River ecosystems are one of the most altered ecosystems worldwide and a main stressor is human water use, including the construction of dams and other instream structures. Dams degrade rivers by influencing one or more of the three determinants of community assembly: dispersal constraints, environmental constraints (abiotic conditions), and biotic constraints (interactions among species). Environmental flow releases aim to mitigate flow alterations and restore rivers downstream of dams by returning some elements of the pre-regulation flow regime, resulting in an ecosystem that resembles a more natural state. We examined the responses of benthic macroinvertebrate communities in riffles and pool-edge habitats to an environmental flow regime (EFR) in the highly regulated Snowy River below Jindabyne Dam over 17 years (2000 – 2016). Benthic macroinvertebrate communities significantly changed in response to prolonged increases in the magnitude of all components of the flow regime. The composition of the benthos in the Snowy River became more dissimilar to pre-EFR communities and more closely resembled those of near-by unregulated rivers. These changes were apparent in both riffle and pool-edge habitats and were most pronounced when flow magnitudes were highest. We found that the EFR was likely to have alleviated environmental constraints more so than dispersal constraints. The relative abundances of functional feeding groups in riffle and pool-edge habitats indicated the EFR altered abiotic conditions or resources that previously limited macroinvertebrate abundances. Scrapers increased as flows increased and in contrast, the relative abundances of gatherers declined with prolonged high flows. For both groups the changes could be attributed to frequent high flow events mobilising substantial amounts of fine sediment and scouring benthic substrates. There was no evidence that dispersal processes played a major role in the observed alterations to community composition in the Snowy River.