An increasing trend in global streamflow has been variously attributed to global warming, land use, and a reduction in plant transpiration under higher CO₂ levels. To separate these influences for the coterminous United States, we use a set of over 1000 United States Geological Survey stream gauges primarily from small, minimally disturbed watersheds to estimate annual streamflow per unit area since 1920 on a uniform grid. We find that changing precipitation, which is not clearly correlated with greenhouse gas concentrations or global warming, explains most of the interannual and longer term variability in streamflow. While streamflow has indeed increased since 1920, this increase has not been steady but rather concentrated in the late 1960s, when precipitation increased. Since the early 1990s, both precipitation and streamflow show nonsignificant declining trends. Multiple regression of streamflow against precipitation, temperature and CO₂ suggests that higher CO₂ levels may increase streamflow, presumably from lower transpiration due to the physiological plant response to CO₂, but that this positive response is offset by concomitant increasing evaporation due to global warming. The net impact of the opposing climate and physiological effects of CO₂ emissions for streamflow is close to zero for the coterminous United States taken as a whole, but shows regional variation. Streamflow at a given amount of annual precipitation has decreased in the Pacific west, where most precipitation occurs in winter. Suppression of plant transpiration through higher CO₂ levels may be particularly important for sustaining high streamflow in recent decades in the Great Plains, where precipitation is concentrated during the growing season.