It is now widely accepted that boreal rivers and streams are regionally significant sources of carbon dioxide (CO2), yet their role as methane (CH4) emitters, as well as the sensitivity of these greenhouse gas (GHG) emissions to climate change, are still largely undefined. In this study, we explore the large-scale patterns of fluvial CO2 and CH4 partial pressure (pCO2, pCH4) and gas exchange (k) relative to a set of key, climate-sensitive river variables across 46 streams and rivers in two distinct boreal landscapes of Northern Québec. We use the resulting models to determine the direction and magnitude of C-gas emissions from these boreal fluvial networks under scenarios of climate change. River pCO2 and pCH4 were positively correlated, although the latter was two orders of magnitude more variable. We provide evidence that in-stream metabolism strongly influences the dynamics of surface water pCO2 and pCH4, but whereas pCO2 is not influenced by temperature in the surveyed streams and rivers, pCH4 appears to be strongly temperature-dependent. The major predictors of ambient gas concentrations and exchange were water temperature, velocity, and DOC, and the resulting models indicate that total GHG emissions (C-CO2 equivalent) from the entire network may increase between by 13 to 68% under plausible scenarios of climate change over the next 50 years. These predicted increases in fluvial GHG emissions are mostly driven by a steep increase in the contribution of CH4 (from 36 to over 50% of total CO2-equivalents). The current role of boreal fluvial networks as major landscape sources of C is thus likely to expand, mainly driven by large increases in fluvial CH4 emissions.