How green is hydroelectic power?

You'd hope that hydroelectric power would be fairly sound way of producing electricity and one that had a fairly low carbon footprint once you've constructed the dam.

However interesting new research indicates that the flooding of valleys to create the reservoirs are increasing the amount of methane emissions due to the trees and vegatations rotting under the water. The problem isn't too bad in cold climates such as the UK, but is proving to be a major problem in countries such as Brazil who have heavily invested in hydroelectic.

The Guardian covers this in more detail in a story called Hidden Dangers