Mill Creek Reconnection – Pahsimeroi Valley, Idaho

Project Summary

Mill Creek flows across an alluvial fan and historically all streamflow was intercepted by a large irrigation diversion ditch resulting in complete disconnection from Big Creek (tributary to the Pahsimeroi River). The primary goal of this project was to restore connectivity between Mill Creek and Big Creek to increase late season flows in Big Creek and ultimately restore flows to a severely dewatered section of the Pahsimeroi River. The long-term goal is to have sufficient flow in Big Creek to allow wild steelhead to have access to prime spawning habitat. Mill Creek supports a population of genetically pure westslope cutthroat trout.

In 2014 and 2015, Geum Environmental Consulting, Inc. teamed with WestWater Consultants, Inc. to design and construct a stream reconnection project on Mill Creek for the Big Creek Ranches, LLC and Idaho Chapter of Trout Unlimited. The reconnection and restoration design included construction of almost 2,000 feet of new channel, plugging of relic overflow channels to concentrate flow into the main channel, construction of a siphon for the irrigation to convey irrigation water under the new channel, and revegetation of the new floodplain and streambanks. The project was completed in winter 2015 and Mill Creek is now connected to Big Creek for the first time in a century. Mill Creek has conveyed surface water to Big Creek year-round since project completion.

Services that Geum provided for the Mill Creek Reconnection Project include:

• Wetland delineation and report to support project permitting
• Geomorphic and ecological site assessments to support restoration design
• Coordination and data collection for hydrology analysis
• Coordination of survey data collection and generation of terrain surfaces using ArcGIS
• Channel design in ArcGIS using field and design inputs
• Developed an 80% bid package and contractor support documents
• Provided construction oversight