E-Letter responses to:

reports: Christer Nilsson, Catherine A. Reidy, Mats Dynesius, and Carmen Revenga Fragmentation and Flow Regulation of the World's Large River Systems Science 2005; 308: 405-408 [Abstract] [Full text] [PDF]

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Published E-Letter responses:

Large dams are only part of the truth

Christian Wolter   (24 May 2005)

Large dams are only part of the truth 24 May 2005
Christian Wolter, Fish Ecologist Leibniz-Institute of Freshwater Ecology and Inland Fisheries Respond to this E-Letter: Re: Large dams are only part of the truth	Nielsson et al. (Science 308: 405), have summarized interesting facts about 292 large river systems around the world; however, in their general findings, the authors did not added substantially to their previous work (1). Focusing only on large dams will heavily underestimate human degradations of large rivers, because of an estimated ratio between large and small dams of 1:15 to 1:20 (2). In the United States, the national register reported a fourfold larger surface area of all reservoirs than does the large dam register of the ICOLD (3). Even low- head dams of about 2 m completely prohibit migrations of aquatic organisms especially fish and, thus, fragment populations and communities. Accordingly, the figure of 48% unfragmented rivers dramatically ignores the ecological risks in one of Earth’s most severely human degraded ecosystem types. Additionally, the distance between the river mouth and the first dam should also include an ecologically highly relevant parameter, because almost the first dam impassable for aquatic organisms prevents diadromous species from entering the catchments and causes their disappearance or extinction. I’m further wondering how a virgin annual discharge is measured if the hydrologic regime of a river has been changed completely due to irrigation and increased evaporation and green water.