Lemhi RM32 Habitat Enhancement Project, Confluence Phase

The Confluence Phase of the Lemhi River Mile 32 (LRM) Habitat Enhancement Project will improve 1 mile of habitat on the Lemhi River and 0.19 miles on Hayden Creek. Hayden Creek is the most productive tributary of the Lemhi River, containing up to 40 percent of the spawning Chinook Salmon in the basin each year along with populations of Steelhead and Bull Trout. This section of the Lemhi River is currently a single channel system with high gradient and little to no rearing and overwintering habitat. This project includes raising riffles in the existing Lemhi River, grading inlets to relic side channels, grading new side channels and floodplains, stabilizing streambanks, and installing instream wood structures which will improve and expand the riparian corridor and create off channel habitat and a side channel network allowing juvenile salmonids the opportunity to stay in the Lemhi basin longer before moving downriver. Native riparian plantings and seedings will be completed to help improve existing riparian habitat and weed management will be conducted to ensure native plants will grow back. The project area will be fenced upon completion to help protect plantings and ensure there is time for the area to recover.

The project will also improve the L-40 diversion to eliminate an existing wing dam and headgate infrastructure and improve operational functionality. The current L-40 diversion has a wing dam that requires regular in-stream maintenance and repair in order to force the water to the fish screen and irrigation ditch. The irrigator goes into the Lemhi River two or more times a year to ensure the wing dam is allowing the diversion to function properly. This project will install a new headgate onto a screened ditch to eliminate the need for a wing dam and annual in-stream maintenance. This action is captured under the metric irrigation practice improvement , but there is no water savings or additional fish passage associated with the action.

In June 2025 an amendment was completed to extend the end date, and increase funds and scope of work to include work on the Island Phase of the LRM 32 project. This phase is in conjunction with PCSRF project 006 24 SA, and actions from this agreement will excavate new channels in a section of the Lemhi that is primarily a single-channel system, with one side channel diverging around a large island. The existing channel is characterized by steep gradients with fast flowing water and limited rearing or overwintering habitat. The project will enhance habitat complexity by raising riffles within the mainstem Lemhi, grading inlets to re-engage relic side channels, creating new channels and floodplains, and installing in-stream wood structures. These efforts will expand the riparian corridor and establish a network of off-channel and side-channel habitats, giving juvenile salmonids more areas for rearing and overwintering in the Lemhi.

Trout Unlimited is the project sponsor and is working in partnership with the U.S. Bureau of Reclamation, Lemhi Soil and Water Conservation District, The Nature Conservancy, and the Upper Salmon Basin Watershed Program.

Expanding and enhancing side channel habitat will improve habitat capacity for juvenile Chinook Salmon and steelhead. Reactivating relic side channels and grading new channels will widen the riparian corridor and provide more habitat for fish, while wood structures will add habitat complexity. Constructing riffles in the existing Lemhi River channel will lead to more floodplain activation which will increase the amount of fish habitat and improve the current riparian vegetation. The projects location downstream of spawning grounds on the Lemhi River and Hayden Creek provides an opportunity to provide habitat for rearing parr and overwintering pre-smolts, keeping them in the Lemhi River longer.