The convection and dispersion of pollutants in a deltaic estuary system are calculated using several interfaced computer programmes. The basic programme is a Dronkers type one dimensional hydrodynamic model, which is interfaced with a model of the salt water intrusion in the seaward reach. The importance of Pitt Lake, a large fresh water but highly tidal lake, is discussed, inasmuch that it integrates water quality changes over long periods of time. The salt wedge analysis reveals the nature of the interfacial stress which is not a function of velocity or velocity squared. It is shown that the interfacial stress is a function of entrainment, rather like a Reynolds stress, and plots are given of interfacial stress against local Richardson number multiplied by local upper layer velocity.