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Dams impact the connectivity of fish populations in rivers, sometimes resulting in distinct populations with different characteristics below and above dams.

The Boundary Dam, located on the Pend Oreille River near the U.S.-Canada border in northeast Washington State prevents upstream passage of fish species, including westslope cutthroat trout and mountain whitefish. Researchers from the USGS, University of British Columbia, and Idaho State University evaluated the rationale for moving individuals of these species upstream of the dam. The scientists reviewed pre-dam habitat connectivity and considered the ecological and evolutionary characteristics of existing populations above the dam. There was little evidence of historical connectivity for either species. Prior to the Boundary Dam’s construction, two natural barriers—Metaline Falls and Z Canyon-- separated populations below and above the dam. The authors concluded that moving fish over Boundary Dam is not warranted if management goals include maintaining natural patterns of connectivity or conserving the characteristics of existing local populations.   

Dunham, J.B., Taylor, E.B., and Keeley, E.R., 2022, Evolutionary and ecological connectivity in westslope cutthroat trout (Oncorhynchus clarkii lewisi) and mountain whitefish (Prosopium williamsoni) in relation to the potential influences of Boundary Dam, Washington, Idaho, and parts of British Columbia: U.S. Geological Survey Open-File Report 2022-1084, 22 p.