A major discovery by scientists on man-made lakes could forever change how we think of water reservoirs that we rely on for fresh water and for producing hydropower. And it could change the perception of hydropower as a clean energy source in a world that desperately needs one.

That’s because the study is claiming that water reservoirs have a dirty secret: they actual produce a huge amount of carbon dioxide and may actual be helping warm our Earth. That’s based on an analysis of more than 200 studies the examine how these man-made reservoirs contribute to greenhouse gas emissions. And the contribution isn’t minor: scientists estimate that our reservoirs produce a gigaton of carbon dioxide each year, which would eclipse the annual greenhouse gas emissions put out by Canada, according to a statement from the University of Washington.

How does this happen? Basically, when reservoirs are created, a valley or plain is flooded, and organic material in the soil is gobbled up by microbes in the water, which produces methane and carbon dioxide. And the process continues, as reservoirs are typically hooked up to rivers that continue to deliver fresh organic matter.

The statement reads: “Writing in next week’s journal BioScience, the WSU researchers say reservoirs are a particularly important source of methane, a greenhouse gas that is 34 times more potent than carbon dioxide over the course of a century. Reservoir methane production is comparable to rice paddies or biomass burning, both of which are included in emission estimates of the Intergovernmental Panel on Climate Change, the leading international authority on the subject.”