Carbon dioxide (CO 2) evasion from streams and rivers to the atmosphere represents a substantial flux in the global carbon cycle. The proportions of CO2emitted from streams and rivers that come from terrestrially derived CO2or from CO2produced within freshwater ecosystems through aquatic metabolism are not well quantified. Here we estimated CO2emissions 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 CO2production 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 CO2produced by aquatic metabolism contributes about 28% of CO2evasion from streams and rivers with flows between 0.0001 and 19,000 m 3 s -1. We mathematically modelled CO2flux from groundwater into running waters along a stream-river continuum to evaluate the relationship between stream size and CO2source. Terrestrially derived CO2dominates emissions from small streams, and the percentage of CO2emissions from aquatic metabolism increases with stream size. We suggest that the relative role of rivers as conduits for terrestrial CO2efflux and as reactors mineralizing terrestrial organic carbon is a function of their size and connectivity with landscapes.