This study discusses the state of the flow field and the diffusion of concentration under the interaction of a stream debounchment and sea currents at a river mouth. Three‐dimensional unsteady conservation equations are used to describe the interaction of the flow field. The dynamic factors considered include the effects of buoyancy, inertia, hydrostatic pressure and mixing turbulence. Turbulence effects are modeled using an eddy viscosity through the Reynold Approximation. For convenience in calculation the coordinate system transform is used, combining the horizontal stretching transform and the sigma‐coordinate transform.