A global synthesis reveals the role of strategic hydropower planning in mitigating adverse impacts of reservoir flooding

Hydropower is an integral component in the global transition to decarbonized energy systems. However, the construction of hydropower dam leads to a range of adverse socio-environmental consequences as a result of reservoir flooding. Here, we provide a global synthesis that quantifies key impacts associated with future hydropower development and examines how strategic hydropower planning can mitigate these adverse effects, including aboveground biomass loss, terrestrial biodiversity impact, greenhouse gas emission, evaporation loss, and human displacement. Our findings indicate that the prospective dams in tropical regions contribute the majority (72–82 %) of environmental impacts worldwide, while dam-induced displacement predominantly (64 %) occurs in the northern temperate zone. 52–99 % of the variations in these impacts could be explained by the hydropower capacity per unit of reservoir area (power density), suggesting that systematic planning that prioritizes higher power densities could significantly mitigate negative socio-environmental externalities. Basin-wide hydropower planning could reduce 83–89 % of avoidable socio-environmental costs compared with uncoordinated hydropower expansion. Large rivers in tropical Africa and southeastern Asia show the greatest potential for impact mitigation through strategic hydropower planning, where optimized dam portfolios could reduce adverse effects by more than 90 % compared to suboptimal dam portfolios. By integrating watershed, river network, and dam characteristics, we propose a hierarchical framework that disaggregates the primary drivers of dam impacts across multiple spatial scales. This framework provides practical guidelines for strategic dam siting and facilitates a win-win outcome by meeting energy demands while minimizing the impacts on surrounding ecosystems and local communities.