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Free-flowing rivers (FFRs) support diverse, complex and dynamic ecosystems globally, providing important societal and economic services. Infrastructure development threatens the ecosystem processes, biodiversity and services that these rivers support. Here we assess the connectivity status of 12 million kilometres of rivers globally and identify those that remain free-flowing in their entire length. Only 37 per cent of rivers longer than 1,000 kilometres remain free-flowing over their entire length and 23 per cent flow uninterrupted to the ocean. Very long FFRs are largely restricted to remote regions of the Arctic and of the Amazon and Congo basins. In densely populated areas only few very long rivers remain free-flowing, such as the Irrawaddy and Salween. Dams and reservoirs and their up- and downstream propagation of fragmentation and flow regulation are the leading contributors to the loss of river connectivity. By applying a new method to quantify riverine connectivity and map FFRs, we provide a foundation for concerted global and national strategies to maintain or restore them. A comprehensive assessment of the world’s rivers and their connectivity shows that only 37 per cent of rivers longer than 1,000 kilometres remain free-flowing over their entire length.

Protecting the world’s freshwater resources requires diagnosing threats over a broad range of scales, from global to local. Here we present the first worldwide synthesis to jointly consider human and biodiversity perspectives on water security using a spatial framework that quantifies multiple stressors and accounts for downstream impacts. We find that nearly 80% of the world’s population is exposed to high levels of threat to water security. Massive investment in water technology enables rich nations to offset high stressor levels without remedying their underlying causes, whereas less wealthy nations remain vulnerable. A similar lack of precautionary investment jeopardizes biodiversity, with habitats associated with 65% of continental discharge classified as moderately to highly threatened. The cumulative threat framework offers a tool for prioritizing policy and management responses to this crisis, and underscores the necessity of limiting threats at their source instead of through costly remediation of symptoms in order to assure global water security for both humans and freshwater biodiversity. Dual threat to river biodiversity and water security Access to fresh water is essential for all life forms, but water security for humans and biodiversity are often seen as competing priorities. A new analysis of the threats to the world's rivers breaks new ground by accounting for a broad array of stressors and their downstream effects, from both human and biodiversity perspectives. A subsequent analysis of investments in water resources offers insights into the sources of global disparities in human water security that separate rich from poor. The authors conclude that rivers are in a state of crisis, and that nearly 80% of humanity lives in areas where threat levels are relatively high. River-dwelling species face similarly great challenges around the world. Achieving a sustainable solution to these problems, the authors say, will require creative solutions that jointly address water security for humans and biodiversity, and that treat underlying causes rather than merely symptoms. Water security affects human wellbeing both directly and indirectly, through its effects on biodiversity. Here, a global map has been generated that shows threats to both direct and indirect water security from a full range of potential stressors. Technological investments have also been incorporated. The map shows that nearly 80% of the world's population is exposed to high levels of threat to water security. Investment enables rich nations to offset high stressor levels, but less wealthy nations remain vulnerable.

An Amendment to this paper has been published and can be accessed via a link at the top of the paper.An Amendment to this paper has been published and can be accessed via a link at the top of the paper.

Nature 467, 555–561 (2010) In this Article, the full present address for author P. B. McIntyre was inadvertently missing from the bottom of the page. The correct present address is: Center for Limnology, University of Wisconsin, Madison, Wisconsin 53706, USA. This has been corrected in the online PDF.

Major rivers worldwide have experienced dramatic changes in flow, reducing their natural ability to adjust to and absorb disturbances. Given expected changes in global climate and water needs, this may create serious problems, including loss of native biodiversity and risks to ecosystems and humans from increased flooding or water shortages. Here, we project river discharge under different climate and water withdrawal scenarios and combine this with data on the impact of dams on large river basins to create global maps illustrating potential changes in discharge and water stress for dam‐impacted and free‐flowing basins. The projections indicate that every populated basin in the world will experience changes in river discharge and many will experience water stress. The magnitude of these impacts is used to identify basins likely and almost certain to require proactive or reactive management intervention. Our analysis indicates that the area in need of management action to mitigate the impacts of climate change is much greater for basins impacted by dams than for basins with free‐flowing rivers. Nearly one billion people live in areas likely to require action and approximately 365 million people live in basins almost certain to require action. Proactive management efforts will minimize risks to ecosystems and people and may be less costly than reactive efforts taken only once problems have arisen.

