Bubbly Creek is a tributary of the Chicago River, IL. It is used mainly as a Combined Sewer Overflow (CSO) route to the Chicago River. The CSO events from the Bubbly Creek carry sewers into the main channel and have a great impact on the overall water quality downstream. The sewer water mixed with particles usually has a higher density than the ambient water in the main channel. Upstream intrusion of sewer water is possible during the CSO event. In this paper, the three-dimensional environmental fluid dynamics code (EFDC) is used to simulate the CSO event in Bubbly Creek. The main purpose is to investigate the extent of the upstream intrusion and its implications for the water quality effects on the Chicago River. The results show that the front location is a function of the relative CSO flow and the CSO particle concentration.