<p>Drought impedes the lateral flux of carbon along continental river networks to the oceans due to reduction in water flow. Here we show that, in addition to lower flow, drought also exacerbates secondary carbonate formation and atmospheric CO₂ emissions. We report sub-hourly field measurements of major ion chemistry in the upper Sangamon River in North America, revealing that drought accelerates the characteristically rapid pace of carbonate mineral formation at the watershed scale. Over 195 consecutive days of moderate hydrologic drought, we observe a 17% decrease in annual riverine lateral carbon flux due to secondary carbonate formation, in addition to the restrictions imposed by low flow rates, indicating a natural limit to the inorganic carbon carrying capacity of rivers. We use historical datasets and a compilation of global river geochemistry measurements to show that this phenomenon is reproducible and widespread during periods of drought.</p>