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Carbon dioxide (CO₂) evasion from streams and rivers to the atmosphere represents a substantial flux in the global carbon cycle^,,. The proportions of CO₂ emitted from streams and rivers that come from terrestrially derived CO₂ or from CO₂ produced within freshwater ecosystems through aquatic metabolism are not well quantified. Here we estimated CO₂ emissions from running waters in the contiguous United States, based on freshwater chemical and physical characteristics and modelled gas transfer velocities at 1463 United States Geological Survey monitoring sites. We then assessed CO₂ production from aquatic metabolism, compiled from previously published measurements of net ecosystem production from 187 streams and rivers across the contiguous United States. We find that CO₂ produced by aquatic metabolism contributes about 28% of CO₂ evasion from streams and rivers with flows between 0.0001 …