In order to understand water chemistry after impoundment of the Three Gorges Reservoir (TGR), the authors assessed the hydrogeochemical parameters (water temperature, pH, conductivity, dissolved O2), major element composition, and the carbon stable isotopic value of dissolved inorganic carbon (δ13CDIC) in summer and winter at various depths in the water column at TGR in 2009. In reservoir water, the DO values are lower in winter than in summer, but the pH values and conductivity values are higher. Ca2+ and Na+ are the dominant cations and HCO3− and SO42− are the dominant anions. In surface waters, the δ13CDIC values are more negative in summer than they are in winter, whereas the dissolved inorganic carbon (DIC) concentrations are relatively lower. In the water column, the DIC and δ13CDIC values do not change significantly with water depth or seasons. The DIC content shows a dilution effect in summer and is not modified by phytoplanktonic activity or photosynthesis. Even after the dam obstructed flow, the chemical profile of water in TGR is similar to that of the natural rivers before impoundment. Therefore, in this study, the water at TGR still had riverine characteristics and was still a heterotrophic system.