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Changes in the flows of rivers would have a direct impact on the amount of hydropower generated, because hydropower production decreases with lower flows. Because of the ambiguous projections of changes in future river flow, studies of the impacts of climate change show ambiguous effects on hydropower production. As a general rule, however, a 1 percent decrease in runoff produces a greater than 1 percent decrease in hydropower production. Not only does less water run through the turbines, but the lower reservoir levels reduce the water pressure and hence the power produced by a given amount of water. In the Colorado River's lower basin, for example, a 10 percent decrease in runoff reduces power production 36 percent. (Climate Change and the Colorado River Basin.)
 The Columbia River basin produces about a third of the nation's hydropower (U.S. Bureau of the Census, 1992). Recent studies suggest that the most likely impact of climate change in this region would be greater winter flows and lower summer flows, with annual flows about the same as today. Such a change may be beneficial in the Pacific Northwest, because electricity has a greater demand in winter; in most regions, however, demand is greater in the summer. Other studies examining the implications for the Great Lakes, the Tennessee Valley, the Atlanta area, the Missouri River, and the Colorado Basin have mostly found that a decline in hydropower is more likely than an increase.
Hydroelectric power is a major source of electricity in only a few regions, and even there, it accounts for a declining fraction of all electricity. Thus, changes in hydropower would not cause major disruptions in most regional power grids.
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