The importance of including floodplain effects in one-dimensional river channel hydraulic computations is well known. However, in morphodynamic modeling, these are often partially ignored by using a single effective discharge to drive bed elevation changes. This neglects both changes in the flow and sediment transport fields at overbank discharges and long term change in channel capacity (and thus effective discharge) caused by deposition on or erosion from either the channel bed or the floodplain. This paper presents a model for reach-averaged channel bed and bank top elevation evolution that specifically accounts for changes in channel depth over time, the model considers two grain sizes: one for sand, which interacts primarily with the bed, and one for mud, which interacts only with the floodplain. The model also describes the evolution of the proportion sand and mud in the floodplain deposits. Sediment transport and floodplain deposition are driven by a simple gradually varied flow solution. Erosion from the floodplain is represented as a net loss associated with channel migration. Because overbank deposition is strongly affected by flow, effective floodplain deposition and in-channel sediment transport are obtained by integrating results from an entire flow duration curve. In the absence of bed elevation changes, the channel and floodplain co-evolve toward a stable bankfull geometry where overbank deposition just equals floodplain erosion.