Hydroelectric power's dirty secret revealed
...resulting in a build-up of dissolved methane. This is released into the atmosphere when water passes through the dam's turbines. (...) In effect man-made reservoirs convert carbon dioxide in the atmosphere into methane. This is significant because methane's effect on global warming is 21 times stronger than carbon dioxide's.
In practical terms, I read that as: we must extract this methane and burn it. Post peak, the carbon released from using this particular and limited source (dammed water) should be sustainable within the carbon cycle. Overall emissions will be dropping.