An integrated programme of fieldwork and modelling was set up to improve model predictions of contaminant dispersion in rivers. The practical work investigated the turbulent structure of the River Severn in order to define parameters for the particle-tracking (random-walk) model. The model was then used to simulate dispersion on two reaches of the River Severn, and this paper describes the modelling work carried out. The results show promising agreement with measured concentrations of dye at several sections on the river.