The river bed stability analysis at the Wkra estuary reach were presented in this paper. When a flood appears, channel and river bed erosion can reach a significant size. During such a situation, flow and water velocity exceed critical values for particular bed sediment grain sizes.This causes the breakage of bed armouring, and then bed mass motion begin. For estimation of the sandy Wkra River bed the criteria of average velocity and bed armouring were applied with the use of ARMOUR software. The two Wkra River cross-sections at km 1+000 and km 3+200 distance were analszed. The calculation of the characteristics of bed sediment velocities was performed on the basis of, among other factors, probable river flow data, longitudinal slope, and grain size distribution of the bed. The results of the calculations were compared with average velocities in the analysed cross-sections. The results indicate the there is a threat to the stability of Wkra River bed, especially during long-lasting high water stages. The prognosis of river bed stability loss performed by use of ARMOUR software shows that the bed armour breakage will take place in both considered cross-sections at relatively low water stages (about 40 cm), and mass motion of bed sediments in cross-section P-2 will occur at a flow of slightly lower than Q50% ,and in cross-section P-1 at flows greater then Q0,3%.