The hydrologic processes of cascading hydroelectric reservoirs differ from those of lakes due to the importance of inflows and outflows that vary with energy demand. These heat and water advection terms are rarely considered in energy balance analyses of water bodies, even though reservoirs are common (especially in North America), and as such can affect the regional climate. This study provides a comprehensive assessment of the water and energy balance of the 85-km2 northern reservoir Romaine-2 (50.69°N, 63.24°W), mean depth 44 m, highlighting the significant contribution of the advection heat fluxes. The water balance input was mainly controlled by upstream (turbine) inflows (77.6%), while lateral (natural) inflows and direct precipitation represented 21.2% and 1.2%, respectively. For the reservoir heat balance, net heat advection accounted for an average of 25.0% of the input, of which net radiation was the largest component (73.3%). Taking into account the lack of energy balance closure, latent heat and sensible heat fluxes represented 73.2% and 25.1%, respectively, of the total energy output from the reservoir. The thermal regime was influenced by the hydrological flow conditions regulated by the reservoir management. This played an important role in the evolution of the thermocline and the temperature of the epilimnion, and ultimately in the dynamics of the turbulent heat fluxes. This study suggests that the heat advection term represents a large fraction of the heat budget of northern reservoirs and should be properly considered.