The paper presents investigation results considering morphological changes and sieve curves modifications within the outlet of upland sand-gravel Raba River. Great intensity of the processes forming the bed of Raba River in this place are the key to preserve the natural mechanism of dynamical continuity and natural character in the face of general river training and use of both the Raba and Wisla Rivers. Field survey consisted from measurement of geometrical parameters for a main channel it’s bed forms, granulometry of the surface and sub-pavement, combination of riffles and pools. From this analysis the character of dynamic processes modifying river while the bedload transport continues can be described. The samples of riverbed material were taken using in-situ method to obtain the layered structure of the dunes and the sub-surfaced granulometric composition. The local dynamic of the Raba channel leads to the erosion and lowering of the bed level and on some of the locations on accumulation and changes in local thalweg moving it between the banks. The granulometric composition analysis was used to describe the influence of raised water stages on the riverbed and to interpret the main mechanisms determining changes in granulometry and the movement of bedforms. Estimating the dynamic hydraulical values in cross-section was used to list the limitations of the movement of bed forms. The calculations proved critical depth for the bedload movement in the surroundings of the bed form 15 cm but the water depth over and around the form varying from 10 cm to 23 cm making thereby the form as the key element determining the limits between movement and sedimentation of riverbed material.