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

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The eruption of Mount St. Helens in the spring of 1980 caused a large movement of sediment into surrounding rivers, threatening downstream communities in southwestern Washington with flooding.


Following the eruption, the U.S. Army Corps of Engineers, Portland District, implemented several strategies to mitigate the flood risk to downstream communities. The Corps developed a Long-Term Plan to mitigate for increased flood risk. Features of the 1985 plan included constructing the Sediment Retention Structure, levee improvements on the lower 20 miles of the Cowlitz River and as-needed dredging within the lower Cowlitz River.


The original 1985 Mount St. Helens Long-Term Plan recognized that additional actions would be needed in the future. This Draft Supplemental Environmental Impact Statement is an update of the 1985 Long-Term Plan. To review the original, the Final Mount St. Helens, Washington, Feasibility Report & Environmental Impact Statement, for Toutle, Cowlitz and Columbia rivers, you will find a link in the Documents section below. We encourage individuals, organizations, federal and state agencies to review the draft SEIS and provide comments regarding impacts we may have missed or possible actions not included in the draft SEIS. The public comment period has been extended until Oct. 21, 2014. For more information see the "Submit Your Comments" section below.

Mount St. Helens Environmental Impact Statement timeline

Public involvement information

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The Corps of Engineers developed the 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 wants individuals, organizations and federal, local and tribal agencies to review these alternatives and provide comments. It encourages comments relating to actions which may not have been discussed, or impacts not yet reviewed.

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


Comments may be made in writing, either electronically or by mail. You can submit comments on the draft SEIS by email to: MSHLongTermPlan@usace.army.mil or by sending written comments to:

Tim Kuhn
U.S. Army Corps of Engineers, Portland District
P.O. Box 2946
Portland, OR 97208-2946

Ongoing: Agency and tribal coordination

February / April 2013: Scoping process

August 2014: Notice of Availability for Draft EIS

Aug. 22 2014: Public Comment period opens

Sept. 10, 2014: Informational open house: Cowlitz County Expo Center, 5 – 7:30 p.m.

Sept. 17, 2014: Informational open house: Toutle High School, 5 – 7:30 p.m.

Oct. 21, 2014: Public Comment period extended to Oct. 21, 2014

Winter 2015: Respond to public comments and prepare Final EIS

Spring 2015: Publish Final EIS and sign Record of Decision

The Corps welcomes comments on this draft SEIS. Toprovide information about the MSH sediment management project or provide comments on the draft SEIS, the Corps hosted two informational public meetings in September 2014.  


Open house informational meetings:

Sept. 10, 2014, 5 p.m. to 7:30 p.m.

Cowlitz County Expo Center

1900 7th Ave, Longview, Wash.

 Sept. 17, 2014, 5 p.m. to 7:30 p.m.

Toutle High School

5050 Spirit Lake Hwy., Toutle, Wash.


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

To join the Mount St. Helens SEIS email list, email us at MSHLongTermPlan@usace.army.mil and you will be sent periodic updates in the form of informational emails. Information in the updates may include availability of the new information, publication of the draft Supplemental Environmental Impact Statement or the final SEIS.

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.