Thesis: Estuaries are dynamic systems that ensure the transition of water and contaminants from the river to the sea, including radionuclides released by nuclear installations in normal or accident situations.137Cs is a recurrent radionuclide in accidental situations and persistent in the environment. It has a strong affinity with particles in a fluvial environment but may be desorbed in the salinity gradient, a mechanism promoting its dispersion at sea and its transfer to marine organisms. The institute for radiation protection and nuclear safety (IRSN) has among its missions the description of the transfer of radioactivity into the environment. One tool is the numerical modeling of these transfers. For aquatic environments two models reproduce transfers in fluvial and marine environments without real coupling.This thesis work sets up this coupling on the Rhône-Mediterranean continuum for cesium 137 from a general box-model for estuaries. The latter takes into account the water exchanges (diffusion, advection) and 137Cs desorption that can occur when particles from the river come into contact with salt water. Laboratory experiments and a review of the literature allow to define the key parameters to assess the intensity of desorption. The dispersion at sea of a radioactive plume emitted from the Rhône in the event of an accidental release upstream of the river is modeled for the 6 most recurrent hydroclimatic scenarios (river flow and wind conditions) obtained by a fuzzy clustering algorithm.