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