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Streams and rivers emit petagrams of CO2 yet there is little known about how discharge (Q) variability impacts stream CO2 at broad scales. This paper analyzed changes in pCO2 and CO2 effluxes (F) with Q in 814 conterminous US streams and rivers. Half of the streams and rivers (49%) show positive pCO2–Q responses, while significantly more (80%) show increasing F with Q. While F tends to increase with Q at all stream orders (SO), pCO2 decreases in small streams and rivers (SO 1–6) and increases in large rivers (SO 7–10). Small streams and rivers tend to release rather than store CO2 at higher Q due to highly responsive gas transfer velocity (k). While high F and F–Q responsiveness reflect strong terrestrial connectivity in small streams (especially SO 1–3), increasing pCO2 and F in large rivers are due partly to increased net heterotrophy and river–corridor exchanges.