3 Gorges Gift That Keeps On Giving - Summer Drawdowns Release Methane From Marshes - Nature

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Marshland created when China's Three Gorges Reservoir is partially drained during the summer may be a significant source of the powerful greenhouse gas methane, researchers say. The findings, published in the Journal of Geophysical Research1, are among the latest to raise questions over the green credentials of hydropower.

Scientists have become increasingly concerned about the greenhouse gases released by submerged grass and trees when land is flooded to create dams. When such organic matter decays, it releases methane and carbon dioxide, which contribute to global warming. Methane is particularly troublesome as it has more than 20 times the warming impact of CO2.

Previous studies of methane emission have focused on dam components themselves, such as the power-generating turbines — where the gas fizzes out of solution as reservoir water is fed into them — and also on the reservoir surface, where methane is released either through slow diffusion or as bubbles. But many dams are partially drained at different times of year for maintenance purposes, such as silt removal, exposing 'drawdown' areas that have previously been underwater.

"Little attention has been paid to the drawdown regions, probably because they usually constitute a small part of the reservoir surface area," says Huai Chen, an ecologist at Chongqing University in China, who was first author of the study.

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http://www.nature.com/news/2009/090929/full/news.2009.962.html