Particle image velocimetry(PIV)technique is used to accurately determine the location of shear layer and separation zone based on vorticity. Meanwhile, the vortices in the horizontal plane at the confluence are systematically observed and analyzed. It is found that the vortex density within both the shear layer and the separation zone is larger near the water surface and near the bottom than that in the middle of water depth. The vortex density increases with the increase of tributary discharge when the discharge of main channel is fixed. The high density of vortices near the bottom is caused by the wall turbulence. In addition, the shear layer generated by flow shear is dominated by vortices with small swirling strength, while the separation zone generated by the flow separation is dominated by vortices with medium and small swirling strength.