As an environmentalist, I thought I was pretty well informed about the causes of global warming and the sources of greenhouse gases. Recently I watched the documentary Damnation. The movie describes our love affair in early part of the last century of dams and then discusses the current state of dams their many weaknesses.
I think most people are aware that dams stop fish from moving upstream and have destroyed the livelihoods of many native people. What I did not know prior to watching this documentary is that dams are a significant source of greenhouse gas emissions.
Greenhouse gases are primarily methane and carbon dioxide. The rotting organic matter in a reservoir is generating both of these gases. The organic matter is from both the plants that were originally flooded over when the reservoir was built but also new material when silt and plant material is washed in from the watershed above. Brazilian researchers estimated in 2007 that methane from dams is responsible for around 4% of human-caused climate change. Estimates range from 20% to significantly more of all man made methane is coming from the surface of reservoirs. Methane is 30 times more potent than carbon dioxide as a heat trapping gas.
Despite a deluge of rain in December, California is in a record drought but there are a lot better solutions than building and maintaining dams. Off stream reservoirs are better for holding water as they do not interfere with the fish life cycle. Even more compelling is that the silt is much less likely to build up as the water that goes into a reservoir is taken off the top of the river flow. Better yet is groundwater recharge placing the water underground in existing natural aquifers. Even better yet is simple water conservation such as washing your Tesla with one cup of water.
Stanford University has a 125 year old dam on its property, the Searsville dam. The reservoir is so filled with silt that it only has 10% of its original capacity. The aging dam and reservoir no longer provide drinking water, flood control or public recreation access. Stanford had stated they would determine the fate of the dam in 2014 but failed to do so. The Beyond Searsville Dam organization is a 501(c)(3) non-profit organization and has more information about this particular dam and its impacts. Many more dams like Searsville exist throughout the United States.