Sub-Reach 1 Implementation, Phase 2, Lower Lemhi Rehabilitation Project

The Idaho Department of Fish and Game (IDFG) and partners completed phase 2 of the sub reach 1 (SR1) restoration project that is within the larger Lower Lemhi River Rehabilitation Project. This priority reach of the Lemhi River is a primary location for restoration actions designed to increase survival of juvenile Chinook Salmon and steelhead. Both summer parr and overwinter pre-smolt life-stages are survival bottlenecks in the Lemhi River. Existing conditions in the Lemhi River and the project reach are characterized by an incised and straightened channel form, reduced off-channel habitats, armored banks, infrequent floodplain access, simplified bed form, and reduced riparian vegetation along both banks. This project increased mainstem and lateral channel length, increased frequency and duration of seasonal floodplain activation, stabilized eroding streambanks, constructed relic beaver pond complexes, and installed large woody debris complexes of various types. The L-10 diversion and associated fish screen were relocated (no metrics reported for this action as there were no additional fish benefits related to the relocation) and rip rap was removed to provide space for floodplain and lateral channel habitats.



Three amendments were completed- one to extend the end date and add line-items to help pay for and install minor irrigation infrastructure components; one to increase the grant award to facilitate construction of an additional riffle on the project to help control water surface elevations and activate the floodplain, and pay for additional large woody material acquisition and potted plants; and one to decrease the award amount and extend the award end date. The total award amount was decreased because IDFG paid more of the implementation costs using BPA funds and because costs associated with revegetation were delayed to a future grant award. No revegetation actions took place under this contract

The SR1 project will develop a more natural floodplain and restore natural river processes that will lead to better habitat conditions for ESA listed fish. Project objectives include extending and developing complexity in existing river channels, development of multiple lateral habitats, development of a substantially larger floodplain, and riparian zone restoration and protection. Currently, this reach lacks proper stream channel structure and complexity, the bed load consists of large homogeneous substrate unsuitable for spawning, water velocities are very high, and there are no slow water or lateral habitats suitable for fish rearing and growth. Project actions are expected to address this condition and substantial 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. Development of an active floodplain and construction of complex fish habitat will directly increase habitat capacity for juvenile life stages of salmon and steelhead. Project objectives include reactivating previously occupied floodplain and riparian areas, and developing in-river habitat complexity through multiple types of engineered wood treatments. Currently, in the lower Lemhi River, the habitat quality is poor, the main stem is a single channelized “thread” that contains long swift riffle segments with consolidated large bed load material with little or no lateral habitat. As such, through the Lemhi Effectiveness Monitoring Program, IDFG documents low overwintering fish survival rates in this portion of the river. Project actions are expected to substantially improve conditions for this critical juvenile life stage, and this should translate into improved freshwater productivity, where more juveniles will leave the Lemhi River sub-basin per each adult that enters the system to spawn.