The correlations between annual precipitation, evaporation, temperature, and annual carbon export (bicarbonate, dissolved organic carbon, and particulate organic carbon) are established for the Ohio, upper Mississippi, and Missouri watersheds and coefficients that predict C export on the basis of precipitation, evapotranspiration, and seasonal temperature are provided. Interannual variation in carbon export is controlled by the level of precipitation and evapotranspiration, the discharge to precipitation ratio (D:P), the concentration of the major carbon pool, the rating curve (discharge versus carbon concentration plots), and seasonal temperature. Within a watershed, precipitation has the strongest correlation with annual carbon export for all carbon pools. In the upper Mississippi and Ohio, levels of evapotranspiration can predict the majority of the residuals of precipitation versus carbon export. Bicarbonate demonstrates the largest response in yield to annual precipitation variation for each watershed. Rating curves, however, indicate that dissolved organic carbon and particulate organic carbon generally increase with increasing discharge, while bicarbonate decreases, causing a larger percentage response to precipitation for the organic carbon pools. Across watersheds the discharge to precipitation (D:P) ratio is the dominant determinant of how carbon yields respond to changes in precipitation, and watersheds with a high D:P ratio should demonstrate large changes in carbon yields with forecasted changes to precipitation.