This paper presents an environmental analysis of the impact of an emergency discharge of untreated municipal sewage from the sewage system in Warsaw. This study was based on monitoring water quality in the Vistula River at stations located before and after the discharge site. Statistical analysis, including correlation analysis and Friedmann’s test, was performed for the data covering eight measurement stations and sixteen chemical parameters. The results confirm the negative impact on the water quality in the river, especially in the mixing zone. In the mixing zone, a statistically significant increase in pollution was found for TN, N-NO3−, N-NH4+, TP, P-PO43+, SS, EC, COD, and TOC. The results of the analysis show the self-cleaning capacity of the river ecosystem and illustrate the response to a sudden, excessive influx of pollution. The direct impact of the untreated sewage discharge was found on a section of the river 1200 m after the place of the failure. The good ecological status and drinking water standards are not met in the mixing zone. The supply of water to the people requires the use of highly effective physical and chemical treatment.