Impacts of Low-Head Dams on Fish Assemblages and Habitat in Two Illinois Rivers

Artificial impoundments such as dams have the potential to change river habitat type and hydrologic regimes by shifting riverine lotic habitats to lentic habitats. These changes can influence fish movement and dispersal, leading to changes in fish community assemblages over time. Two low-head dams on two Illinois rivers have been in place for nearly a century and are scheduled to be removed in 2015. To determine how these dams impact biotic communities and habitat quality, we semiannually assessed fish assemblages and habitat characteristics in below dam, above dam (pool), and upriver sites using DC electrofishing since 2012. Below dam sites on both rivers had the highest diversity while pool sites had the lowest (p=0.0005). Habitat quality scores were lowest in pool sites (p=0.002), indicating that dams have altered habitat immediately upstream of the dams. Our data suggest these different habitat types favor different functional groups of fishes, influencing the dispersal of habitat and trophic groups. Genetic analyses will be conducted on select fish species to evaluate potential genetic differentiation between populations above and below dams; these data will be presented as a part of this project. Overall, data indicate that the presence of dams influences habitat type and fish assemblages.