The confluence of flow in open channel often forms complex flow structure, thereby affecting the water and sediment movement and river bed deformation at the confluence zone. The flow movement at the confluence zone of the Baisha River and the Minjiang River was analyzed based on the two-dimensional mathematical model which was verified by the measured data. The results show that:the variation of incoming water of mainstream and tributary will cause different backwater zone, mixing zone, extrusion zone and separation zone. With the increasing of discharge ratio R(the ratio of tributary to mainstream), within the backwater zone, the top support action of tributary becomes stronger, and the water level is high; within the mixing zone, the flow velocity decreases because of the enhanced interaction between tributary and mainstream; within the extrusion zone, the position of the highest velocity is closer to right bank and the high speed belt is narrower with the enhancing of the extrusion. Under the compression effect fromthe mainstream, the flow direction of tributary which flows into mainstream with right-angle will change observably. However, the curved bank of the Baisha River can make the tributary flow along the riverbank and adjust the flow direction simultaneously, which will restrain the formation of the separation zone effectively. In the two comparisoncases that the tributary stays the same while the mainstream flow changes, the greater the difference between the mainstream and the tributary, the stronger the mainstream's backwater effect is to the tributary, the longer the distance from the supporting point to the confluence zone, and the larger the backwater zone is. This study can provide theoretical basis for flood control and river regulation of the Dujiangyan reach by revealing the flow characteristics of mainstream and tributary when they impact, extrude and mix with each otherat the confluence zone.
