Abstract: From the 2011-2017 field surveys of the Yellow River source region, bedload deposition was found to be the leading pattern in the inlet section of an oxbow lake on the Black River of the Zoige basin, but the deposition process and its control factors remain unclear yet. This paper gives a theoretical analysis to clarify this issue, draws a sketch of its relevant physics, and constructs a conceptual model of deposition process. This model is used in the numerical calculations and verified against survey data. Numerical results show that the deposition rate at the inlet section is a function of its upstream sediment supply, the length of its sand plug, and the diversion angle between the new and old channels, and that the time span of the inlet deposition stage is shorter in the case of a shorter plug, more supply, or larger angle. Application of the model to the simulations of deposition and formation of another oxbow lake in the upper Black River achieves a good agreement with measurements. It also reveals a modeling error in the lake formation during the four flood seasons of 2013-2016 and its causes ? unsteady incoming flow and sediment and their large variation ranges.