This paper presents the results of a laboratory study on the flow regimes at a rectangular drop with a subcritical approach flow, when the tailwater depth approaches the height of the drop or exceeds it. It was found that the flow at the drop could be in either the impinging jet or surface flow regime. An empirical scheme has been developed for predicting the occurrence of either of these regimes, which was found to be affected by the direction of change of the tailwater depth.