Exposure assessment enhanced by mathematical modeling has become an accepted technique for evaluating the environmental risk associated with the release of toxic substances into various environmental media. Mathematical models for analyzing the behavior of toxic substances in river systems range from simplified, plug-flow transport procedures to dynamic water and sediment routing procedures incorporating characterization of the physical, chemical, and biological processes that collectively determine the toxicant concentrations in time and space. Four levels of such models can be identified: (1) steady-state, constant flow; (2) plug flow, time-of-travel analytical solution; (3) quasi-dynamic routing incorporating simple sediment interaction with the bed; and (4) dynamic routing incorporating dynamic, process-oriented sediment interaction with the bed. In levels 1, 3, and 4, chemical exchange with the bed is handled as a linear dispersive interchange. In each of the four levels, sorption, volatility, hydrolysis, photolysis, oxidation, and bacterial degradation of organic toxicants are included. These four levels of models were compared by predicting pesticide fate and transport within the Yazoo River system using candidates from each level.