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

The Idaho Department of Fish and Game (IDFG) will complete Phase 2 of the Sub-reach 1 (SR1) project that is contained within the 2.2 mile contiguous Lower Lemhi River Rehabilitation Reach. SR1 is one of 4 sub-reaches previously identified for aggressive channel and floodplain rehabilitation, and is located in the lower Lemhi River. The project goal is to expand the floodplain while creating substantial lateral side channel habitat with multiple complex woody debris structures. Because of its large scale, restoration actions in the Lower Lemhi Rehabilitation Reach were partitioned into subreaches to most efficiently meet project goals and objectives. SR1 is approximately 1.25 miles in length with opportunities to expand the floodplain width and function substantially over baseline conditions. Due to its relatively large scale, SR1 was further partitioned into four project phases. Phase 2 will build upon Phase 1 efforts, and will more than double the amount of available fish habitat. The landowner is allowing development of the entire valley floor north of the current river, and has removed all irrigation infrastructure and the transferred water rights to another location for this purpose. The reach is protected by a donated conservation easement held by the Lemhi Land Trust, which will protect these habitat enhancements while allowing natural processes to proceed.



Project actions designed to address the project goals and objectives include: 1) mechanically construct more than 2 miles of additional river channel, including a realigned main stem and lateral channels, to increase stream sinuosity and lateral complexity, 2) remove and/or relocate irrigation diversions and rip rap, 3) grade river banks, floodplain areas, and benches to establish a larger floodplain/riparian forest that will allow natural stream channel processes to occur, 4) install engineered log jams and other woody debris structures throughout the existing and newly created stream channels to provide channel structure and roughness, 5) stabilize streambanks using logs, log revetments, and vegetation, and 6) conduct riparian vegetation plantings.

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 homogenous 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.