Lower Lemhi River Phase 1 Adaptive Management

The Idaho Department of Fish and Game will continue implementing the Lower Lemhi River Rehabilitation Project. This large-scale multi-phase effort is designed to improve floodplain function, increase habitat capacity to support important life stages of ESA fish and allow natural river processes to occur in a 2.4 mile segment of the lower Lemhi River. Due to its relatively large scale, the reach was partitioned into 4 sub-reaches. The Lower Lemhi River Phase 1 Adaptive Management project will support 0.4 mile of adaptive management actions that are necessary for Sub-reach 1 (SR1) to provide improved floodplain activation and water conveyance. The anticipated outcomes are enhanced rearing and overwinter conditions for Endangered Species Act (ESA) listed juvenile Chinook Salmon and steelhead.



Specific actions for SR1 include: 1) excavation of a series of floodplain swales to further convey river flow onto the floodplain, 2) modification of the main river channel and associated side channels to increase floodplain conveyance, 3) construction of engineered riffles at prescribed locations, 4) grading river banks and benches to establish a larger floodplain/riparian area, 5) installation of brush bank treatments and willow baffles to provide floodplain structure and roughness, 6) completion of irrigation infrastructure improvements as necessary to offset acreage used for river restoration and access road surfacing for construction and cattle use (no metrics realized from these actions), and 7) revegetation using willow cuttings, floodplain willow baffles, and seeding.

The goal of the Lower Lemhi Rehabilitation project is to enhance floodplain function and allow natural river processes to occur. This will diversify habitat condition and increase capacity to support critical juvenile life stages of Chinook salmon and steelhead. Project actions address the poor habitat condition of the existing river, resulting in activation of floodplain habitats at various flow intervals, expanding and enhancing riparian habitat, and increasing floodplain connectivity. The proposed project expands on previous efforts completed in Phase 1 to diversify hydraulic and structural complexity. As a result, Chinook salmon and steelhead freshwater productivity is expected to increase, with more juveniles leaving the Lemhi River sub-basin in better condition for each adult female entering the watershed to spawn.



Previous rehabilitation efforts include completion of Sub-reach 2 in 2016 and Sub-reach 3 in 2018 that addressed a total of 1.2 miles of degraded river. Ongoing rehabilitation actions prescribed for Sub-Reach 1 expand on these efforts, providing opportunities to develop a floodplain for multiple life stages of fish at a much larger scale. A large segment of the valley floor within this reach is being utilized. The property is also protected through a donated conservation easement held by the Lemhi Regional Land Trust, providing the opportunity for the quality of habitat to improve through natural river processes.