Abstract: Sediment deposition in Three Gorges Reservoir(TCR)is likely to increase significantly after the filling of TCR.The causes of reservoir sedimentation pattern in the vicinity of Three Gorges Project(TCP)during the early filling of TCR are examined using the observed data and a 3D mathematical model for flow and sediment transport.Results show that the sedimentation characteristics in the vicinity of TCP are not only related to the 3D properties in flow patterns,but also related to the characters of deposited sediment particles during the early filling of TCR.The formation of fluid mud is the result of deposited fine sediment particles in the vicinity of TCP during early filling.The particle is in a fine form and the initial dry unit weight of deposited sediment is also small,which exhibit a typical feature of fluid mud.The sedimentation volume value nearby TCP is tremendous due to the lower flow velocity.The deposited sediment particles in the form of fluid mud slide move downward to the lowest position of cross-sections under the influence of gravity,and then accumulate there in a horizontal form following the thalweg line.