The influence on hydraulic countermeasures in the Chernobyl zone on the radioactive contamination of the Pripyat river and Dnieper reservoirs is studied on the basis of the set of mathematical models of radionuclide aquatic transport. The models are validated using post-accidental monitoring data. The most efficient countermeasure for water protection after the Chernobyl accident was the dam construction on the left-bank Pripyat River floodplain at the Chernobyl Nuclear Power Plant. The effectiveness of the continuation of this project by the right-bank dam construction is evaluated by means of simulating the ⁹⁰Sr concentration in the Pripyat River and the Dnieper reservoirs.