Six reservoirs located in the Western United States (F. D. Roosevelt, Dworshak, Wallula, Shasta, Oroville, and New Melones) were sampled in order to estimate their greenhouse gas (GHG) emissions. Two types of fluxes were assessed: (1) diffusive fluxes of methane (CH4) and carbon dioxide (CO2) at the air/water interface and (2) degassing fluxes of CH4 and CO2 from water passing through the turbine spillways. Diffusive flux measurements indicated that the surface of the reservoirs were a source of CH4 during the sampling period (from +3.2 to +9.5 mg CH4 m−2 d−1). Oroville (+1026 mg CO2 m−2 d−1) and Shasta (+1247 mg CO2 m−2 d−1) surfaces were also sources of CO2. In contrast, the surface of all the other reservoirs constituted sinks for CO2 (from −349 to −1195 mg CO2 m−2 d−1). Degassing fluxes ranged from +0.003 to +0.815 t CH4 d−1, and from +16 to +324 t CO2 d−1. Daily GHG budgets ranged from +0.146 to +2.228 t CH4 d−1, and from −15 to +224 t CO2 d−1. Degassing fluxes represented an important term of these budgets. A significant correlation was observed between the magnitude of CO2 diffusive fluxes and the water pH (R2 = 0.81; p < 0.0001). All other correlations between GHG diffusive fluxes and independent variables tested were weak and/or not significant. Finally, while attempting to resolve the spatial variability in diffusive fluxes, we were able to cluster reservoirs neither according to geological nor ecological criteria.