Downstream Riffle Research Articles

Changes in the Downstream Insect and Amphipod Fauna Caused by an Impoundment with a Hypolimnion Drain1

Mill Creek, a small stream in southwestern Wisconsin, was impounded to create a 150-acre recreational lake. This action had a pronounced effect on the insect and amphipod fauna of a riffle 600 ft below the dam, and also affected the fauna of a riffle 2 miles downstream. Prior to impoundment both riffles were inhabited by a diverse insect fauna that included mostly Ephemeoptera, Trichoptera, Diptera, and Coleoptera. After impoundment the reduction of the number of species in the upstream riffle was highly significant, and previously dominant families such as Baetidae (Ephemeroptera), Hydropsychidae (Trichoptera), and Elmidae (Coleoptera) were replaced mostly by increased populations of Simulium vittatum Zetterstedt (Diptera), Gammarus pseudolimnaeus Bousfield (Amphipoda), and Chironomus and other genera of Chironomidae (Diptera). The impoundment affected the downstream riffle similarly, but to a lesser degree. Large increases in phosphorus and nitrogen concentrations and increased siltation as a result of impoundment were probably the most important factors in altering the insect and amphipod populations.