Risk assessment of intentional or accidental discharges of toxic substances into river systems requires combined hydraulic and chemical modeling. Periodic discharges of known volumes with low radioactivity by the Beznau nuclear reactor (Switzerland) serve as validation tracers for both river flow and chemical speciation simulation. Validation of the former has been achieved by comparison of modeled and measured arrival times of radiocobalt along a 65 km transect with a maximum reaction period of 24 hours. Modeled breakthrough curves coincide well with measurements collected during three field campaigns, in spite of the fact that sorption and sedimentation processes were not activated during simulation. This gives indirect evidence of inefficient cobalt sorption. Particle/solution distribution measurements carried out during breakthrough allow further validation of our speciation approach, which is based on Tableau setup of inorganic reactions combined with sorption and organic complexation. Modeled and measured speciation results confirm recent observations of enhanced complexation of cobalt with dissolved organic substances, which significantly reduces particle sorption. The large variability of conditional stability constants for sorption and complexation reactions, for sorption site densities, and for organic ligand concentrations explains the variability of published particle-solution distribution coefficients.