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River Fragmentation by Dams Print E-mail
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Impact classifications, synthesized from channel fragmentation and flow regulation data, for 292 of the world’s largest rivers.

Map description and dataset provided by: Umeċ University, Landscape Ecology Group

DescriptionRiver Fragmentation by Dams

The map shows 292 of the world’s largest river systems (LRSs), classified as either not affected, moderately affected or strongly affected by dams. White areas represent LRSs that did not meet the discharge criteria for inclusion in the study (VMAD ? 350 cms). Grey areas represent LRSs that might meet VMAD criteria for inclusion in the study but were data deficient.


Results of the analysis show that there are 105 strongly affected, 68 moderately affected, and 119 unnaffected large river systems. Unaffected river systems are those without dams in their catchments, but dams in tributaries may not be disqualifying if flow regulation is <2% of the VMAD. A river system is never considered unaffected when there are dams in the main channel, and it is never classified as strongly affected if there are no dams in the main channel. All river systems with no more than one-quarter of their main channel length left without dams are considered strongly affected. Please see Nilsson et al. 2005 for further explanation.


The dataset for this map was provided by: Cathy Reidy and Christer Nilsson, Umeċ University, Landscape Ecology Group


Data collection for this project began in the late 1980s, with results for the northern third of the world initially published in 1994. Subsequent work on the project led to the 2005 publication which presents results for the entire world, as well as biogeographic and socio-economic analyses.


Please cite this map as: "Nilsson, C., Reidy, C.A., Dynesius, M., Revenga, C., 2005. Fragmentation and flow regulation of the world’s large river systems. Science 308, 405–408. Source: GWSP Digital Water Atlas (2008). Map 25: River Fragmentation by Dams (V1.0). Available online at http://atlas.gwsp.org."

Contact Information

Cathy Reidy, This e-mail address is being protected from spam bots, you need JavaScript enabled to view it

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