Small hydroelectric dams, despite their "green" reputation, are creating "hotspots" of greenhouse gas emissions, a European researcher says.

The large reservoirs of water behind the world's 50,000 large dams are a known source of methane, Andreas Maeck of the University of Koblenz-Landau in Germany said, coming from decaying organic matter in the sediments that accumulate behind them.

Methane is one of the greenhouse gases that trap heat near Earth's surface and contribute to global warming, but has a warming effect 25 times more powerful than carbon dioxide, another greenhouse gas.

Maeck and his colleagues decided to examine methane releases from the water impoundments behind smaller dams that store water less than 50 feet deep.

They analyzed methane release from water behind six small dams on a European river.

"Our results suggest that sedimentation-driven methane emissions from dammed river hot spot sites can potentially increase global freshwater emissions by up to 7 percent," they reported.

Such emissions are likely to increase due to a boom in dam construction fostered by the quest for new energy sources and water shortages, they said.

The study was reported in the American Chemical Society's journal Environmental Science & Technology