Mount St. Helens

Long-Term Sediment Management Plan

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.

The Corps' Ongoing Role

The 1985 Long-Term Plan recognized that additional actions would be needed in the future to maintain the authorized levels of flood risk reduction. The Corps is currently evaluating how to best manage sediment to provide authorized levels of flood risk reduction through the year 2035, in a manner that does not jeopardize the continued existence of any endangered or threatened species or result in the destruction or adverse modification of designated critical habitat.

USACE has conducted a limited re-evaluation of sediment management in the North Fork Toutle River and is proposing to implement updated sediment management measures to manage flood risk to established levels for the Washington cities of Castle Rock, Lexington, Kelso, and Longview through the year 2035. In addition to a No Action alternative, three sediment management alternatives were identified and evaluated that would address the flood risks associated with sediment build up through 2035. They are:
• Cowlitz River dredging only
• SRS spillway and embankment raise
• A phased construction plan (preferred alternative) consisting of:
  o SRS spillway crest raises
  o Grade-building structures
  o Dredging, as needed

The FSEIS also includes an evaluation of the fish conservation measures that were developed to ensure the proposed sediment management plan will not jeopardize the continued existence of any endangered or threatened species or adversely modify or destroy the species’ designated critical habitat. Fish conservation measures evaluated include:
• Replace the fish collection facility on the North Fork Toutle River operated and maintained by the Washington Department of Fish Wildlife (WDFW) and establish a fish release site on Deer Creek
• Modify the fish collection facility on the North Fork Toutle River and establish a fish release site on Deer Creek

Environmental impacts of the no action, sediment management alternatives and fish conservation measures are evaluated in the FSEIS.

The Corps’ preferred alternative for managing sediment and flood risk includes phased construction of spillway crest raises at the SRS; grade building structures on the sediment plain upstream from the SRS; and, dredging as needed to maintain the authorized levels of protection. The Corps’ preferred alternative for improving fish passage includes modify the existing FCF and constructing one additional fish release site.

The National Environmental Policy Act of 1969

The National Environmental Policy Act of 1969 is one of the nation’s oldest environmental laws that encourages federal agencies to make environmentally responsible decisions. The Act requires all federal agencies to consider and disclose the environmental effects of their proposed actions in an environmental impact statement.

Collecting the right level of information at the right time is important to developing a plan, requiring early and frequent engagement of all affected federal, state and local agencies, Native American Tribes, and interested groups and individuals. Public review is an important step in the process wherein the public is invited to provide substantive information and identify issues and potentially significant effects of the government's proposal.

These subjects are examples of environmental effects among the biological, physical, natural, social and economic categories considered under NEPA:

Air Quality
Cultural Resources
Floodplain Management
Economic Impacts
Environmental Justice
Land Use
Migratory Birds
Sediment Transport, Deposition, Composition
Social Considerations
Endangered Species Act
Tribal Interests
Water Quality

Public comments help the Corps:

  • Define the breadth of environmental resources and affects to evaluate
  • Identify alternatives to be considered
  • Determine new sources of data or information
  • Identify and eliminate from detailed study issues that are not significant or that have been covered by prior environmental review

Your Role in the Process

The U.S. Army Corps of Engineers (USACE), Portland District, is providing for public review, the Final Supplemental Environmental Impact Statement (FSEIS) for the Mount St. Helens Long-Term Sediment Management Plan. The public review period is intended to provide those interested in or affected by this action an opportunity to review the FSEIS. Upon completion of the public review period, USACE will consider any comments submitted in the preparation of the record of decision (ROD). The ROD will document the USACE’s decision.

All comments received will become part of the administrative record and are subject to public release under the Freedom of Information Act, including any personally identifiable information such as name, phone numbers and addresses.

The FSEIS will be available for public review for 30 days after publication of the notice of availability in the Federal Register by the U.S. Environmental Protection Agency.  Comments must be received or postmarked by September 10, 2018. 

Written comments may be sent via email to or by traditional mail to:

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

Thank you for your interest in this project!