This paper presents a systematic investigation on the transition from supercritical to subcritical flow at an abrupt drop. Over a wide range of experimental conditions various types of flow have been classified and the concept of low and high drops defined. When the flow on the step is supercritical, the direction of the main flow passing over the step can be decided from the momentum equation. The reason why various flow conditions are formed has been explained. The hydraulic conditions required to form various types of flow and their length characteristics have been clarified.