Carbon dioxide (CO2) evasion from streams greatly contributes to global carbon fluxes. Despite this, the temporal dynamics of CO2 and its drivers remain poorly understood to date. This is particularly true for high-altitude streams. Using high-resolution time series of CO2 concentration and specific discharge from sensors in twelve streams in the Swiss Alps, we studied over three years the responsiveness of both CO2 concentration and evasion fluxes to specific discharge at annual scales and at the scale of the spring freshet. On an annual basis, our results show dilution responses of the streamwater CO2 likely attributable to limited supply from sources within the catchment. Combining our sensor data with stable isotope analyses, we identify the spring freshet as a window where source limitation of the CO2 evasion fluxes becomes relieved. CO2 from soil respiration enters the streams during the freshet thereby facilitating CO2 evasion fluxes that are potentially relevant for the carbon fluxes at catchment scale. Our study highlights the need for long-term measurements of CO2 concentrations and fluxes to better understand and predict the role of streams for global carbon cycling.