Hydropower impacts on riverine biodiversity

Hydropower is a rapidly developing and globally important source of renewable electricity. Globally, over 60% of rivers longer than 500 km are already fragmented and thousands of dams are proposed on rivers in biodiversity hotspots. In this Review, we discuss the impacts of hydropower on aquatic and semi-aquatic species in riverine ecosystems and how these impacts accumulate spatially and temporally across basins. Dams act as physical barriers that disrupt longitudinal connectivity and upstream-downstream movement of species. Impoundment creates still-water habitats upstream of dams and leads to declines in lotic-adapted species. Intermittent water releases modify the natural flow, sediment and thermal regimes in downstream channels, altering water quality, substrate structure and environmental cues that are vital for species to complete their life cycles, resulting in reduced reproduction success. Moreover, retention effects of reservoirs and flow regulation alter river-floodplain exchanges of water, sediment and nutrients, modifying the habitats on which riverine species depend. Improvements to flow regulation, fishway design and sediment redistribution can mitigate these ecological impacts. Future research should support reforms to dam operations and design adaptations to balance renewable electricity development and biodiversity conservation through systematic basin-scale planning, long-term monitoring, adaptive management and involving multiple actors in decision-making. Hydropower is a renewable energy source that can contribute to growing energy demands. This Review considers the ecological consequences of hydropower plants on riverine systems and emphasizes the urgent need to mitigate ecological impacts to ensure sustainable development.