Sub-Reach 4 Design and Implementation, Lower Lemhi Rehabilitation

This project completed final design and construction of habitat treatments prescribed for Phase 1 of the Sub-reach 4 (SR4) project area that is contained within the 2.2 mile contiguous Lower Lemhi River Rehabilitation Reach. SR4 is a 0.4 mile reach and the lowermost section of the 4 sub-reaches previously identified in a conceptual design for rehabilitation. Substantially degraded stream and riparian habitat exists throughout its limits, primarily due to road and agricultural development. IDFG partitioned the reach into 4 sub-reaches because of the large scale, substantial anticipated resources required, and to allow managers to efficiently address unique challenges found throughout this reach of the Lemhi River.

In 2017, high water caused significant erosion and loss of a portion of a ranch road that resulted in the flooding of adjacent and downstream properties in the SR4 reach. The Idaho Department of Fish and Game (IDFG) coordinated with the landowner to address this problem by developing a two phased fish habitat project that would increase habitat quantity while also controlling future flooding. This contract implemented Phase 1, comprised of a 0.1 mile section of the SR4 reach. IDFG removed the riprap levee from the ranch road, and increased the amount of instream fish habitat by constructing woody bank jams perpendicular to the flow to create river channel complexity and hydraulic diversity, and installing a bank roughening structure to create rearing habitat while providing bank stability. An alcove was also constructed to provide off-channel habitat. Successful implementation of these actions increased the quantity of lateral fish habitat that improved conditions for summer parr rearing and over-wintering pre-smolt life stages of Chinook salmon and Steelhead. Revegetation actions were completed using a certified weed free seed mix and irrigated to recover vegetation as quickly as possible.

Design and construction of Phase 2 will incorporate actions completed during Phase 1, and will be implemented in beginning in 2022. The projects proposed end date was erroneously entered as 2021. The project finished on time for a 2020 end date.

The goal of the Lower Lemhi River Rehabilitation Reach, including SR4, is to develop a more natural floodplain while restoring natural river processes and function. Project objectives include extending and developing complexity in existing river channels, development of a substantially larger floodplain, and riparian zone restoration and protection. Currently, this reach does not effectively support most life stages of salmon and steelhead. It lacks proper stream channel structure, function, and structural complexity, sediment consists of large homogenous substrate unsuitable for spawning, water velocities are high, and there are no lateral habitats suitable for fish rearing and growth. As such, Lemhi monitoring efforts document low fish survival rates in this river segment.

SR4 treatments will contribute substantially to improving freshwater productivity of Chinook salmon and steelhead in the lower Lemhi River. As with the uppermost Sub-reach 1 project (Lower Lemhi River Rehabilitation Reach), SR4 river bank elevations are only slightly higher than the river, providing conditions sufficient for inundation of the floodplain. Thus, more cost efficient techniques can be applied to develop an active floodplain with anabranching channels. IDFG, using Bonneville Power funding, previously developed a 60% design for this river segment. The design addressed parr/rearing and overwinter life stages by developing several side channels, deepening the river depth and reducing current velocities, and installing ample large woody debris structures that provide immediate fish habitat while stabilizing eroding banks. Furthermore, the riparian areas will be managed for riparian health and function.