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 quality in a 171-km reach of a midwestern warmwater river that was fragmented by 15 dams into a series of free-flowing and impounded habitats. Dams impounded 55% of the river’s surface area within the study reach and influenced distributions of 30 species of fish by restricting upstream movements. Values for the Illinois index of biotic integrity (IBI) were higher in free-flowing areas (mean IBI 5 46 out of a possible 60 at below-dam and midsegment free-flowing locations) than impounded areas (mean IBI , 31 for above-dam and midsegment impounded locations). Likewise, scores from a macroinvertebrate condition index (MCI) were higher at stations in free-flowing reaches (mean MCI . 415 out of a possible 700) than in nearshore areas of impounded reaches (mean MCI , 210). Ponar dredge samples taken only from open-water impounded areas showed an offshore invertebrate community that consisted almost entirely of tolerant oligochaetes and chironomid larvae. Qualitative habitat evaluation index (QHEI) scores indicated good-quality habitat in free-flowing areas (mean QHEI . 70 out of a possible 100) and severely degraded habitat at impounded sites (mean QHEI , 45). In impounded reaches, dissolved oxygen and pH showed wide daily fluctuations (2.5–18.0 mg/L and 7.0–9.4 units) and often failed to meet Illinois water quality standards. In free-flowing portions of river, fluctuations in these parameters were less extreme and water quality standards typically were met. We found little evidence of cumulative effects of dams; however, our data suggest that low-head dams adversely affect warmwater stream fish and macroinvertebrate communities by degrading habitat and water quality and fragmenting the river landscape. These results should aid river managers and stakeholders in determining appropriate restoration practices (i.e., dam removal versus fish passage structures) for warmwater rivers and streams that contain low-head dams. Free-flowing rivers have been characterized as having a gradient of physical conditions that elicit gradual changes in biotic communities from headwaters to the river mouth (river continuum concept; Vannote et al. 1980). Due to disruptions in natural flow caused by dams and their associated impoundments, few U.S. rivers remain free flowing throughout their lengths (Ward and Stanford * Corresponding author: [email protected] 1 Present address: Illinois Department of Natural Resources, 8916 Wilmot Road, Spring Grove, Illinois 60081, USA. Received November 7, 2003; accepted December 27, 2004 Published online July 20, 2005 1983). Past ecological research related to dams has focused on lotic reaches directly below dams (Ward and Stanford 1979; Bain et al. 1988; Ligon et al. 1995; De Merona and Albert 1999), mainstem reservoirs directly above dams (Ellis 1941; Hall 1971; Hall and Van Den Avyle 1986), fish communities upstream of impoundments (Martinez et al. 1994), fish and invertebrate migration (Clay 1995; Benstead et al. 1999; Pringle et al. 2000), and environmental impacts from hydroelectric development (Efford 1975; Baxter 1977). From this large body of work, we know that dams can have dramatic effects on rivers and aquatic biota by altering water quality and habitat, disrupting nutrient cycling and sediment transport, and blocking fish and invertebrate movements.