Most studies dealing with greenhouse gas (GHG) emissions from large boreal reservoirs were conducted during the ice-free period. In this paper, the potential methane (CH4) and carbon dioxide emissions are estimated for two hydroelectric reservoirs, as well as for a small experimental reservoir from boreal latitudes (northern Quebec, Canada) at the ice break-up event through diffusion (diffusive fluxes) and release of bubbles (bubbling fluxes). The results of this preliminary study suggest that the winter diffusive fluxes at the air–water interface of the sampled reservoirs represent < 7% of their cumulative carbon emissions during the ice-free period. Furthermore, the release upon ice-break of CH4 bubbles accumulated under the ice cover during the winter could represent 2% of the summer carbon emissions from hydroelectric reservoirs in northern Quebec. The results presented herein suggest that the GHG emissions upon ice break-up from the boreal reservoirs investigated are a small, but non-negligible, component of their annual GHG emissions.