Middle Lemhi River – Henry Reach

The Middle Lemhi River Henry Reach is a 0.65 mile segment of the Lemhi River located approximately 2 miles downstream of the confluence of Hayden Creek and the Lemhi River. The pre-treatment project condition included a confined river segment that was largely isolated from its historic floodplain. Bank armoring (e.g. rip rap) previously installed to stabilize stream banks adjacent to a pasture and irrigation diversion substantially limited lateral floodplain interactions. This reach was void of woody structure, offered minimal lateral habitat such as alcoves or secondary channels, and contained high water velocities that offered poor quality rearing habitat for summer parr and winter pre-smolt life stages.

The Middle Lemhi River Henry Reach expanded approximately 0.65 miles of the Lemhi River into more of a natural floodplain to encourage natural river processes and increased habitat capacity for juvenile life stages of ESA listed fish. Objectives included increasing river sinuosity, developing habitat complexity, encouraging floodplain engagement, development of secondary channels, and enhancing riparian zones. Project actions included excavation of multiple channels and pools, installation of instream wood structures, construction of a channel inlet and engineered riffle to provide water to a developed floodplain, channel banks were graded to increase lateral flooding potential, and logs and vegetation were used to stabilize the streambank. Once construction was completed, all disturbed areas were rehabilitated by planting willow stakes, seeding, and placing loose woody debris that was generated when the channels were excavated. Cattle exclusion fencing was installed to prevent livestock from entering the project.

This project received an amendment to increase funds and the project scope in November 2019. The database has been revised to reflect the increase in funds and the additional metrics. Note that this project was multi-year funded from FY16 and FY18.

Completion of the Henry Reach project is expected to improve freshwater productivity of Chinook salmon and steelhead in the Lemhi River. Substantial lateral fish habitat has been created that regularly floods during certain times of the year. Floodplain benches containing thick stands of riparian vegetation are now bisected with multiple channels, and the sinuosity of the Lemhi River has been increased. Furthermore, the project is located just downstream of two primary Chinook salmon production areas. IDFG has determined that some of the highest juvenile densities in the watershed occur in this area (IDFG unpublished data). Thus, the relative benefits of this project are expected to be high. Immediately upstream of the project site, the Lemhi River is a heavily incised single thread channel with no functional summer rearing or wintering habitat. As such, the project will function as a “catcher’s mitt”, providing much needed habitat for large numbers of rearing parr that were displaced in the spring from upstream production areas, and for abundant pre-smolts moving out of the production areas in the fall in preparation for overwintering.