Species traits have been identified as a component of biodiversity that is worthy of investigation as a tool for exploring ecological effects of anthropogenic disturbances. We used meta-analysis to show how selected species traits of Ephemeroptera and Plecoptera can be used to examine the ecological effects of flow and water-level regulation in freshwater ecosystems used for hydropower generation. We hypothesized that hydrological regulation would favor an opportunistic ecological strategy and bring about increased similarity in species composition across samples. We found higher reproductive capacity, faster life cycles, and a tendency toward a changed composition of functional feeding groups in lotic stonefly assemblages in response to hydrological regulation. However, the effects of hydrological regulation on mayfly species traits and lentic stonefly species traits were limited. We found lower species turnover in mayflies and stoneflies from regulated than from unregulated lotic and lentic sites, a result suggesting selection of a subset of mayflies and stoneflies from the regional species pool by flow and water-level regulation. The limited support for our predictions probably reflects the comparatively low diversity of aquatic insect species in Norwegian freshwater ecosystems for biogeographical and historical reasons, phenotypic plasticity of the insects' life histories and feeding habits, low trait diversity, trade-offs among species traits in a marginal region, restrictions to trait combinations and, perhaps, random extinction of mayflies and stoneflies caused by hydrological regulation. We advocate inclusion of species-trait variables that may affect ecosystem-level ecological processes in environmental assessment studies because a better understanding of trait-mediated ecological functioning should facilitate assessment of the ecological consequences of anthropogenic perturbations of freshwater ecosystems.