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

Project Narrative

The Idaho Department of Fish and Game (IDFG) implemented Phase 3 of Sub-reach 1 (SR1) that is contained within the 2.5 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 (007 19 SA) was constructed in 2021, Phase 2 (016 20 SA) was completed in July 2024, and Phase 4 (008 22 SA), expected to be complete by fall 2026. Habitat treatments focused on increasing habitat complexity for Chinook Salmon summer parr and overwintering pre-smolts by addressing the limiting factors of simplified hydraulics, increased water velocities, a channelized planform, a perched floodplain, impaired riparian habitat/ shading, and reduced hyporheic exchange. Habitat treatments included construction of multiple side channels, meanders in the mainstem river, installation of large woody debris and pools, streambank stabilization, and levee removal. The outcomes for Phase 3 are reduced average slope (existing 0.65% to 0.45%), increased channel length from 0.6 miles to 1.5 miles, increased frequency of floodplain activation from approximately every 5 years (Q5) to approximately every 1.5 years (Q1.5), increased floodplain area inundated from 4 acres (i.e., area of the channel in the project area pre-project) at Q1.5 to 8 acres at Q1.5, and net increase in wetland acreage from 2 acres to 6 acres. The complexity and scale of this project required several changes from the originally proposed scope. Water management challenges significantly increased the cost and timeline, and two extensions were completed. A ford through the Lemhi River on the project site was proposed to be replaced with a railcar bridge; however, it was later determined that a larger bridge was necessary, and this was installed under a separate funding award. This project also experienced ice jams in the winter of 2022/2023 and 2023/2024, causing water to overtop levees at the upstream extent of the project and upstream of the project. An adaptive management plan to reduce ice risk on the project site was developed and funded as PCSRF project 021 23 SA. Several additional project changes on future phases and potential additional adaptive management actions are likely necessary as the team continues learning and adapting to this dynamic situation. The large sum of returned funds were intended for bridge installation and revegetation actions. Revegetation will occur later, under a separate PCSRF grant award (014 24 SA), after adaptive management is complete. This project was multi-year funded with PCSRF grant funds from FY17, 18, 19 and 21. FY17, FY18, and FY19 funds were spent out within their respective grant periods. Due to a large return of funds at project closeout, FY21 funds were underspent, causing the bulk of funding to come from FY19. This was an unintended and unanticipated consequence resulting from this projects’ many challenges, outlined above. 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.