Big Springs Lemhi River Confluence Restoration Project Phase III

Project Narrative

The Big Springs Lemhi River Confluence Restoration Project Phases I & II were completed in 2018-19. At the end of summer 2019, it was apparent that the first section of the project needed to be reworked due to initial calculations of late season overland flooding being more prominent than originally thought. The design underestimated the flow regime and too much floodplain was accessed during non-spring flows. Roughness coefficients and excess small woody debris were causing too much floodplain access during the non-flood season. In spring of 2020, Phase III construction began to adjust the earlier sections of the project on the Lemhi River by re-positioning in-stream riffles, widening the channel and modifying areas with woody debris. Some willow clumps were removed and wetland sod was placed in the floodplain to reduce flooding in areas that the original design did not account for. The L-58C diversion, previously located 30 feet downstream of the confluence of Big Springs and the Lemhi River was relocated to a more suitable location that benefits the project area and allows for potential side channel habitat design in future phases of Lemhi River restoration. This project received an amendment to increase funds by $8,384. The confluence section of Big Springs and the Lemhi River was lacking riparian habitat, was over-straightened, and over widened. The project area has been heavily impacted by historic land use practices which have reduced channel complexity, floodplain connection, and riparian vegetation resulting in degraded habitat for anadromous fish. Overall project goals were to improve habitat for limiting life stages of ESA-listed fish, restore natural channel processes to maintain diverse habitat, and to address shade targets set in the Conservation Easement. The overall project aimed to maximize short-term and long-term riparian and shade conditions, reduce the channel width to a more appropriate geometry, increase in-stream hydraulic diversity and velocity gradients, increase pool size and frequency, increase interstitial space along margins, and increase in-stream cover. These goals were accomplished on the restoration of 4700 feet of Big Springs Creek and 3500 feet of the Lemhi River. The design increased sinuosity and added multi-threaded channel segments, narrowed over-widened areas, and added in-stream structure to improve fish habitat and floodplain connectivity; excavated pools and created constrictions; utilized existing mature riparian vegetation; and planted willows and sedges to enhance riparian conditions over time. This final phase of the project will adjust and enhance the originally constructed 1650 feet of the Lemhi River.