Step spillway flow phenomenon is known for about 100 years. Subject of special interest in terms of application in engineering practice is the change of submerged bottom flow conditions for step spillway and surface flow conditions when depth behind the step spillway is increasing. In reference literature and scientific publications there are often theoretical and empirical dependences of determining hydraulic depth, corresponding to flow rates’ changing limits behind the downstream by step. But empiric formulas have clear range of application. Results of experimental findings on investigation of flow rates’ changing limits (plane problem) behind the downstream by step spillway, taking into account small Froude numbers, are given. Experiments were carried out on hydraulic model. The empirical dependences for upper limit evaluating the formation of surface flow conditions and submergence of surface and surface-bottom flow conditions are obtained. Dependences are in line with experimental findings.