By John Ormiston, Shirley Sorenson, Gordon Maus, Canyon Creek Irrigation District Board
The Canyon Creek Board of Directors would like to respond to the recent letter to the editor regarding rebuilding Canyon Lake Dam.
In 2018, our Board requested authority from the Bitterroot National Forest to rehabilitate (not maintain) the dam we actually own (it was built by District landowners in 1891) at Canyon Lake. We propose to rebuild the upstream face of the dam as a long-term solution to seepage and piping that have resulted in emergency repairs of sinkholes threatening failure of this high hazard dam. The project, designed by qualified dam engineers, involves removal of the large boulder riprap from the entire face of the dam, smoothing the surface and installing a state-of-the-art liner system and a new outlet structure. A geomembrane liner (essentially a thick layer of engineered plastic) will water-proof the surface of the dam, soil imported from selected areas will provide a smooth surface, a heavy geotextile (fabric) will cushion the liner and riprap will be replaced to protect the liner system and prevent surface erosion. The project will result in an easy to maintain structure for at least 50 years, the minimum expected life span of the liner. The Forest Service completed the required Environmental Analysis in early 2020 so the project can move forward after the lake empties in late August.
Original construction involved then state-of-the-art equipment (horses, fresnoes, ore cars and manpower) very likely for more than one season. The resulting dam probably seeped water from the very beginning because technology, expertise, and site materials prevented construction of a low permeability core. District records from the turn of the century do not exist, but we know the landowners added length and height to achieve the current configuration in 1918.
As stated in the referenced letter, a rehabilitation project using primitive equipment was accomplished in the early 2000s. When the project was completed the outfitter hired to move the necessary primitive equipment and camp supplies stated he would never again take a pack string over the trail to Canyon Lake. Montana Conservation Corps personnel hiked to the site for two, two week shifts to provide necessary labor to partially accomplish removal and replacement of the outlet pipe and pour a partial concrete core for the dam. During the project it became clear the volume of necessary excavation and concrete necessary to complete the project would require mechanical equipment. A mini-excavator and motorized wheelbarrows were flown to the dam to prepare for concrete pouring. Concrete was flown to the site to add to the top of the old concrete core but did not anchor to bedrock. Seepage continued through the dam, under the core, and since then three sinkholes, all related to seepage through the dam, required emergency repairs with mechanical equipment to alleviate the threat of breach failure. Due to the emergency nature of the necessary repair of the high hazard dam, the Forest granted permission to use helicopters to ferry equipment and camp supplies to the site. The Wilderness Act of 1964 clearly makes an exception for use of motorized equipment “including measures required in emergencies involving the health and safety of persons within the area.” While the current project does not qualify as an emergency, sealing the seepage through the dam should alleviate the seepage problem and thus the recurrence of sink holes that threaten imminent failure of the high hazard dam.
The Board of Directors of the Canyon Creek Irrigation District strongly believe we need to use state-of-the-art technology and equipment to repair the dam structure and minimize impact on the Wilderness resource as well as threatened impacts on downstream resources and people should the high hazard dam fail.