Sub-Reach 1 Implementation, Phase 3, Lower Lemhi Rehabilitation Project

The Idaho Department of Fish and Game (IDFG) will implement Phase 3 of the Sub-reach 1 (SR1) project that is contained within the 2.2 mile contiguous Lower Lemhi River Rehabilitation Reach. SR1 is one of 4 sub-reaches in approx. 1.25 miles previously identified for aggressive channel and floodplain rehabilitation. Due to its relatively large scale, SR1 was further partitioned into four project phases. Phase 1 was constructed in 2021, Phase 2 construction commenced in October 2021 and is ongoing.



The Phase 3 project goal is to spatially expand the current floodplain while improving natural river processes, building upon previous SR1 phases. Implemented habitat treatments will include constructing multiple side channels, a more sinuous mainstem river, more complex instream habitat in the form of large woody debris and pools, streambank stabilization, and a more functional riparian zone. Removal of levees and grading the existing floodplain in combination with raising the surface height of the river and constructing side channels, will allow for lateral inundation from floodwaters at annual intervals. This will improve habitat diversity and complexity by allowing more natural processes to evolve. To further improve instream habitat, an existing ford will be replaced by a pre-fabricated bridge to eliminate open water crossings that transport equipment and livestock (captured in riparian metrics under water gap/livestock bridge). The project will install fencing to remain in place for 3-5 years until vegetation grows above browse height; due to the temporary nature, fencing metrics have not been included.



This project is multi-year funded with grant funds from FY17, 18, 19 and 21. FY17 funds were spent out within that grant award period, and have been updated as spent in the database. Note that this project is ongoing and those spent funds do not reflect the total cost of the project.

Development of a large active floodplain will substantially increase habitat capacity for critical juvenile life stages of Chinook salmon and steelhead. The project 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 Sub-reach 1 (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.