Climate Change Shrinks Some of the World's Largest Rivers

Several of the rivers channeling less water serve large populations, such as the Yellow River in northern China, the Ganges in India, the Niger in West Africa, and the Colorado River in the southwestern United States.

By contrast, the scientists reported greater stream flow over sparsely populated areas near the Arctic Ocean, where snow and ice are rapidly melting.

The water level in the river Ganges is low at Kanpur in the state of Uttar Pradesh, India. Farms have cropped up in the dry river bed. February 17, 2008. (Photo by Vivek Jishtu)

"Reduced runoff is increasing the pressure on freshwater resources in much of the world, especially with more demand for water as population increases," says lead author Aiguo Dai a scientist with the National Center for Atmospheric Research in Boulder. "Freshwater being a vital resource, the downward trends are a great concern."

The research team examined stream flow from 1948 to 2004 and found significant changes in about one-third of the world's largest rivers. Of those, rivers with decreased flow outnumbered those with increased flow by a ratio of about 2.5 to 1.

The rivers in the study drain water from every major landmass except Antarctica and Greenland and account for 73 percent of the world's total stream flow.

Dai and his co-authors combined stream flow measurements with computer-based stream flow models to fill in data gaps.

They found that over the 64 years studied annual freshwater discharge into the Pacific Ocean fell by about six percent, or 526 cubic kilometers - approximately the same volume of water that flows out of the Mississippi River each year.

The Colorado River is the primary river of the American Southwest.(Photo credit unknown)

The annual flow into the Indian Ocean dropped by about three percent, or 140 cubic kilometers.

But annual river discharge into the Arctic Ocean rose about 10 percent, or 460 cubic kilometers.

In the United States, the Columbia River's flow declined by about 14 percent during the study period, due to reduced precipitation and higher water usage in the West.

The Mississippi River, however, has increased by 22 percent over the same period because of greater precipitation across the Midwest since 1948, the scientists said.

Some rivers, such as the Brahmaputra in South Asia and the Yangtze in China, have shown stable or increasing flows. But they could lose volume in future decades with the gradual disappearance of the Himalayan glaciers feeding them, the authors warned.

"As climate change inevitably continues in coming decades, we are likely to see greater impacts on many rivers and water resources that society has come to rely on," says NCAR scientist Kevin Trenberth, a co-author of the study.

Many factors can affect river discharge, including dams and the diversion of water for agriculture and industry. These researchers found, however, that the reduced flows in many cases appear to be related to global climate change, which is altering precipitation patterns and increasing the rate of evaporation.

The results are consistent with previous research by Dai and others showing widespread drying and increased drought over many land areas.

Fisherman on the Niger River near Timbuktu, Mali (Photo by JNDB)

The study raises wider ecological and climate concerns.

Discharge from the world's great rivers results in deposits of dissolved nutrients and minerals into the oceans.

The freshwater flow also affects global ocean circulation patterns, which are driven by changes in salinity and temperature and which play a vital role in regulating the world's climate.

Although the recent changes in the freshwater discharge are relatively small and may only have impacts around major river mouths, Dai said the freshwater balance in the global oceans needs to be monitored for any long-term changes.

Scientists have been uncertain about the impacts of global warming on the world's major rivers. Studies with computer models show that many of the rivers outside the Arctic could lose water because of decreased precipitation in the mid-latitudes and lower latitudes and an increase in evaporation caused by higher temperatures. Earlier, less comprehensive analyses of major rivers had indicated, however, that global stream flow was increasing.

The findings will be published May 15 in the American Meteorological Society's "Journal of Climate."

The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under sponsorship by the National Science Foundation.

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