Bitcoin mining powered by hydroelectric is "green" - right?

I was at a Bitcoin Meetup last year, and the point of Bitcoin being "green" - as in the use of green energy came up.  

We consider that hydroelectric power is fairly green energy.  Or is it?  Billy Garrison raised an interesting point which I had never heard before.  That the reservoirs for dams release CO2.  I had to look into this because it sounds like an interesting phenomenon.

I found this paper: Greenhouse Gas Emissions from Reservoir Water Surfaces: A New Global Synthesis

Sure enough - hydroelectric dams produce greenhouse gasses.  It's not a lot but "... these emissions still contribute significantly to global budgets of anthropogenic CO2 equivalent emissions."

Diablo Dam © Steven Pavlov / https://commons.wikimedia.org/wiki/User:Senapa / CC BY-SA 3.0

Where do the GHGs come from?

The paper suggests 3 sources.

  1. Microbial decomposition of organic matter during the initial flooding of the reservoir,
  2. Variability in the depth of the reservoir (shoreline decay)
  3. Runoff of man-made material into the reservoir from the local land.

It is entirely feasible that hydroelectric dams are not as green as I thought!  I learned something new.