Sub-Reach 1 Implementation, Lower Lemhi Rehabilitation Project

Project Narrative

The Lower Lemhi Rehabilitation project is a 2.5 mile contiguous river segment that Idaho Department of Fish and Game has been developing for fish habitat restoration. Sub-Reach 1 (SR1), a 1.5 mile reach, is one of 4 sub-reaches within this segment where aggressive habitat treatment actions were prescribed to restore floodplain function and reestablish natural river processes. Project objectives included realigning the Lemhi River to create a more sinuous and complex system containing multiple side channels, grading the floodplain to increase lateral inundation, creating habitat complexity using multiple woody debris treatments, and improving riparian quality and function. Over a span of 2 years, contractors reconstructed the mainstem into a much more sinuous configuration that resulted in more hydraulic and geomorphic diversity, constructed many types of lateral channels of varying lengths and widths, and removed many thousands of cubic yards of material from the floodplain to meet design grades. Using a prescribed gradation of cobbles and boulders, engineered riffles were constructed to raise surface water height at strategic locations. Rip rap and levees were removed from designated areas that previously channelized the river, and natural wetland sod was salvaged and relocated to other areas. Existing streambanks were stabilized with logs, revetments, and vegetation. Finish grading occurred along stream banks, all disturbed areas were capped with topsoil and seeded, and willow stakes were planted throughout the project. Lastly, an irrigation diversion was strategically relocated to increase the amount of habitat accrued by this project, and a new fish screen was installed by the IDFG Screen Program. The goal of SR1 is to develop a more natural floodplain while restoring natural river processes and function. Project objectives identified to achieve this goal include extending and developing complexity in existing river channels, development of a substantially larger floodplain, and riparian zone restoration and protection. Pre-project, this reach lacked proper stream channel structure and function, instream structural complexity, a bed load of large homogenous substrate unsuitable for spawning, high water velocities, and there were no slow water or lateral habitats suitable for fish rearing and growth. Through the Lemhi Effectiveness Monitoring Program, IDFG documented low fish survival rates in this portion of the river. Project actions are expected to increase habitat capacity for key juvenile life stages that is currently limiting recovery of ESA listed fish throughout the Lemhi basin (Office of Species Conservation, 2019). The condition and scale of SR1 will contribute substantially to improving freshwater productivity of Chinook salmon and steelhead in the lower Lemhi River. Unlike the other subreaches, bank elevations in SR1 are only slightly higher than the river surface, providing conditions sufficient for inundation of the floodplain during average water years. Thus, more cost efficient techniques can be applied to develop an active floodplain and anabranched complex river segment.