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A mathematical model was developed to predict the impact of dairy and beef cattle feedlot runoff on receiving streams. The mathematical expressions used in the model describing runoff quantity and quality were not only a function of single rain or snow precipitation events but also consecutive events prior to the runoff occurrence. The runoff quantity and quality were also a function of feedlot surface. Computer simualtions indicate that pollutants from feedlot runoff may have a significant impact on receiving streams during winter months. Runoff from feedlots located within the study area, however, had little impact on water quality in the summer. The computer imulations were compared with field data collected within a subdrainage system of Cache Valley, Utah. Concentrations of pollutants within the streams were higher in summer. This is believed due to mixing of stored pollutants in the stream sediments with the overlying water.