Description of RAND's water-quality simulation model and a heat transport model that has been incorporated into it in order to determine the effect of thermal discharge on aquatic ecosystems, an important issue in the increased construction of U.S. nuclear power plants. In addition to the hydrodynamic equations (continuity and momentum) used in the water-quality simulation model, a set of generalized mass-heat-balance equations, coupled to one another through a reaction matrix, are derived. Specifics of surface heat exchange processes, responsible for external sources or sinks of thermal energy into the waters of a bay or estuary, can be inserted into the reaction matrix in various ways. One is to include the entire surface heat exchange in the source term. This approach is now being incorporated into the water-quality simulation model, and numerical and graphical results for specific estuaries currently under investigation will be reported shortly.