AbstractThe current research of carbon cycling in inland waters lacks a sound knowledge of carbon outgassing from small streams. As a consequence, these compartments of the terrestrial water cycle might still be seriously underrepresented in estimates of global carbon transfer budgets from land surfaces. This study investigated carbon sources and sinks of a mountain river in a silicate catchment. For this purpose, the high‐relief Gravona river, in the western part of the island of Corsica (France) in the Western Mediterranean, was investigated for field parameters, dissolved inorganic carbon (DIC) and its stable carbon isotopes (δ¹³C_DIC). The source region was characterized by low DIC contents and high δ¹³C_DIC values that decreased further downstream due to increasing soil respiration. Associated increasing DIC concentrations and elevated seasonality in the lower river section also indicated more respiration and subsequent DIC‐input by weathering. The aqueous partial pressure (pCO_2(aq)) was lowest at the source of the Gravona river and samples from the cold season even showed undersaturation that led to uptake of atmospheric CO₂. Further downstream, the seasonality of pCO_2(aq) increased and was particularly pronounced near the river mouth where CO₂ degassing took place. Average DIC flux along the river was 0.129 Gg C year⁻¹ and was almost equal to CO₂ degassing from the river surface with 0.128 Gg C year⁻¹. Our study showed that on an annual basis the river is an overall weak to medium source of CO₂ to the atmosphere even though headwater parts of the river seasonally act as CO₂ sinks.