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Skagit River Rehabilitation of Flood Control Works
Skagit County, Washington, United States
The U.S. Army Corps of Engineers (USACE) Seattle District (NWS) completed a combined total of 60 emergency levee repairs in 2007 and in 2011 on the lower Skagit River. Habitat features that could function as riverine edge habitat were incorporated with the repairs; the riverine edge habitat had been lost over the years as a result of numerous riverbank modifications within the lower Skagit River Valley. Thirteen levee repair sites with mitigation features were subsequently studied to evaluate whether they sufficiently compensated for the loss of habitat functions—specifically those functions impacting juvenile salmon—in terms of meeting project goals. Overall, most features examined in 2017 were functioning as planned, providing rearing and refuge habitat as well as foraging opportunities; the features also improved the riparian corridor.
Article cover: Woody debris is an essential component for salmon rearing and refuge habitat complexity. Large logs with rootwads were laid on the riverward bench of the levee as a mitigation feature that delivers benefits to fish during high river flows. (Photo by USACE Seattle District)
Producing Efficiencies
To provide compensatory mitigation for any detrimental effects of levee repair on edge habitat, the USACE NWS and project sponsors developed a multi-purpose tool, the Habitat Capacity Mitigation Tool (HCMT), to achieve the most significant on-site impact reductions and to evaluate off-site mitigation opportunities. The concept of focusing on riverine edge habitat was developed over the course of several Skagit Technical Working Group meetings. The HCMT combined the goal of establishing and meeting target conditions, the factors of salmon’s density dependence and the Skagit River’s carrying capacity, and the need for mitigating for edge habitat impacts.
Using Natural Processes
To improve river edge complexity and in-stream rearing habitat, laybacks, single logs along the levee toe, unanchored woody debris piles, and anchored rootwads were installed. As a means of providing refuge opportunities, edge complexity during high-flow events was increased by installing double and triple willow lifts, habitat weirs/groins, fish benches, and layback transition zones. Foraging opportunities were provided by establishing dense vegetation communities along the levee slope and in the riparian areas at the top of the bank by installing single willow lifts and bank plantings. The riparian corridor was enhanced by planting native tree species landward of the levee to provide river shading and a seed source for natural recruitment in the riparian zone.
Broadening Benefits
The Skagit River has been designated as critical habitat for three salmonid species listed under the Endangered Species Act (Chinook, steelhead, and bulltrout). The lower Skagit River provides migratory and rearing habitat for eight anadromous salmonid species. Due to the significance of the Skagit River to salmonid species and as a stock source for salmon populations in other Puget Sound rivers, USACE NWS mitigation efforts focused on the habitat requirements of these species. Salmon, in particular, are important for several reasons: they are a barometer of ecosystem health, they bring ocean nutrients to rivers and streams, and they are a source of food to other wildlife and humans. The cultures, livelihoods, and diets of Pacific Northwest Native American tribes have always focused on salmon. Important from an economic standpoint as well, a robust salmon population supports the region’s commercial, recreational, and tribal fishing industries.
Promoting Collaboration
While USACE NWS initiated the concept and calculation method of the Habitat Capacity Mitigation Tool (HCMT), a technical working group developed the multiple features and principles for the HCMT’s implementation. Representatives from NWS, the Skagit Diking Districts, the Skagit River System Cooperative, the National Marine Fisheries Service, and the U.S. Fish and Wildlife Service prioritized impacts and determined which environmental enhancement features were sufficient to provide compensatory mitigation.
