and when needed. To determine whether a next phase would need to be constructed, the Corps would monitor hydrologic and sediment conditions in both the sediment plain and the lower Cowlitz River and decide whether conditions trigger the need for action. The three phases of the Phased Construction Alternative are sequential and are listed below in order of implementation:

  • Phase 1: First SRS spillway crest raise

  • Phase 2: Second/final SRS spillway crest raise

  • Phase 3: Grade building structures

For more information about the alternatives and how they were developed, please review the Alternatives section in the SEIS.


The Corps released its Draft Supplemental Environmental Impact Statement on Aug. 22, 2014. The public review and comment period closed on Oct. 21, 2014. 

Ongoing: Agency and tribal coordination

February / April 2013: Scoping process

August 2014: Notice of Availability for Draft EIS

Aug. 22 2014: Public Comment period opened

Sept. 10, 2014: Informational open house: Cowlitz County Expo Center 

Sept. 17, 2014: Informational open house: Toutle High School

Oct. 21, 2014: Public Comment period closed

Winter 2015: Respond to public comments 

Fall 2017: Publish Final EIS and sign Record of Decision

The Corps held information meetings about the sediment management project in September 2014.  

Public scoping meetings were also held in Toutle Lake and Longview, Wash., in March 2013.

To add your name to the Mount St. Helens SEIS email list for project updates, email us at

The initial stage of the SEIS process, scoping was used to identify issues, alternatives, and impacts that were addressed in the NEPA analysis. The Corps proposed four alternative plans for managing long-term sediment management.

Onetime SRS and spillway raise by 40 to 50 feet.

Annual dredging of the lower 20 miles of Cowlitz River.

Adaptive approach that includes additional raises of SRS Spillway (up to 23 feet); construction of small scale structures upstream of SRS (similar to GBS Pilot project effort) and infrequent dredging in the lower Cowlitz River if large events occur.

No action.

When considering public comments, the Corps focused on the four topics that scoping is designed to address:

  1. Is the geographic area to be analyzed in the SEIS sufficient to capture potential effects from the proposed alternatives?

  2. Have all potentially affected resources and the extent of analysis for those resources been identified?

  3. Are there known resources that may be adversely impacted by the proposed alternatives?

  4. What specific measures can we consider to avoid, minimize and mitigate effects of our proposals?


These and other questions helped the Corps develop the alternatives in the draft SEIS and includes public comments received during the scoping process.

Contact us

U.S. Army Corps of Engineers, CENWP-PM
ATTN: Mike Turaski
P.O. Box 2946
Portland, OR 97208-2946

(503) 808-4704