One method for performing exposure assessments is to use mathematical models that predict the transport and transformations of toxic substances in the environment. To run these models, a variety of input data on receiving water-body properties and water-quality parameters that define a range of environments and influence the chemical-specific rate coefficients must be developed. A data base that contains selected hydrologic and water quality parameters has been developed by EPA for 15 major U.S. river basins. Called the Canonical Environments Data Base, the file brings together information developed by public and private organizations on each reach of the rivers as well as individual lakes and reservoirs in the basins. Coupling raw data with a data base management system in the environmental file will facilitate entry into and deletions from the data base and development of data input to the Exposure Analysis Modeling System (EXAMS) and other similar exposure assessment mathematical models. (Copyright (c) 1988 by Marcel Dekker, Inc.)