Sub-Reach 1 Implementation, Phase 4, Lower Lemhi Rehabilitation Project

The Idaho Department of Fish and Game will implement Phase 4 of the Sub-reach 1 (SR1) project. SR1 is one of 4 sub-reaches previously identified for aggressive channel and floodplain rehabilitation along a 2.2 mile segment of the lower Lemhi River. Phase 1 was constructed in 2021, construction of Phase 2 is ongoing, and Phase 3 construction will begin in early November 2022. Phase 4 will complete the project, and is expected to begin in early summer 2023.



Phase 4 builds upon previous SR1 phases, with a goal of spatially expanding the current floodplain and diversifying instream habitat throughout the 0.65 mile project reach. Habitat treatments include excavation of the existing floodplain to allow for lateral inundation of flow under a range of conditions, constructing side channels, developing a more sinuous mainstem river, installing large woody debris in channel and on the floodplain, and streambank stabilization. Revegetation actions of planting and seeding will rehabilitate riparian habitat as quickly as possible.

Development of a large active floodplain will substantially increase habitat capacity for critical juvenile life stages of Chinook salmon and steelhead. Currently, this reach lacks proper stream channel structure, substrate consists of large cobble unsuitable for spawning, water velocities are high, and slow water or lateral habitats that are optimal for fish rearing and growth are limited. Project actions will address this condition and provide habitat complexity for key juvenile life stages that is currently limiting recovery of ESA listed fish throughout the Lemhi basin.



Increased habitat capacity at the scale of SR1 is expected to improve Chinook salmon and steelhead freshwater productivity throughout the Lemhi watershed. Proposed aggressive habitat treatments will create hydraulic and structural habitat diversity in the lower Lemhi River, resulting in increases in fish abundance and survival. Lemhi Effectiveness Monitoring studies documents low overwintering fish survival rates in this portion of the river. Project actions are expected to substantially improve conditions for this critical juvenile life stage, with more juveniles surviving and leaving the Lemhi River sub-basin in better condition per each adult that enters the system to spawn.