Effects of multiple low-head dams on fish, macroinvertebrates, habitat, and water quality in the Fox River, Illinois

We examined the effects of low-head dams on aquatic biota, habitat, and water qualityin a 171-km reach of a midwestern warmwater river that was fragmented by 15 dams into a seriesof free-flowing and impounded habitats. Dams impounded 55% of the river\u27s surface area withinthe study reach and influenced distributions of 30 species of fish by restricting upstreammovements. Values for the Illinois index of biotic integrity (IBI) were higher in free-flowing areas(mean IBI = 46 out of a possible 60 at below-dam and midsegment free-flowing locations) thanimpounded areas (mean IBI \u3c 31 for above-dam and midsegment impounded locations).Likewise, scores from a macroinvertebrate condition index (MCI) were higher at stations in freeflowingreaches (mean MCI \u3e 415 out of a possible 700) than in nearshore areas of impoundedreaches (mean MCI \u3c 210). Ponar dredge samples taken only from open-water impounded areasshowed an offshore invertebrate community that consisted almost entirely of tolerant oligochaetesand chironomid larvae. Qualitative habitat evaluation index (QHEI) scores indicated good-qualityhabitat in free-flowing areas (mean QHEI \u3e 70 out of a possible 100) and severely degradedhabitat at impounded sites (mean QHEI \u3c 45). In impounded reaches, dissolved oxygen and pHshowed wide daily fluctuations (2.5û18.0 mg/L and 7.0û9.4 units) and often failed to meet Illinoiswater quality standards. In free-flowing portions of river, fluctuations in these parameters wereless extreme and water quality standards typically were met. We found little evidence ofcumulative effects of dams; however, our data suggest that low-head dams adversely affectwarmwater stream fish and macroinvertebrate communities by degrading habitat and waterquality and fragmenting the river landscape. These results should aid river managers andstakeholders in determining appropriate restoration practices (i.e., dam removal versus fishpassage structures) for warmwater rivers and streams that contain low-head dams