Abstract: Tributary-mouth channels of mountain rivers are affected by the game between the tributary and main stream, the water and sediment characteristics of which are characterized by both natural rivers and the reach affected by backwater from the main stream. Analyzing the water and sediment conditions of the tributary and main stream, the confluence characteristics, the backwater effects is of great significance for studying the tributary-mouth channel regulation. Taking mouth of Minjiang River as an example, a joint distribution function model of daily mean discharge during the recession period of Jinsha River and Minjiang River was constructed based on the Copula function theory and a function for the low water tributary-mouth channel formation was established using measured hydrologic and sediment data of Jinsha River and Minjiang River during 1960—2020. A quantitative method for determining the shoal-regulation stage of tributary-mouth channel was proposed. The study produced some important results: ① The annual runoff and sediment runoff showed significant decreasing trends, whereas the runoff during the dry season showed significant increasing trends in the Lower Minjiang River affected both by the large-scale development and construction of reservoirs in the upper reaches and human activities. ② The backwater effect of the Jinsha River on the mouth of Minjiang River was generally stronger during the flood season than that during the dry season which provided favorable conditions for scouring the channel during the period from the end of the flood season to the dry season and maintaining the mouth channel depth of Minjiang River. ③ The backwater height at mouth of Minjiang River increased with the increase of the confluence ratio, and there was a significant positive linear correlation between them. ④ The regulation stage should follow the principle of being 1.5 meters above the designed minimum navigable stage in the fluctuant backwater zone upstream of Yangjiaoshi, and should gradually increase from 1.5 to 2.0 meters above the designed minimum navigable stage in the reach between Yangjiaoshi and Yangjiatan.