The paper considers the building of a stochastic model to describe the dynamics of dissolved oxygen and biochemical oxygen-demand concentrations in water streams. The water stream is divided into several reaches and the dispersion between the reaches is considered by treating the concentration of each pollutant advected across each interface as the output of an interface dynamical system. A method is proposed for identifying the model and the interface system's dynamics. The proposed procedure is applied to obtain a dynamic model for the water quality in Åbu-Qîr Bay, Alexandria, Egypt, using actual data; and a prediction of the water quality levels is made with the model.