Lower Eighteenmile Creek Culvert Replacement

The Lower Eighteenmile Culvert to Bridge project removed a fish passage barrier on the lower end of the Hawley and Eighteenmile Creek watershed. Pre-project, a 36” corrugated culvert conveyed Eighteenmile Creek under State Highway 29, barring anadromous fish species from migrating upstream. Two undersized overflow culverts also conveyed water under SH29. During high water, flow backed up upstream of the culverts and often ran parallel to SH29 before spilling into lower Texas Creek. This project replaced the culverts with a bridge span that creates a natural stream simulation to provide passage to 10 miles of habitat for juvenile and adult life stages of Chinook salmon and Steelhead.

Project construction was originally scheduled for August 2018, however, due to issues with fabrication of the bridge structure and errors found in submittals from the sub-contractor, bridge fabrication was delayed. bridge for more than 2 months, necessitating rescheduling the start date to July 2019. Additional construction actions on dry ground were completed by May 2020. Increased costs occurring during the design and construction phases necessitated multiple amendment requests by IDFG to increase the total amount of the contract and to extend it.

Hawley and Eighteenmile Creeks are important tributaries that form the headwaters of the Lemhi River. Collectively these creeks contain more than 30 miles of fish spawning and rearing habitat that is currently unavailable to ESA-listed anadromous and freshwater resident salmonids. Of the two tributaries, Hawley Creek is the most important biologically because there is more flow in the middle and upper reaches and contains the higher potential for important habitat for juvenile and adult life stages of Chinook salmon and steelhead. Hawley Creek has a large snow-melt driven surface water component and is also fed by springs with excellent water quality.

Re-establishing connection between the Lemhi River and its tributaries is an important conservation measure that has been identified by the USBWP Technical Team. It is also identified as a key measure to be implemented in the Snake River Basin Adjudication Lemhi River Framework and Habitat Actions Plan as well as in the Salmon Subbasin Management Plan. Large segments of historically used habitat are currently unavailable to Chinook salmon and steelhead because of irrigation water withdrawals and fish passage barriers.