AbstractHydropower generation induces rapid and frequent fluctuations of hydrodynamic parameters in rivers downstream of hydroelectric impoundments. On the FontauliéGre river (ArdéGche basin, France), water releases come from the Loire basin and the flow varies from 1.3 to 20 m3/s for one reach and from 0.1 to 20 m3/s for another reach. A reference site upstream was chosen to compare the communities with two downstream impacted sites. These two sites enabled two different base flow situations to be studied simultaneously and also the effect of base flow enhancement at one site. Benthic and epilithic samples were collected at the three sites on three dates. Epilithon development was linked to hydraulic regimes, with a major increase in algal biomass, dominated by filamentous algae, linked to the minimum base flow level and to the duration of periods without peaks. The macroinvertebrate communities of impacted reaches were less diversified and more specialized, with predominance of one or two taxa, whereas communities in upstream reaches were more diverse. The different morphological units specificity was attenuated in hydropeaking situations. In the case of a very low base flow, the lentic units with very low velocities (less than 4 cm s−1) had different fauna. The downstream algal development explained part of the change in benthic community structure in trophic groups (i.e. dominance of scrapers) in the site with the higher base flow. When the base flow was too low, a trophic discordance was observed and excess algal biomass represented a limiting factor. Thus, this study indicated that base flow level and duration had important effects on epilithic development and on invertebrate trophic structure in terms of mean velocities and of number of days without pe