Accurate quantification of CO₂ flux across the air-water interface and identification of the mechanisms driving CO₂ concentrations in lakes and reservoirs is critical to integrating aquatic systems into large-scale carbon budgets, and to predicting the response of these systems to changes in climate or terrestrial carbon cycling. Large-scale estimates of the role of lakes and reservoirs in the carbon cycle, however, typically must rely on aggregation of spatially and temporally inconsistent data from disparate sources. We performed a spatially comprehensive analysis of CO₂ concentration and air-water fluxes in lakes and reservoirs of the contiguous United States using large, consistent data sets, and modeled the relative contribution of inorganic and organic carbon loading to vertical CO₂ fluxes. Approximately 70% of lakes and reservoirs are supersaturated with respect to the atmosphere during the summer (June–September). Although there is considerable interregional and intraregional variability, lakes and reservoirs represent a net source of CO₂ to the atmosphere of approximately 40 Gg C d^–1 during the summer. While in-lake CO₂ concentrations correlate with indicators of in-lake net ecosystem productivity, virtually no relationship exists between dissolved organic carbon and pCO_2,aq. Modeling suggests that hydrologic dissolved inorganic carbon supports pCO_2,aq in most supersaturated systems (to the extent that 12% of supersaturated systems simultaneously exhibit positive net ecosystem productivity), and also supports primary production in most CO₂-undersaturated systems. Dissolved inorganic carbon loading appears to be an important determinant of CO₂concentrations and fluxes across the air-water interface in the majority of lakes and reservoirs in the contiguous United States.