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Damming the World's Rivers
River sediment discharged from the Mississippi River.
Image courtesy of James P.M. Syvitski
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Humans are having a major impact on the world's rivers, according to two new studies in the 15 April 2005 issue of the journal Science. We've built dams in more than half of the large river systems and have decreased the amount of sediment flowing to the world's coasts by nearly 20 percent.
In one study, "Fragmentation and Flow Regulation of the World's Large River Systems," Christer Nilsson and colleagues surveyed how dams have fragmented the world's largest river systems and affected the flow of water. The authors report that 172 out of 292 of the world's large river systems are affected by dams, including the eight systems that are most biogeographically diverse.
In a second paper, "Impact of Humans on the Flux of Terrestrial Sediment to the Global Coastal Ocean," James Syvitski and colleagues compared the amounts of sediment transported by rivers today with amounts transported during "pre-Anthropocene" times, before major environmental impacts by humans. They found that soil erosion caused by human activity has increased rivers' sediment transport, but human-made reservoirs also trap sediment, reducing transport. Overall, sediment transport has decreased, especially in African and Asian rivers.
Kathy Wren
15 April 2005
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