An equation is derived for the distribution of a solute in an estuary where river flow and tides are predominant factors and where the dynamics and mixing can be described by a simple physical model. This model is subsequently generalized in various ways and it is found that the basic form of the diffusion equation is unchanged. An equation is derived which allows for time dependence of the varioUB quantities involved and for introduction of a solute into the estuary by an external agent. Some solutions of this equation are studied.