PORTLAND, Ore. – The U.S. Army Corps of Engineers has completed the final supplemental environmental impact statement (EIS) for its long-term sediment management plan for Mount St. Helens. It is available for review and comment until Sept 10.
In the document, the Corps’ preferred alternative is the phased construction of two incremental spillway crest raises at the sediment retention structure (SRS) on the North Fork Toutle River, followed by the construction of grade-building structures on the sediment plain upstream from the main structure. Dredging of the lower Cowlitz River would be performed as needed.
In addition, to meet obligations under the Endangered Species Act (ESA), the Corps’ preferred alternative is to modify the Fish Collection Facility (FCF) to meet current National Marine Fisheries Service (NMFS) design criteria and establish a new fish release site.
The Corps and local sponsors have been managing flood risk at Mount St. Helens since the volcano’s 1980 eruption triggered a debris avalanche, which deposited more than three billion cubic yards of sediment into the Toutle River basin. To reduce flood risk, the Corps completed a long-term management plan that incorporated sediment management.
From this 1985 plan, Congress authorized the Corps to build, operate and maintain an SRS at North Fork Toutle River and perform associated actions to help reduce flood risk to communities downstream. Because the structure blocks upstream fish passage, the Corps also constructed a FCF just downstream, which is operated and maintained by the Washington Department of Fish and Wildlife (WDFW). Fish collected at the facility are transported by tanker trucks upstream for release.
The 1985 plan recognized that additional actions would be needed in the future to maintain the authorized levels of flood risk protection. The final supplemental EIS describes why the Corps’ ongoing involvement is critical, and lays out its plan for the coming decade and beyond.
“Our preferred alternatives take into account numerous changes to conditions in and around the project area since the original 1985 decision document was published and will allow the Corps to continue managing flood risk with communities along the lower Cowlitz River through 2035,” said Mike Turaski, project manager for the long-term sediment management plan. “In addition, this plan lays out the fish conservation measures to support the survival of endangered or threatened species.”
Such measures include establishing a third upstream fish release site at Deer Creek and the modification or replacement of the WDFW fish collection facility just downstream from the retention structure.
The new release site would be located in the Deer Creek tributary of the NF Toutle River. This site would increase critical spawning habitat for coho and steelhead collected at the FCF and transported above the SRS to spawn. After construction of the Deer Creek release site, fish trapped at the FCF would be released in one of three release sites on any given day: Hoffstadt/Bear Creek, Alder Creek, and Deer Creek.
The adaptive nature of the phased construction proposal is part of a cost-effective strategy for managing flood risk based on sediment in-filling conditions behind the retention structure, conditions in the lower Cowlitz River and the budgeting cycle for funding, explained Turaski.
The two spillway crest raises described in the phased construction proposal would increase the total height of the spillway by up to 23 feet. Once the spillway crest has been fully raised, grade-building structures described in the same proposal could be built well upstream of the retention structure to further assist in sediment management.
“Raising the spillway crest increases the sediment trapping efficiency at the sediment retention structure,” said Turaski. “Meanwhile, grade-building structures will help increase the rate of sediment accumulation, further raising the efficiency of the main structure.”
For each of the three phases, the decision to begin construction would depend on the results of the Corps’ flood protection level monitoring in leveed areas along the lower Cowlitz River — namely, in the Washington communities of Castle Rock, Kelso, Lexington and Longview. Each construction phase would only be launched if the authorized flood protection level for any of these leveed areas is at risk of not being met and the Corps determines the level of protection could not recover naturally.
The review period is intended to provide an opportunity for the public to review the analysis, the preferred alternatives and make comment as appropriate. The final document includes responses to comments received on the original and the revised draft supplemental EIS.
Public involvement is a vital part of the Corps’ planning process because it allows the agency to work with the public to better inform its decisions. The participation of interested citizens in this process ensures management decisions can be made based on all available information. Public comments allow the Corps to meet its public service goals while lowering the chances of unintended consequences to affected interests.
The final supplemental EIS is available for review and comment at: https://usace.contentdm.oclc.org/digital/collection/p16021coll7/id/7638.
Additional information may be found at the Mount St. Helens section of the Portland District website at http://www.nwp.usace.army.mil/mount-st-helens/.