Reference Reach Phase 1 Seifert Bank

Project Narrative

As late as the early 2000s, the project area on the Lemhi River contained a sinuous river that inundated relatively good quality floodplain and riparian habitat. Over the past decade, a series of high-water events has caused the primary river channel to erode laterally into a pasture and migrate away from the functional floodplain area. Because the eroding banks lack stabilizing vegetation or any woody debris, managers expected the erosion to continue, resulting in total isolation of this once active floodplain.
The Reference Reach Phase 1 Seifert Bank project improved streambank and habitat conditions for Snake River Sp/Su Chinook Salmon and steelhead summer rearing and overwintering juveniles in this eroding reach. Project actions focused on installing engineered wood structures along the eroding bank to create lateral fish habitat while providing bank stabilization. Large woody debris structures installed in the active river channel re-direct flow back into previously occupied areas. A flow deflector log jam, log barbs, small habitat log structures, bank roughening structures, and an extended large engineered log jam were all installed. A historic channel was used as the bypass during construction, and was left open to function as a side channel. The historic channel did not require excavation and therefore is not reported in the metrics under creation/connection to off-channel habitat. Vegetation was planted in and on the streambank to establish a riparian zone and streambank stability. A jack fence was constructed to replace the old fence that had washed out and exclude cattle from the project area. Floodplain grading did not occur as proposed as the grading sections were based on outdated aerial imagery. The feasibility of getting equipment to the area, and a more robust existing riparian habitat than expected resulted in a wait and see approach. If the installation of instream large wood structures passively floods the area, no additional work will be completed. If that isnt successful, the area may be addressed during a larger, multi-phase project with a 2024 installation date. The project came in under budget and ahead of schedule due to elimination of the floodplain grading and bids coming in lower than expected. Project objectives of the Reference Reach Phase 1 Seifert Bank project included developing in-river habitat complexity through multiple types of engineered wood treatments. Currently, in the lower Lemhi River, the habitat quality is poor, the mainstem is a single channelized thread that contains long swift riffle segments with consolidated large bed load material and little or no lateral habitat. As such, through the Lemhi Effectiveness Monitoring Program, IDFG 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, which has been determined to currently limit the recovery of ESA listed fish throughout the Lemhi basin (Office of Species Conservation, 2019). This should translate into improved freshwater productivity, where more juveniles will leave the Lemhi River sub-basin per each adult that enters the system to spawn.