This App calculates the critical dissolved oxygen deficit, concentration, and downstream distance from a point source load of Biochemical Oxygen Demand (BOD) in a river using the Streeter-Phelps equations. The method used is documented in the US EPA document, EPA/600/6-85/002a (Water Quality Assessment: A Screening Procedure for Toxic and Conventional Pollutants in Surface and Ground Water). This App may be used as a screening tool to determine the potential for dissolved oxygen water quality standards to be violated. If results suggest that the dissolved oxygen sag is close to or below the water quality standard, then a more sophisticated model would be used to derive appropriate effluent limits.