The Amazon, Congo, and Mekong river basins, home to one-third of the world's freshwater fish species, are undergoing rapid hydropower development. While these projects meet energy needs, they often overlook biodiversity and fisheries impacts. New tools and data can help balance energy and environmental goals. Current assessments focus on local impacts but neglect cumulative effects of multiple dams on hydrology and ecosystems. Large dams reduce fish diversity, block migration, and alter flood patterns, harming fisheries and ecosystems. Ecological changes also affect deltas, estuaries, and marine ecosystems. Dam planning lacks transparency, and many countries lack proper protocols for hydroelectric projects. Large dams often exceed cost estimates and fail to mitigate environmental impacts. Hydropower in the Amazon, Congo, and Mekong has significant biodiversity and social impacts, including displacement and deforestation. The Belo Monte dam in Brazil may cause severe biodiversity loss. The Congo has fewer dams but relies heavily on hydro-power. The Inga dams could significantly impact the Congo's ecosystem. The Mekong has many planned dams, threatening fish migration and food security. Long-term planning is needed to balance hydropower and biodiversity. Basin-scale planning, using biodiversity and ecosystem data, is essential for sustainable development. Institutions should require assessments that consider cumulative impacts and climate change. Proper planning is crucial to avoid species extinction and ecosystem decline in these mega-diverse rivers.The Amazon, Congo, and Mekong river basins, home to one-third of the world's freshwater fish species, are undergoing rapid hydropower development. While these projects meet energy needs, they often overlook biodiversity and fisheries impacts. New tools and data can help balance energy and environmental goals. Current assessments focus on local impacts but neglect cumulative effects of multiple dams on hydrology and ecosystems. Large dams reduce fish diversity, block migration, and alter flood patterns, harming fisheries and ecosystems. Ecological changes also affect deltas, estuaries, and marine ecosystems. Dam planning lacks transparency, and many countries lack proper protocols for hydroelectric projects. Large dams often exceed cost estimates and fail to mitigate environmental impacts. Hydropower in the Amazon, Congo, and Mekong has significant biodiversity and social impacts, including displacement and deforestation. The Belo Monte dam in Brazil may cause severe biodiversity loss. The Congo has fewer dams but relies heavily on hydro-power. The Inga dams could significantly impact the Congo's ecosystem. The Mekong has many planned dams, threatening fish migration and food security. Long-term planning is needed to balance hydropower and biodiversity. Basin-scale planning, using biodiversity and ecosystem data, is essential for sustainable development. Institutions should require assessments that consider cumulative impacts and climate change. Proper planning is crucial to avoid species extinction and ecosystem decline in these mega-diverse rivers.