Behind the study are Christer Nilsson, Cathy Reidy and Mats Dynesius at Ume 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 Ume 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
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Contact: Karin Wikman
karin.wikman@adm.umu.se
46-090-786-6024
Swedish Research Council
14-Apr-2005