Middle Lemhi River – Henry Reach, Phase 2

Project Narrative

The Middle Lemhi River – Henry Reach, Phase 2 will design and construct habitat treatments prescribed for the second phase of the Henry Reach project. The 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. Phase 1 addressed the previously degraded habitat condition by realigning the mainstem river and constructing side channels containing multiple complex large woody debris structures, and adjusting the river and floodplain grade to increase their interaction. Phase 2 will augment Phase 1 actions by constructing a series of additional lateral habitats while further improving instream structure and floodplain interaction. Phase 2 will spatially expand the current floodplain while improving channel complexity and hydraulic diversity. Existing levees will be removed. Specific habitat treatments will include constructing relic beaver dam complexes (found under other engineered structures in metrics) to increase off channel habitat, installing more complex instream habitat in the form of large woody debris, creating instream pools, and further raising river surface water to further increase floodplain interaction. Riparian zone plantings of willow and cottonwood are prescribed to accelerate growth of riparian vegetation in disturbed areas, along with seeding. Further development of the relatively large active floodplain created during Phase 1 of the Henry Reach project will increase habitat capacity for critical juvenile life stages of Chinook salmon and steelhead. In the Lemhi watershed, habitat condition has been altered to the extent that the capacity of the system is unable to support critical life stages of juvenile fish. In most areas, water velocities are high, and slow water habitats that are optimal for fish rearing and growth are non-existent. Project actions will address this condition and provide habitat complexity for key juvenile life stages that is currently limiting recovery of ESA listed fish throughout the Lemhi basin. Increased habitat capacity is expected to improve Chinook salmon and steelhead freshwater productivity throughout the Lemhi watershed. The Henry Reach is located within a segment of the Lemhi River that contains some of the highest densities of juvenile salmon (IDFG unpublished data). Any increase in hydraulic and structural habitat diversity in this area is expected to result in a proportionally higher increase in fish abundance and survival. As such, more juveniles will leave the Lemhi River sub-basin in better condition per each adult that enters the system to spawn.