In the debate about hydroelectric power, one of the most contentious issues involves greenhouse gases. Like tidal and wave power, hydroelectric power does not burn fuel and so does not generate carbon dioxide emissions directly. Some commentators have therefore argued that hydroelectric power would reduce greenhouse gas emissions and moderate climate change. For example, Kim Murphy in a 2009 Los Angeles Times article noted that "The ability of the nation's aging hydroelectric dams to produce energy free of the curse of greenhouse gas emissions and Middle Eastern politics has suddenly made them financially attractive—thanks to the economics of climate change."18
There is some question, however, about whether hydroelectric dams do in fact reduce greenhouse emissions. When a dam is built, the reservoir created behind it floods a large land area. The result is that a large number of land plants end up underwater. Because the water levels of the lake continually rise and fall, new plant matter on the banks is continually growing and then being submerged. This means that the lake is continually being supplied with dead plant matter. This dead plant matter is broken down by bacteria underwater, where there is no oxygen, in a process that produces a gas called methane.
The methane dissolved in the water is held in solution by the high pressure of all the water above it. Thus, in normal circumstances, the methane would enter the atmosphere only slowly, through the surface of the lake. When the dam gates are open, however, and the water rushes through, that pressure is suddenly relieved. The effect is like opening a bottle of Coca-Cola, with gas
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