Abstract: A system for determining the thermal characteristics of a cooling system and controlling the cooling system in response to the determined thermal characteristics. The flow pattern of a cooling reservoir having a surface exposed to the atmosphere is represented by a plurality of water segments each representing a specified time interval, preferably equal time intervals. A thermal characteristic of the segments is determined at least periodically, preferably in response to the prevailing thermal transfer characteristics at the surfaces of the segments, and the flow pattern of the cooling reservoir is modified to accommodate the required cooling needs of the system and to prevent the discharge of cooling water into natural reservoirs if the thermal characteristics of the cooling water do not permit such discharge because of set standards. A method for at least periodically calculating the temperature of each segment from known and assumed thermal data through a trial iteration process is also disclosed.