Dams Control Most of the World's Large Rivers

UMEA, Sweden, April 15, 2005 (ENS) - More than half of the world's large rivers are fragmented and regulated by dams, including all the largest and the most biologically diverse rivers, according to new research from the University of Umea in Sweden and the Nature Conservancy in the United States.

The study shows that flow in 172 of the 292 largest rivers is regulated by dams, and that this number would be larger if irrigation were included. There are dams in the world's 21 largest rivers and in the eight rivers that are biologically and geographically most diverse.

More than 45,000 dams over 15 meters (49 feet) high have been constructed and together they can store more than 6,500 cubic kilometers of water - equal to 15 percent of the annual freshwater runoff in the world.


China's Three Gorges Dam controls the Yangtse River. (Photo courtesy NASA)
The Three Gorges dam on the Yangtze River in China is the world's largest. When completed in 2009, the dam will funnel water through 20 generator stations. The dam itself will be the largest in the world 181 meters (594 feet) high, and the lake formed behind it has displaced more than a million people.

Europe has the highest proportion of dam impacted rivers, says the study's principal researcher Christer Nilsson, a landscape ecology professor in the Department of Ecology and Environmental Science at the University of Umea.

The region that includes Australia, New Zealand and Papua New Guinea has the largest proportion of free-flowing rivers, he said.

Overall, says Nilsson, how severely dams impact free-flowing rivers is related to population density and economic development.

The few river systems that buck this trend are in places such as northern Canada, where dams were built in sparsely populated areas for the export of electricity and/or water, Nilsson says.

The rivers in temperate forests and savannahs belong to the highest impact class, whereas many rivers in the tundra and in northern coniferous forests still remain free-flowing.


Professor Christer Nilsson is in the Department of Ecology and Environmental Science at the University of Umea. (Photo courtesy U. Umea)
Over 300 dams are considered giants, standing over 150 meters (490 feet) high or storing more than 25 cubic kilometers of water.

"Humans have drastically changed many rivers by impoundments and diversions to meet the needs of water, energy and transportation. Such exploitation belongs among the most dramatic, deliberate impacts that humans have had on the natural environment," the Umea research group said in a statement.

The study's results will affect the assessment of how future climate changes and the constantly increasing use of water will impact the rivers' ecosystems in different parts of the world.

The article, entitled "Fragmentation and Flow Regulation of the World's Large River Systems" and is published in the journal "Science" today.

Other scientists on the study are Cathy Reidy and Mats Dynesius at Umea University and Carmen Revenga of the Nature Conservancy.

The research project was funded by WWF Sweden, the UNESCO/World Water Assessment Programme, the United Nations Environment Programme, and World Resources Institute. Nilsson was last autumn funded by the Swedish Research Council to continue his study.

For the next stage of their research, Nilsson said, " I am leading a new project about the effects of global warming on wetland vegetation, and a global project on how river exploitation threatens the fish fauna."

He and Reidy will begin to explore the survival status of freshwater fish in rivers that are either free-flowing or affected by habitat loss. For this project, the scientists will investigate species extinctions caused by habitat losses.

"Some rivers," said Nilsson, such as the Amazon, La Plata and Congo, have several hundreds or thousands of fish species. In addition, in the Amazon and Congo rivers more than half of the species are endemic, which makes them especially vulnerable."

The upcoming research will predict the global distribution of freshwater fish that are threatened by extinction. In addition, the team will attempt to determine the global distribution of river systems that are affected by habitat loss because of dams.