Abstract: Sedimentation pattern is one of the important factors affecting reservoir capacity and reservoir sediment flushing. In the near-dam area of Xiaolangdi Reservoir on the Yellow River, fine-grained muddy deposits are easy to flow like fluid. In this paper, the rheological property and flow pattern of muddy deposits were investigated. It was found out that the flow pattern of low density muddy deposits is that of Bingham flow, and its fluidity will decrease quickly when its density exceeds 1.25 g/ cm³. Base on analysis of shear stresses at interfaces between water and muddy deposits and between muddy deposits and channel bed, governing equations for the movement of muddy deposits were established. A simulation method for fine-grained sedimentation processes was developed by coupling the model for water flow and sediment transport with the model for muddy deposits movement. Calculation results indicate that sedimentation pattern of fine-grained particles in the Xiaolangdi Reservoir can be simulated accurately by considering the effect of muddy deposits movement. This investigation could improve our understanding of sediment deposition mechanisms in reservoirs, especially for fine-grained sediment.