Middle Lemhi River – Hayden Reach Phase 1

Project Narrative

The Lemhi Soil and Water Conservation District (LSWCD) designed and constructed habitat treatments on a 0.33 mile segment of the Lemhi River located near the confluence of Hayden Creek and the Lemhi River. Pre-project habitat conditions included a straightened channelized river segment confined by State Highway 28 that contained no instream woody structure, no side channels or other lateral habitat, a disengaged floodplain, and limited riparian vegetation. The project increased habitat capacity for juvenile salmon and steelhead by constructing a new alignment for the Lemhi River (including pool excavation) to increase sinuosity, raising the surface water elevation of the river by installing engineered riffles (unanchored boulders) to improve river and floodplain function, adding large wood instream to provide channel structure, roughness, and complexity, floodplain grading to increase lateral floodplain connectivity, streambank stabilization, and grass seeding and plantings of native herbaceous and woody species to rehabilitate the riparian zone. This was Phase 1 of a 2-Phase project focusing on habitat rehabilitation along approximately 0.70 miles of the Lemhi River. Phase 1 was implemented under this contract, with treatments constructed along 0.33 miles of the Lemhi River. This project received amendments to extend and to decrease awarded funds. Due to an icing event on a restoration project downstream, this project along with several others were reviewed by an interagency flood risk team. It was determined that this project had a significant chance of having ice problems like those that had occurred downstream, and some of the proposed project actions were scaled back. It is likely this project will undergo a redesign and some sort of adaptive management under Phase 2. Phase 2 will be completed under PCSRF project 017 21 SA, being sponsored by Idaho Fish and Game. Developing a large portion of this property into more of a natural floodplain while restoring natural river processes and function will directly increase habitat capacity for juvenile life stages of salmon and steelhead. Improving the current condition by increasing river sinuosity, developing habitat complexity in the form of woody debris structures, constructing side channels, and riparian zone restoration will result in improved survival for all freshwater juvenile life stages. Furthermore, the magnitude of these affects will be substantial since project is located at the downstream end of the Lemhi primary Chinook salmon production area. Immediately upstream of the project site, the Lemhi River is a heavily incised single thread channel with no functional summer rearing or wintering habitat. As such, the project is ideally located to function as a catchers mitt, providing much needed habitat for rearing parr that were displaced by spring high flows from upstream production areas, and for pre-smolts moving out of the production areas in the fall in preparation for overwintering. The conceptual design directly addresses these life stages by developing instream habitat and floodplain diversity.