An analysis of the stream flow data for 925 of the world’s largest rivers over the last 56 years has found that rivers in some of the most populated regions are losing water. The authors of the study suggest that the reduced flow is linked to climate change, and say that it could put water and food supplies in jeopardy in the future.

The scientists, from the National Center for Atmospheric Research, examined stream flow from 1948 to 2004 and found significant changes in about a third of Earth’s largest rivers. Some of the rivers actually increased flow, but the losers outnumbered the gainers by a 2.5 to 1 margin. The Colorado, Niger, Yellow, and the Ganges rivers, all serving large populations, were among those found to be channeling less water.

“Reduced runoff is increasing the pressure on freshwater resources in much of the world, especially with more demand for water as population increases. Freshwater being a vital resource, the downward trends are a great concern.” – NCAR scientist Aiguo Dai, lead author

Among the factors affecting river discharge are dams and agricultural and industrial diversion of water, but the researchers found that in many cases, the reduced flows appear to be related to climate change. The altered precipitation patterns and increased evaporation rate from an increase in average temperatures contribute to increased drought conditions. The impact on future water and food supplies may be severe in the affected areas.

“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.” – NCAR scientist Kevin Trenberth, co-author

The study found that from the years 1948 to 2004, the annual freshwater discharge into the Pacific dropped by around 6%, and the annual flow into the Indian Ocean fell by 3%, but the annual river discharge into the Arctic Ocean rose by about 10%. In the U.S., the flow of the Columbia River dropped by about 14% due to reduced precipitation and high water usage, while the Mississippi River’s flow has increased by 22% over the same period.

Other concerns were raised by the study, such as the effect that reduced freshwater flow can have on the ocean’s circulation patterns, which also affects climate regulation. The lead author says that although the changes in stream flow appear relatively small and may only impact the areas around the mouth of the rivers right now, monitoring of these freshwater flows is necessary to measure long term changes.

The rivers analyzed in the study account for 73% of the world’s total stream flow, and drain water from every major landmass except Antarctica and Greenland. The results will be published on May 15 in the American Meteorological Society’s Journal of Climate.