More than half of the world's large rivers are fragmented and regulated by dams. The largest and the most biologically and geographically diverse rivers are all affected. This is shown by a global study that is published in this week's issue of the journal Science.
Behind the study are Christer Nilsson, Cathy Reidy and Mats Dynesius at Umeå University and Carmen Revenga at The Nature Conservancy in the U.S.
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. Many of the ecological effects of dams are relatively well known. Despite this fact, there has so far not been any overview of how this impact is distributed globally.
The Umeå based research group now presents an overview of how the world's large rivers are regulated and fragmented by dams. The researchers examined the world's rivers with a mean annual flow of at least 350 m3/s (e.g., larger than the Torne River in northern Sweden). The only regions for which accurate data have not been available are Indonesia and a small part of Malaysia.
The study shows that flow in 172 of the 292 largest rivers is regulated by dams. 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. 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.
– When comparing continents, Europe has the highest proportion of strongly impacted rivers whereas Australia, including New Zealand and Papua New Guinea, has the largest proportion of free-flowing rivers. Overall, the degree of impact relates 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, says Christer Nilsson.
Today, there are more than 45 000 dams over 15 meters high and that together can store more than 6500 km3 of water. This equals 15 percent of the annual freshwater runoff in the world. Over 300 dams are considered giants, over 150 meters high or storing more than 25 km3 of water. The recently built Three Gorges dam on the Yangtze River in China is the largest – 181 meters high and with a storage capacity of over 39 km3.
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 name of the article is Fragmentation and Flow Regulation of the World's Large River Systems and is published in Science on April 15, 2005.
The research project has amongst others been funded by WWF Sweden, United Nations Educational, Scientific and Cultural Organization (UNESCO)/World Water Assessment Programme, United Nations Environment Programme (UNEP) and World Resources Institute. Christer Nilsson was last autumn funded by the Swedish Research Council to continue his study. His research group will now investigate how the dams have affected the vulnerability of freshwater fish.
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