The 5-day biochemical oxygen demand (B.O.D.5) contained in wet weather urban discharges from combined sewer overflows and separate storm runoff can result in depletion of dissolved oxygen in receiving streams below acceptable levels. This study was undertaken to determine the effects of the wet weather and dry weather discharges on minimum dissolved oxygen (D.O.) levels in the Red River, at Winnipeg, Manitoba, and to predict the effects of various pollution control strategies, such as wet weather flow treatment, dry weather flow treatment, and low flow augmentation, on the D.O. levels of the Red River. The Storage Treatment Overflow Runoff Model (STORM) was used to predict B.O.D.5 loadings in urban runoff for 1977. These loadings were input to a computer based planning level river quality model developed for this study, using the Streeter-Phelps formulations to predict minimum D.O. concentrations in the Red River. The effect of continuous discharges were examined using a dry weather flow model. Both models were calibrated to measured data... It was concluded that the D.O. level in the river is sensitive to variations in dry weather and wet weather flow treatment. A minimum of secondary treatment of dry weather flows is necessary to keep average minimum D.O. above 5 mg/L. The treatment of wet weather flows will reduce the number of violations of a D.O. concentration of 5 mg/L.