Abstract Detroit River flow reversals were investigated using a water surface gradient analysis in conjunction with Detroit River unsteady flow models. Three cases and five highly probable cases were simulated to occur between 1900 and 1986; the most recent episode occurred in April 1984. Flow reversals are likely only during St. Clair River ice jams, when the water supply to Lake St. Clair is severely restricted. The reversals appear to be of limited duration, less than 12 hours, with maximum flows less than 4,200 m 3s −1. Flow reversals were most common during the first 40 years of this century and 46 years separate the last two occurrences. The decreased frequency probably results from the 7.6 m and 8.2 m navigation dredging projects on the St. Clair River. The use of the Gibraltar water level gage to represent the mouth of the river was found to be critical for the analysis.