The study by the National Center for Atmospheric Research suggests the reduced stream flow potentially threatens future supplies of food and water.
The scientists, examining stream flow from 1948 to 2004, said they found significant downward changes in about one-third of the world's largest rivers, including the Yellow River in northern China, the Ganges in India, the Niger in West Africa and the Colorado in the southwestern United States.
In contrast, the scientists reported greater stream flow over sparsely populated areas near the Arctic Ocean, where snow and ice are rapidly melting.
"Reduced runoff is increasing the pressure on freshwater resources in much of the world, especially with more demand for water as population increases," said NCAR scientist Aiguo Dai, who led the study. "Freshwater being a vital resource, the downward trends are a great concern."
Although many factors can affect stream flow, the scientists said reduced flows in many cases appear linked with global climate change, which is altering precipitation patterns and increasing evaporation.
The research will appear in the May 15 issue of the American Meteorological Society's Journal of Climate.
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