Sudden changes in flow, downstream of plants managed by hydropeaking, modify the living conditions of biological communities and may alter their biological parameters (reproduction, survival, growth, displacement). The high interannual variability of these communities and of the hydropeakings has to be taken into account to assess the global impact of hydropeaking on their sustainability. For that, a comprehensive monitoring of relevant physical and biological parameters was conducted over several years on 13 French rivers, downstream 19 hydropower plants (24 monitoring sites). The majority of these environmental monitoring was conducted before and after the implementation of mitigation measures. The change in spatial-temporal structure of the biological assemblages (fish, macroinvertebrates) at the 24 sites was analyzed. The nature and the degree of the ecological impact varied according to the site. Alterations of living conditions were more or less accentuated in function of the morphology of the site and of hydropeaks characteristics. The phenomena of stranding occurred only on sites with alluvial benches or secondary channels. The degree of impact seemed to be lower for macroinvertebrate community than for fish. Regarding the efficiency of mitigation measures, a sufficient minimum discharge and a limitation of high and fast hydropeakings during sensitive periods of spawning and recruitment were two key mitigation measures. Punctual actions on the morphology were complementary for some sites. To better design mitigation measures adapted to the type of reaches, tests of several mitigation measures have to be pursued and will be associated to environmental multiannual monitoring.