Bi-directional flow observed in the main branch of the Chicago River is due, in most cases, to density currents generated by density differences between the water in the North Branch Chicago River and the Chicago River. An upward-looking 600-KHz acoustic Doppler current profiler was installed by the U.S. Geological Survey (USGS) in the center line of the Chicago River at Columbus Drive at Chicago, IL, to characterize these flow conditions. Bi-directional flow was observed eight times in January 2004 at Columbus Drive. Three bi-directional flow events, with temperature stratification of approximately 4°C, were also observed on the North Branch Chicago River. Analysis of these data indicates that the plunging point of the density current moves upstream or downstream on the North Branch Chicago River, depending on the density difference. Complementary water-quality and meterologic data from January 2004 help confirm the mechanism causing the formation of the density currents.