Dirty Hydropower?

Is there no human activity that won't produce greenhouse gases? New Scientist has an article (subscription required) indicating that hydropower, once considered clean, may actually create more -- and worse -- greenhouse gases than fossil fuels!

Hydroelectric dams produce significant amounts of carbon dioxide and methane, and in some cases produce more of these greenhouse gases than power plants running on fossil fuels. Carbon emissions vary from dam to dam, says Philip Fearnside from Brazil's National Institute for Research in the Amazon in Manaus. "But we do know that there are enough emissions to worry about."

In a study to be published in Mitigation and Adaptation Strategies for Global Change, Fearnside estimates that in 1990 the greenhouse effect of emissions from the Curuá-Una dam in Pará, Brazil, was more than three-and-a-half times what would have been produced by generating the same amount of electricity from oil.

This is because large amounts of carbon tied up in trees and other plants are released when the reservoir is initially flooded and the plants rot. Then after this first pulse of decay, plant matter settling on the reservoir's bottom decomposes without oxygen, resulting in a build-up of dissolved methane. This is released into the atmosphere when water passes through the dam's turbines.

Unbelievable! One wonders, though: don't rivers have decaying matter at their bottoms anyway? How much more does a dam really create? And if this is true, isn't this a good excuse to kill all the world's beavers and dynamite their dams?