"There is a major controversy brewing on how carbon neutral large scale
hydroelectricity really is. It has been known for a long time that dams
emit both methane and CO2. The question has always been, how much of
those emission are net? According to the International Rivers Network
<http://www.irn.org/pdf/greenhouse/FizzyScience2006.pdf> (PDF), studies
by ecologist Philip Fearnside of the National Institute for Research in
the Amazon (INPA) have shown that net methane emissions from hydropower
are slightly higher than those from burning natural gas.
"Rivers generally have organic matter trapped in their silt and mud.
This rots in the dark, wet environment, producing methane that's
dissolved in the water. In undammed rivers, bacteria consume the
methane, converting it to CO2. (CO2 is a much less potent greenhouse gas
than methane.) According to Fearnside's research, the higher pressure
created by large-scale dams forces the dissolved methane out.