Despite the recognized importance of reservoirs and dams, global datasets describing their characteristics and geographical distribution are largely incomplete. To enable advanced assessments of the role and effects of dams within the global river network and to support strategies for mitigating ecohydrological and socioeconomic costs, we introduce here the spatially explicit and hydrologically linked Global Reservoir and Dam database (GRanD). As of early 2011, GRanD contains information regarding 6862 dams and their associated reservoirs, with a total storage capacity of 6197 km 3 . On the basis of these records, we estimate that about 16.7 million reservoirs larger than 0.01 ha - with a combined storage capacity of approximately 8070 km 3 - may exist worldwide, increasing Earth's terrestrial surface water area by more than 305 000 km 2 . We find that 575 900 river kilometers, or 7.6%% of the world's rivers with average flows above 1 cubic meter per second (m 3 s −1 ), are affected by a cumulative upstream reservoir capacity that exceeds 2%% of their annual flow; the impact is highest for large rivers with average flows above 1000 m 3 s −1 , of which 46.7%% are affected. Finally, a sensitivity analysis suggests that smaller reservoirs have substantial impacts on the spatial extent of flow alterations despite their minor role in total reservoir capacity.

Despite the recognized importance of reservoirs and dams, global datasets describing their characteristics and geographical distribution are largely incomplete. To enable advanced assessments of the role and effects of dams within the global river network and to support strategies for mitigating ecohydrological and socioeconomic costs, we introduce here the spatially explicit and hydrologically linked Global Reservoir and Dam database (GRanD). As of early 2011, GRanD contains information regarding 6862 dams and their associated reservoirs, with a total storage capacity of 6197 km 3 . On the basis of these records, we estimate that about 16.7 million reservoirs larger than 0.01 ha – with a combined storage capacity of approximately 8070 km 3 – may exist worldwide, increasing Earth's terrestrial surface water area by more than 305 000 km 2 . We find that 575 900 river kilometers, or 7.6% of the world's rivers with average flows above 1 cubic meter per second (m 3 s −1 ), are affected by a cumulative upstream reservoir capacity that exceeds 2% of their annual flow; the impact is highest for large rivers with average flows above 1000 m 3 s −1 , of which 46.7% are affected. Finally, a sensitivity analysis suggests that smaller reservoirs have substantial impacts on the spatial extent of flow alterations despite their minor role in total reservoir capacity. Read More: http://www.esajournals.org/doi/abs/10.1890/100125

Despite the recognized importance of reservoirs and dams, global datasets describing their characteristics and geographical distribution are largely incomplete. To enable advanced assessments of the role and effects of dams within the global river network and to support strategies for mitigating ecohydrological and socioeconomic costs, we introduce here the spatially explicit and hydrologically linked Global Reservoir and Dam database (GRanD). As of early 2011, GRanD contains information regarding 6862 dams and their associated reservoirs, with a total storage capacity of 6197 km³. On the basis of these records, we estimate that about 16.7 million reservoirs larger than 0.01 ha – with a combined storage capacity of approximately 8070 km³ – may exist worldwide, increasing Earth's terrestrial surface water area by more than 305 000 km². We find that 575 900 river kilometers, or 7.6% of the world's rivers with average flows above 1 cubic meter per second (m³ s⁻¹), are affected by a cumulative upstream reservoir capacity that exceeds 2% of their annual flow; the impact is highest for large rivers with average flows above 1000 m³ s⁻¹, of which 46.7% are affected. Finally, a sensitivity analysis suggests that smaller reservoirs have substantial impacts on the spatial extent of flow alterations despite their minor role in total reservoir capacity.