The potential for degassing of carbon dioxide stemming from the passage of water through turbines of hydroelectric reservoirs was studied in two boreal reservoirs (La Grande 2 and La Grande 3) located in the James Bay region of Québec, Canada. Samples of dissolved CO₂ were taken monthly over a period of 1 year from the main reservoirs, within the hydroelectric facilities from the shaft entering the turbine system and from the exits below the facilities. Diffusive fluxes from the reservoir surfaces were calculated using the thin boundary layer equation. The differences between CO₂ concentrations above and below the dams were used to calculate the amount of degassing per unit of water turbined. Diffusive flux calculations indicated that the reservoirs acted as sources of CO₂ to the atmosphere throughout the sampling period, with fluxes ranging between 80 and 1800 mg CO₂ m⁻² d⁻¹ at LG2 and between 400 and 1500 mg CO₂ m⁻² d⁻¹ at LG3. Degassing calculated from turbining ranged between 5–45 and 5–25 t d⁻¹ at LG2 and LG3, respectively, and represented between <1 and 7% and mean weighted values of <1% of the equivalent fluxes across the air-water interface of the main reservoirs. The quantity of degassing is seasonally defined, with highest rates observed in the winter/spring period, a result of lower water temperature effects on the solubility of CO₂, and the buildup of gases over the winter period due to mineralization of organic matter and the influx from watershed sources due to the springtime melt. Depending on the effluxes occurring at the air-water interface of the main reservoir, degassing can represent a maximum equivalent 16%. This study indicates that the main role of turbining lies in the seasonality of release of GHG rather than the absolute amount.