In 1986 the Lower Cub River Irrigation Company installed a full-spanning diversion structure on the Cub River in Franklin County, Idaho. The structure blocked all upstream fish movement and local observers immediately reported seeing large Bonneville cutthroat trout (BCT) gathered each spring in the pool below the dam as they attempted to migrate to their historic spawning habitats upstream in the Cub River. In addition, outmigrants from the resident populations upstream were trapped and killed in the Cub River irrigation canal as they moved downstream to the Bear River.
During the winter of 2006/2007 Trout Unlimited and a diverse group of project partners completed construction of a fish ladder and installation of fish screens at the Lower Cub River Irrigation Diversion. Partners included the Cub River Irrigation Company, Idaho Department of Fish and Game (IDFG), Franklin County Fish and Game Association, the Bear River Environmental Coordination Committee (ECC), the U.S. Fish and Wildlife Service (USFWS), and the National Fish and Wildlife Foundation (NFWF). Funding was provided by a grant from the Fisheries Restoration and Irrigation Mitigation Act (FRIMA), with critical non-federal matching funds provided by PacifiCorp and the ECC.
The new fish ladder is equiped with a removable fish trap, which Trout Unlimited has been using to monitor upstream fish movement through the new Cub River ladder in 2007. The goal of this monitoring effort is to both evaluate the effectiveness of the project and to ensure that the fish passage structure does not facilitate invasions by non-native species (e.g. rainbow trout, brown trout, walleye). Trout Unlimited technicians measure, weigh, and tag each fish that moves through the fish ladder, and then release them upstream to continue on their way. During the first two years of operation the fish ladder has passed over 400 Bonneville cutthroat trout and 80 native mountain whitefish upstream into habitats that have been inaccessible to them for over 20 years.