The World Commission on Dams (WCD) report documented a number of social and environmental problems observed in dam development projects. The WCD gave particular emphasis to the challenges of properly resettling populations physically displaced by dams, and estimated the total number of people directly displaced at 40-80 million. Less attention has been given, however, to populations living downstream of dams whose livelihoods have been affected by dam-induced alterations of river flows. By substantially changing natural flow patterns and blocking movements of fish and other animals, large dams can severely disrupt natural riverine production systems – especially fisheries, flood-recession agriculture and dry-season grazing. We offer here the first global estimate of the number of river-dependent people potentially affected by dam-induced changes in river flows and other ecosystem conditions. Our conservative estimate of 472 million river-dependent people living downstream of large dams along impacted river reaches lends urgency to the need for more comprehensive assessments of dam costs and benefits, as well as to the social inequities between dam beneficiaries and those potentially disadvantaged by dam projects. We conclude with three key steps in dam development processes that could substantially alleviate the damaging downstream impacts of dams
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