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.　Basinwide　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.