Studying the carbon balance in surface waters gives information on the annual cycles of photosynthesis and respiration. It also provides insight on the water body's capability to serve as a source or sink for atmospheric CO2, which may be essential in evaluating the effects of climate change. The target of this study was the Vantaanjoki River known to have a significant base flow component, located in a densely populated area in southern Finland. The aims of this study were firstly to study if human induced changes are evident in the inorganic carbon quality of the river, and secondly to determine whether the river releases carbon to the atmosphere. These aims were achieved by studying the isotopic composition and contents of dissolved inorganic carbon (DIC) in relation to river discharge. It was evident from the results that the human activities only have mild local and temporal effects on the quality of the DIC in the river. The most important contributors to the changes in the carbon balance are the annual changes in the proportion of the base flow and surface flow components and the escape of CO2 to the atmosphere.