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

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The Corps of Engineers is developing a long-term sediment management plan designed to manage the amount of sediment depositing in the lower Cowlitz River.  The purpose of the Mount St. Helens Sediment Retention Structure and associated features is to maintain flood risk protection at specified levels for the cities of Castle Rock, Lexington, Kelso and Longview, Wash.


The national Environmental Policy Act of 1970 established procedures for federal agencies to document their environmental impact analysis for proposed activities, such as long-term management planning for existing structures (40 Code of Federal Regulations 1500-1508). The existing NEPA document for the Mount St. Helens SRS was published in December 1984 (Final Mount St. Helens, Washington Feasibility Report & Environmental Impact Statement, Toutle, Cowlitz and Columbia Rivers).


The Corps has started drafting a Supplemental Environmental Impact Statement to address environmental changes that have occurred since the original EIS was published.  In order to update the original EIS, the SEIS will incorporate changes to the affected environment that have occurred over the past 30 years.  The SEIS will also analyze potential impacts from four proposed long-term management alternatives not previously discussed.

Mount St. Helens Environmental Impact Statement timeline

Mount St. Helens SEIS Scoping Process

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Scoping is the initial stage of the SEIS process used to identify issues, alternatives, and impacts to be addressed in the NEPA analysis. The Corps proposed four alternative plans for managing long-term sediment management. (Note: these four alternative plans could change based on feedback received during the SEIS scoping process.)

  1. Onetime SRS and spillway raise, by 40-50 feet.
  2. Annual dredging of the lower 20 miles of Cowlitz River.
  3. 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.
  4. No action.

When considering public comments, we focus 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?

If comments provided give input for any or all of these four topics, the Corps can use this feedback in determining the appropriate scope for the SEIS.