BOX 4.4

Global Impact of Dams

Humans have constructed more than 45,000 dams above 14 m in height, which together are capable of holding back about 15 percent of the total global annual river runoff (Vörösmarty et al., 2003). Dams have reduced the total amount of sediment carried to the ocean by about 20 to 30 percent, even though human activities have increased the total sediment production by 30 percent. And dam building continues; between 160 and 320 new dams are built annually, especially in Asia. Dams cause major changes to local, and ultimately worldwide, physical, chemical, and ecological systems and in many cases simply terminate river ecosystem functions.

Despite the negative impacts of dams, demand for power, flood control, and water supply means that many will remain and more will be built. Nonetheless, much can be done to preserve the desired physical, chemical, and ecological characteristics of affected watersheds. For example, the release of water from reservoirs could be designed to mimic important natural functions, such as sediment transport, recruitment of riparian vegetation, and fish reproduction. As we learn more about the long-term consequences of sediment depletion in downstream rivers and coastal environments, we can take action to compensate.

Summary of the impacts of dams on major global river systems. (Top) Geographical distribution of 633 large reservoirs (i.e., those with a storage capacity of 0.5 km3 or greater). (Bottom) Efficiency of basins in trapping suspended sediment. In some basins sediment load is severely restricted by dams along the river course; in some cases virtually all sediment is trapped. SOURCE: Vörösmarty et al. (2003). Copyright 2003 by Elsevier Science and Technology Journals. Reproduced with permission.

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