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Radioactive tracer measurements carried out in three Polish mountain rivers have allowed to determine the critical dimensionaless shear stress, fi, required to set in motion a given size di of particles. The different functions, fm/fi=ϕ(di/dm) (fm—the critical dimensionaless shear stress relative to the mean grain diameter, dm), have revealed that extrapolation of results from some rivers studies is very difficult. An attempt at determination of incipient bed load motion, on the basis of the grain size distribution has been made. The measured grain size distributions before and after flood could be compared with the radio tracer measurements results. The Gessler (1970) function has been applied to determine the armour layer motion beginning. The critical shear stress values were obtained for the all particle diameters at which their motion had begun, fm=0.034-0.040 corresponding to dm=0.07-0.14 m. The standard deviation of the grain size distribution was variable δ=1.8-3.2. The presented investigations concern the floods, when the maximum water flow was Qmax1=5.50 m3/s, Qmax2=2.75 m3/s, Qmax3=5.75 m3/s, Qmax4=5.90 m3/s. The amounts of the transported bed material during these floods were: G1=6.25 T, G2=2.70 T, G3=57.4 T, G4=181.1 T. It was concluded that the proposed method can be used in many similar cases if the incipient motion of bed materials is too difficult to measure, but only when the changes of the river bottom caused by flood are known. [ABSTRACT FROM AUTHOR]