Comparative experiments were performed to examine generation factor of three dimensional structure in periodical large vortices at the boundary between a river flow and vegetation zone along the river. The first comparative experiment showed that the existence of the large vortices itself is most dominant factor rather than the effect of vegetation. The second one indicates that the important mechanism lies in the movement over the depth, not in the turbulence near the bed. And measurement of velocity fluctuations revealed that a strong upward flow is generated slightly behind the center of the vortex.