This paper presents a dynamic box model for the radionuclide behaviour in rivers on medium- and long-term periods (several days to several years). The river is described as a succession of boxes representative of its different reaches. In each reach, the compartments are the water column and three bottom sediment layers. Called interface, the first layer plays a fundamental role for the vertical exchanges of solid radionuclide phases between the water column and the sediment. The second layer results from the consolidation of the previous one. Its interstitial water is mobile and the dissolved radionuclide phases can be exchanged with the water column. It is called active. The last layer results from the consolidation of the active layer. Its interstitial water is slightly mobile and it is assumed that its dissolved radionuclide phases cannot be exchanged. It is called passive. In each compartment, the model computes the temporal evolution of the radionuclide activities in the main abiotic and biotic components. The abiotic components are the water and different matter classes classified according to their deposit kinetics. The biotic components are phytoplankton, zooplankton, and fish distributed in planktonivorous and omnivorous species, in water column and macrobenthos in bottom sediment.