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Humans have significantly affected the flow of half of the world's major rivers by building dams, according to the first detailed global assessment of the activity's impact, published today (15 April) in Science.

Another study, also in today's issue of Science, shows that dam building, among other human actions, has altered the amount of sediment that rivers carry to the coast, with implications for marine ecology, fisheries and coastal erosion.

The first study, led by Christer Nilsson of the Landscape Ecology Group at Umeå University, Sweden, assessed the extent to which dams affect 292 large river systems. These rivers drain 54 per cent of the world's land area and carry 60 per cent of the planet's river-water.

Previous surveys had only looked at rivers in the northern hemisphere. Nilsson and colleagues added to this collection new data from Africa, Asia, Latin America and Australasia.

The team found that river flow has been strongly affected in four of the world's ten largest river systems and moderately affected in the other six.

Fragmenting river systems and controlling their flow with dams has well-known potential impacts on the ecology of rivers. Among other effects, dams disrupt the migration of animals up- and downstream, an impact that has been linked to the extinction of species of freshwater fish.

While areas upstream of dams are flooded to create reservoirs, wetlands downstream can dry out and the fertility of floodplain soil can decline.

The authors insist that the severe ecological effects of building dams "need to be accounted for in global planning for sustainable river management".

They note that large dams are planned or under construction in 46 large river systems, 40 of which are in developing countries.

Almost half of these new dams will be on just four rivers: China's Chang Jiang (also known as the Jangtze River), where 49 new dams are planned, the Rio de la Plata in Latin America (29 new dams), the Shatt Al Arab in the Middle East (26 new dams), and the Ganges-Brahmaputra in South Asia (25 new dams).

Another impact of dams, highlighted by the second study in Science, is that they retain sediment that would normally be carried downstream by rivers.

Led by James Syvitski of the University of Colorado, United States, the study compared the amount of sediment currently carried down rivers around the world to what they would be expected to be with no history of human activity.

They found that while some human activities — such as damming — have decreased the flow of sediment, others have meant that more of it reaches the coasts.

Excessive sediment can suffocate coral reefs and seagrasses, which can make fish populations decline. Less sediment, however, can make coastal area more vulnerable to erosion.

On the whole, say the authors, the positive and negative influences of human activities on water flow balance each other out. The net result, however, is that less sediment is being carried down rivers globally, with considerable differences on the regional level.

In Indonesia, where fewer dams have meant fewer reservoirs in which sediment can build up, more sediment is building up along the coastline as a result of human activities — principally deforestation.

Generally, Africa and Asia have seen the largest reduction in sediment to the coast.

Link to full paper by Syvitski et al

Link to full paper by Nilsson et al

Three Gorges Dam, Hubei Province, China

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