Freshwater resources, because of a host of human assaults, but especially because of dams, are the most degraded of the Earth's major ecosystems. Now the future of every dam on Earth is threatened-- not by environmental protests or economic constraints-- but by the Greenhouse Effect and the world's changing climate. Historical and geological evidence over past millennia indicate that even small changes in climate can cause major changes in the size of floods. Insurers increasingly are convinced that global warming is to blame for the greater frequency and severity of violent storms, floods and droughts since the late 1980s.
Hydrologists cannot predict exactly how much water will flow into a planned reservoir. To make a "best guess," they project past streamflow data into the future. Overestimates of average flows mean that many dams fail to yield as much power and water as predicted, the Buendia-Entrepenas reservoir in Spain is an example.
Sedimentation, despite over 60 years of research, still may be the most serious technical problem faced by the dam industry. In the US, large reservoirs lose storage capacity at an average rate of 0.2% per year, in China the rate is closer to 2.3%. Despite all the uncertainties surrounding reservoir sedimentation, authorities very rarely stop planned projects due to a lack of adequate sediment data.
McCully, Patrick, International Rivers Network
American Rivers produced abstract