Little basis exists for calculating emissions from reservoirs. However, existing information can be organized in such a waay as to draw the best possible conclusions given the limitations of our knowledge. The present paper addresses the amounts, types, and vertical distribution of biomass in areas flooded by reservoirs. Rough inferences are drawn as to emissions resulting from decay of the biomass; but the certainty that can be attached to them is low, due to poor understanding of rates and pathways of decay for flooded biomass. Hydroelectric emmissions are the least well-understood of 'greenhouse' gas emissions from Amazonian deforestation. The ultimate contribution of dams to carbon emissions is the difference between the carbon stock in the forest prior to flooding and that in the reservoir once the forest has decayed and an equilibrium is reached.

Estimates suggest that, per unit of energy produced, greenhouse-gas flux to the atmosphere from some hydroelectric reservoirs may be significant compared to greenhouse-gas emission by fossil-fuelled electricity generation. In Canada, there are about 20,000 km^2 of peatland and upland areas covered by hydroelectric reservoirs with over 11,000 km^2 more planned. We hypothesize that development of hydroelectric reservoirs may increase the flux of CH4 and CO2 to the atmosphere. Our hypothesis is based on past studies which show that both upland forests and peatlands are sites of intense microbial decomposition and greenhouse-gas production when they become covered in water. Depending on the extent of flooding and other physical, chemical and biological features, some reservoirs may be significant emitters of greenhouse gases.

Greenhouse gas (GHG) emissions from hydroelectric dams are often portrayed as nonexistent by the hydropower industry and have been largely ignored in global comparisons of different sources of electricity. However, the life cycle assessment (LCA)of any hydroelectric plant shows that GHG emissions occur at different phases of the power plant's life. This work examines the role of decommissioning hydroelectric dams in greenhouse gas emissions. Accumulated sediments in reservoirs contain noticeable levels of carbon, which may be released to the atmosphere upon decommissioning of the dam. The rate of sediment accumulation and the sediment volume for six of the ten largest United States hydroelectric power plants is surveyed. The amount of sediments and the respective carbon content at the moment of dam decommissioning (100 years after construction) was estimated. The released carbon is partitioned into CO2 and CH4 emissions and converted toCO2 equivalent emissions using the global warming potential (GWP) method. The global warming effect (GWE) due to dam decommissioning is normalized to the total electricity
produced over the lifetime of each power plant. The estimated GWE of the power plants range from 128-380 g of CO2eq./kWh when 11% of the total available sediment organic carbon (SOC) is mineralized and between 35 and 104 g of CO2eq./kWh when 3% of the total SOC is mineralized. Though these values are below emission factors for coal power plants (890 g of CO2eq./kWh), the amount of greenhouse gases emitted by the sediments upon dam decommissioning is a notable amount that should not be ignored and must be taken into account when considering construction and relicensing of hydroelectric dams