When sediment-laden overland flow passes across an area of reducing surface slope or increased hydraulic roughness, there is often a reduction in the sediment load carried by that flow. This paper provides a new model for the description of processes and sediment fluxes through such zones. The model describes simultaneous deposition and reentrainment of a multiclass sediment mix and includes explicit description of the mass and size composition of the deposit. A range of solutions to the developed equation for time-steady inputs is provided. These solutions are the multisediment class case when no reentrainment occurs and the single-sediment class solution when reentrainment is significant. The more general case of multiclass reentrainment and deposition is developed here, but a solution is provided in a subsequent paper. A preliminary evaluation of the model using a laboratory data set is presented. The model is shown to predict well trends of sediment concentration leaving an area of net deposition with incoming sediment concentration and unit discharge. Advantages of this new model over current models include a physically based sorting mechanism and no reliance on an externally invoked transport capacity concept.