Worldwide, dams have had pervasive impacts on their associated river systems. In this study, we examined the effects of a large-scale dam on the downstream macroinvertebrate assemblages of a 65-km stretch of the Chattahoochee River (Georgia, USA) over an 11-year period. We quantified differences among six sites within the study reach using NMS, ANOSIM, invertebrate metrics (Shannon–Weiner diversity, number of EPT taxa, and number of sensitive taxa), and indicator analysis. We compared results from this study to the nearby unregulated Middle Oconee River. There were significant differences among all sites and increases in all macroinvertebrate metrics as distance from the dam increased (and diel flow variation decreased). Indicator analysis supported this pattern: no EPT indicator taxa existed directly below the dam, whereas 17 were indicators at the final site. Similarity between the Middle Oconee and the Chattahoochee was moderate (52%). Our results suggest that impacts of the dam are far-reaching and long-term, and that the macroinvertebrate assemblage does not return to a fully representative state even 65 km downstream. This study contributes to the small but growing field of research on effects of dams over long temporal and spatial scales.

The majority of the world's large river systems is affected by dams. The influences of unnatural regimes induced by flow management are wide-ranging from both biotic and abiotic standpoints. However, many of these effects are not evident over short (1-2years) periods (e.g.impacts of annual flow variation). This study examines the long-term effects of annual flow variation on the macroinvertebrate community in the Chattahoochee River (GA) in the reaches below Buford Dam, the major water control structure on the river. Quarterly, macroinvertebrate samples were taken from 2001 to 2011 using Surber and Hester-Dendy plate samplers at six locations spread across 65km below the dam. Data were analysed via analysis of similarities to determine differences in community composition between high-flow (mean discharge=58.27m(3)/s) and low-flow (mean discharge=26.53m(3)/s) years. Taxa that contributed most to community differences were determined via similarity percentages analyses and subsequent t-tests. Several insect taxa (e.g.Cheumatopsyche and Ceratopsyche caddisfly larvae, Maccaffertium mayfly nymphs and Taeniopteryx stonefly nymphs) were more prevalent under the high-flow regime. Non-insect macroinvertebrates (e.g.Crangonyx amphipods, Tricladida flatworms and Caecidotea isopods) were more abundant under low-flow conditions. In terms of taxon richness, no significant effects of flow regime were detected. Implications of macroinvertebrate patterns for the fishery and ecological health of the river are discussed. Copyright (c) 2014 John Wiley & Sons, Ltd.