In order to map the freshwater reservoir effect (FRE) variability of the Loire River and its tributaries, spatial and temporal carbon isotope (C-13 and C-14) analyses of the dissolved inorganic carbon (DIC) were conducted. Sites were selected to represent the diversity of geological settings, soil type, and land use. Results show a large spatial variability of C-14 FRE ranging between 135 and 2251 +/- 30 yr, objectively correlated to DIC contents and alkalinity. Deeper investigations of the relationship between C-14 activity of DIC and environmental variables show that the geological substrate is the dominant factor in the C-14 reservoir effect, and far more influential than the river flow discharge.