Abstract: Model similarity-rules for confluence between a debris flow and the main river were analyzed, and a series ofmodel experimentswere performed to investigate the regularityof confluence between a debris flowand the main river. The experimental result shows that the movement patterns of a debris flow are different in the intersection region for different relationships between the branch and the main river and can be divided the submerging flowand the stratification flow. In the submerging flow, the velocityof the debris flowhead, submerging intowater, decreases approximately linearly, the changes in its height and length are very small, and there is a linear relationship between the width and the length. All these are quite different from a density current. Furthermore, whenwater flows into the branch, the debris flowheadwill be broken bywater, resulting in the sedimentation of sand and stone. Under that condition, the debris flow entering into the river becomes a stratiform flow.