A simple discrete-time model is proposed to describe dispersion in an estuary, based on tidal exchanges between neighbouring segments. A procedure for parameter estimation and model validation is described which can be implemented with a minimum of field measurements; and the application of the model is demonstrated with data collected from the Hawkesbury River system, near Sydney. The results indicate that the modelling procedure described is capable of producing useful predictions of the response of salinity distribution to changes in fresh water input and tidal amplitude. The method could be most useful in circumstances where limitations on experimental resources preclude the undertaking of a more detailed investigation.