1D model of the Drava River, between 00 + 543 rkm and 69 +118 rkm, was created by the MIKE 11 river-modeling tool. The aim of this model was to analyze the river water quality (WQ). Compared to the real Drava River section, the model was simplified to exclude any tributaries for the purpose of the water quality analysis. The inflow boundary was specified upstream, in addition to the WQ parameters (dissolved oxygen (DO), temperature, ammonia nitrogen, nitrate nitrogen, total phosphorus and orthophosphate, and five-day biochemical oxygen demand (BOD5)). The water level and open boundary conditions were specified at the downstream end of section. The WQ parameters, mentioned earlier, were also defined at the Belišće wastewater treatment plant (WWTP) (54 + 373 rkm) and at the wastewater outlet near Osijek (14 + 580 rkm). The model was then used to simulate the effect on the WQ of the Drava River in a hypothetical extreme scenario: the 2-day failure of the Belišće WWTP. It was assumed that during this time untreated wastewater would be discharged into the river. After running the simulation, the resulting concentrations of DO, BOD5, and ammonia nitrogen were analyzed. The results showed that, according to the concentration values of the untreated wastewater, the concentrations of BOD5 and ammonia nitrogen rapidly increased and the DO concentrations rapidly decreased with the initiation of the WWTP failure. Immediately after the WWTP was repaired, the parameters returned to their pre-failure values. The resulting analysis of the effects of this scenario highlighted the self-purification ability of the Drava River.