Ecological impacts of run-of-river hydropower plants—Current status and future prospects on the brink of energy transition

Abstract The general perception of small run-of-river hydropower plants as renewable energy sources with little or no environmental impacts has led to a global proliferation of this hydropower technology. However, such hydropower schemes may alter the natural flow regime and impair the fluvial ecosystem at different trophic levels. This paper presents a global-scale analysis of the major ecological impacts of three main small run-of-river hydropower types: dam-toe, diversion weir, and pondage schemes. This review's main objective is to provide an extensive overview of how changing the natural flow regime due to hydropower operation may affect various aspects of the fluvial ecosystem. Ultimately, it will inform decision-makers in water resources and ecosystem conservation for better planning and management. This review analyses data on ecological impacts from 33 countries in five regions, considering the last forty years' most relevant publications, a total of 146 peer-reviewed publications. The analysis was focused on impacts in biota, water quality, hydrologic alteration, and geomorphology. The results show, notably, the diversion weir and the pondage hydropower schemes are less eco-friendly; the opposite was concluded for the dam-toe hydropower scheme. Although there was conflicting information from different countries and sources, the most common impacts are: water depletion downstream of the diversion, water quality deterioration, loss of longitudinal connectivity, habitat degradation, and simplification of the biota community composition. A set of potential non-structural and structural mitigation measures was recommended to mitigate several ecological impacts such as connectivity loss, fish injuries, and aquatic habitat degradation. Among mitigation measures, environmental flows are fundamental for fluvial ecosystem conservation. The main research gaps and some of the pressing future research needs were highlighted, as well. Finally, interdisciplinary research progress involving different stakeholders is crucial to harmonize conflicting interests and enable the sustainable development of small run-of-river hydropower plants.