We analyze the hydrodynamics and sediment transport on a mudflat in Willapa Bay, Washington State, United States. Velocity profiles and suspended sediment concentrations were simultaneously measured for 46 days in a major flow-through channel, in a dead-end tributary channel, on the channel bank, and on the adjacent tidal flat, encompassing periods with and without wind waves. A lateral circulation, perpendicular to the direction of the main channel, is observed to be associated with high sediment discharge directed from the channel to the tidal flat at the beginning of flood. This sediment discharge is able to explain the turbid tidal edge, which is a common feature of many tidal flats. An analytical model describing the lateral circulation and a conceptual model describing the sediment spillover from the channel are proposed. According to the model, the tidal flat sediment dynamics are strongly influenced by the sediment input from the main channel during fair weather, a process that is often overlooked in simplified models of tidal flat morphodynamics.