Structural and trophic changes in a benthic macroinvertebrate community downriver from Valparaíso Reservoir (northwestern Spain) were compared before (1986) and after (1990–1991) the creation of that hydropower impoundment. The number of taxonomic groups, total biomass and total density significantly declined under the influence of the new flow regulation, with scrapers (as relative density) being the functional feeding group most adversely affected. The environmental impact decreased with time after the construction of the dam Taxonomic richness and diversity of predators appeared to be the most sensitive biological indicators of the progressive recovery of the macrobenthic community.