As the threat of global warming drives the global population towards finding environmentally-friendly methods of energy generation many people are inclined to suggest hydropower. After all, it is just the harnessing of the potential and kinetic energy within a waterway. It seems like it should be a relatively low-impact source of energy collection. Not to mention that we have been building dams since the seventh millennium BC in order to create artificial bodies of water.  Yet, as we move towards greening our energy resources we must begin to question even the practices that we have long considered an integral component to human development of land. We must begin to ask ourselves what the impacts of these practices are and if we need to change these behaviors in order to protect ourselves from the predicted doomsday of global warming. In recent years, many researchers have begun to take a closer look at the environmental impacts of hydropower dams, only to discover that hydropower does not live up to the label as an environmentally friendly method of energy production. There are many reasons why dam creation is not as environmentally sustainable as we had once thought, but one of the most important factors that we had not considered was methane production in tropical reservoirs.
When a dam is created a large area of land is flooded in order to create reservoir for the dam. In the Amazon Basin the terrain is extremely level, which means that more land typically needs to be flooded in order to generate the energy head needed for reasonable power generation capacity. For example, if a dam is built in a mountainous region the flooded area would be much smaller because you need to flood less land when you make the reservoir deeper. Unfortunately, the large tract of land that is flooded by the reservoir turns from a methane sink to a source. This is because typical Amazonian Basin terrain has been shown to absorb methane, but Amazonian reservoirs produce vast quantities of methane. 
Methane is produced because when the large area of tropical land is flooded in Amazonia in order to make a reservoir there is a large mass of plants that also gets submerged. When all of these plants get submerged there becomes a huge amount of organic carbon that is drawn down to bottom of the reservoirs. Due to the lack of physical mixing and high rate of oxygen consumption the reservoirs contain very low levels of oxygen as one travels down in the water column. The bottom layers of the reservoir are almost completely free of oxygen, which means that decomposition happens anaerobically. When anaerobic degradation of carbon occurs by bacteria at the bottom of the reservoir, methane is produced as a byproduct. Then, this methane forms small bubbles at the bottom that bubble up and diffuse through the water column. This methane is transported directly to the air by the form of bubbling or by off-gassing of the waters. Methane is off-gassed from the water as it is transported through the spillway or when it is transported downstream. 
Unfortunately, this production of methane does not decrease over the lifetime of the dam. Actually, the methane production of a dam usually increases throughout the lifetime of the dam.  This is thought to happen because of the accumulation of organic carbon from upstream in the reservoir and the slow degradation process that occurs. The reservoir traps sediment that is rich in organic material that would otherwise have been transported downstream or to the ocean where it would have probably been degraded in a more oxygen rich environment. Thus, this organic material would have actually degraded and formed more carbon dioxide as byproduct rather than methane.
It is important to note that methane, CH4, has a 21 times higher global warming potential than carbon dioxide.  So, when carbon is degraded in an environment without oxygen, like a reservoir, the global warming potential created by that degradation is 21 times worse than if it had been degraded in an oxygen rich environment.  So, the massive amounts of organic carbon that is transported downstream in Amazonian Rivers is being trapped in these reservoirs and converted into a more severe global warming agent than would have otherwise been formed.
The emissions from many tropical dams have not yet been accurately measured. However carbon emissions from the Petit Saut hydroelectric reservoir, in French Guiana, were quantified for the first ten years of impounding.  Diffusive flux measurements and bubbling emissions were measured directly from the reservoir.  Degassing from the downstream turbines and in the immediate downstream were measured by direct flux measurements.  The total emissions from this one dam were 0.37 +/- 0.1 Mt of carbon for the first three years and then it decreased to a yearly emission rate of 0.12 +/- 0.01 Mt of carbon for the next seven years of measurement.  The carbon emissions were supplied from many regions of the hydroelectric reservoir. On average, 61% of the carbon dioxide emissions occurred as diffusion from the surface of the reservoir, 31% from the estuary immediately downstream, 7% from degassing at the outlet of the dam, and a negligible fraction was supplied via bubbling.  The methane emission sources were distributed quite differently than the carbon dioxide emissions. During the first year of inundation of the reservoir 40% of methane emissions occurred through bubbling to the surface.  However, after the first year an average of 70% of the methane emissions occur as degassing at an aerating weir downstream of the turbines. 
The tropics are especially a bad place for reservoirs to occur because the higher temperatures and flooding of large amounts of biomass leads to high levels of methane production over the lifetime of the dam.  It has been estimated that artificial reservoirs that have been created in the tropics could be emitting about 64 megatons of methane each year, which would account for 90% of the methane emissions that occur in the tropics.  With methane emissions on the rise there are several growing world sources of methane including rice cultivation, coal and gas mining, breeding of animals that ruminate, biomass burning, and landfill spills. Yet, if reservoirs are included in the sources of methane over the entire world surface the world budget of methane would be dramatically increased. If we included the estimated emissions from all tropical reservoirs, then the emission estimates for methane from the entire world surface would be increased by 15 to 20%.  Thus, it is important that we include this source of global warming emissions when we calculate the environmental impact of this energy source.
© Katy Ashe. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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