This research report is part of the Water Pollution Control Research Series which describes the results and progress in the control and abatement of pollution in our nation's waters. The River Basin Model described is a computer-assisted decision-making tool in which a number of computer programs simulate major processes related to water use that take place within a geographical boundary. These processes include migration, housing selection, employment, transportation, shopping patterns, the allocation of leisure time, and water quality determination. Users of the model provide inputs to these programs on behalf of business activities in the economic sector, groups of people or population units in the social sector, and government departments in the government sector. This document includes printed computer outputs (maps showing characteristics of the region which differ geographically, summaries which present information in capsulated form, and detailed information from which the summaries are derived) which provide a yearly report of the status of the simulation and of interactions within the region during the previous year. (SHM)