Arizona’s largest reservoirs generate electricity for a number of western states and are widely considered sources of clean energy. But conservationists are beginning to question how clean that power is.

Although hydroelectric power does not generate greenhouse gasses, researchers have found that large reservoirs emit methane. One study showed that Lake Powell greenhouse gasses were roughly half those of a natural gas power plant, while Lake Mead’s were on par with a coal-fired station.

Gary Wockner is with the nonprofit Save the Colorado, which has asked the Environmental Protection Agency to use the findings in its climate change reports.

"The fundamental issue is that although hydro power does look from the eye to be clean, there’s a lot going on beneath the surface. Whenever vegetation, or any kind of organic material, decomposes, it decomposes and emits carbon and methane,” Wockner said.

Save the Colorado was part of a coalition of environmental groups that sent a petition to the EPA, which was signed by more than 140 conservation groups.