The design of open check dams at mountain rivers for flood protection has an essential impact on the sediment budget of downstream river reaches. A key issue is the bedload transfer hindering and retention which can be influenced by the geometry of the openings of check dams. Contemporary design concepts provoke the retention of too much or insufficient bedload with regard to the downstream transport capacity. This dysfunction can cause continuous maintenance works or endanger downstream infrastructures, respectively. Herein an experimental analysis of the influence of check dam openings on the bedload transport capacity is made as a function of the constriction ratio of the opening width and the mean width of the undisturbed channel. Constriction ratios of more than 0.9 have no significant influence on the bedload transport capacity, while constriction ratios of less than about 1/3 block every grain larger than 1/40 of the channel width.