The Dam Effect

More than half the world's large river systems now have dams, decreasing the amount of sediment that flows to the sea. Above, sediment discharged from the Mississippi River flows into the Gulf of Mexico.

More than 65,000 dams dot the Mississippi River basin.

Soil erosion caused by human activity is markedly higher in modern times compared to the pre-agricultural era. But dams trap the eroded sediment.

In a new study reported in the April 15 issue of the journal Science, researchers say 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. The study was led by Christer Nilsson of the Umeo University in Sweden.

"Humans have extensively altered river systems through impoundments and diversions to meet their water, energy, and transportation needs," the researchers write.

In a second report in the journal, James Syvitski and colleagues at the University of Colorado-Boulder compared the amounts of sediment transported by rivers today with amounts transported before major environmental impacts by humans. They confirmed earlier studies that show soil erosion caused by human activity has increased rivers' sediment transport, but when the trapping effect of reservoirs is factored in, overall sediment transport has decreased -- especially, the scientists note, in African and Asian rivers.

More than 100 billion metric tons of sediment has been trapped by reservoirs constructed mostly in the past 50 years, Syvitski team estimates.

• What Wild Rivers Do

-- LiveScience Staff

Image courtesy of James P.M. Syvitski