We studied the spatial and temporal variation in O2 and dissolved inorganic carbon (DIC) forms concentrations in ten subtropical watersheds located in the state of São Paulo, Brazil, with different degrees of impact by urbanization and land-use changes. Additionally, we used stable carbon isotopic composition of DIC to explain observed patterns. We found that land-cover changes and watershed geology are the main drivers of DIC distribution. Land-cover/use changes influence the riverine DIC in two ways: by replacing the original Cerrado 3 (C3)-type forest vegetation by C4-type vegetation composed of grasses (pasture), and by sugarcane. Most domestic sewage is dumped untreated into rivers in the state of São Paulo. Consequently, in the most densely populated watersheds, sewage is an important source of labile carbon and consequently of DIC to rivers. In terms of geology, although silicate weathering that produces kaolinite is the main type of weathering in the watersheds, the weathering of carbonate cements present in the geological formations of the western portion of the state of São Paulo are also an important source of DIC to rivers.