In-stream open check dams are essential for flood protection at mountain rivers that are characterized by steep slopes (>1%) and sediment-laden flood discharges. These structures comprise an opening acting as a lateral or vertical flow contraction to force sediment retention when floods occur. Otherwise, the contraction should not affect the run-off. Design criteria for the discharge capacity referring to the size and geometry of the opening were previously established. This experimental study reviews the existing formulae for the discharge capacity and analyses the beginning of sediment deposition, with varying channel slopes. The effects of the channel slope on backwater, sediment deposition and local head losses are relevant when free surface flow conditions persist in the opening. A channel slope-sensitive correction factor is introduced for calculating the discharge capacity. The sediment transfer rate through the contraction decreases with increasing backwater and is most sensitive for trans-critical flow conditions. The findings are validated against a case study in the Swiss Alps.