Fine-sediment storage within floodplain systems typically represents a significant component of the catchment sediment budget and a primary control on sediment-associated nutrient and contaminant fluxes at the basin scale. However, quantitative modeling of floodplain sedimentation within whole catchments represents a significant challenge, not least because hydraulic controls on sediment transport and deposition processes operate at fine spatial scales that cannot be resolved by basin-scale models. This paper outlines a new approach to addressing this problem in order to develop a computationally efficient model of floodplain sedimentation, which retains a strong physical basis. The approach involves two stages. First, a simple theoretical model of overbank sedimentation is presented, which quantifies the relationship between floodplain geometry, overbank discharge, and total sedimentation rate. Second, the precise form that such a relationship should take is established by analysis of the output from high-resolution flow and suspended sediment transport models applied to 22 floodplain reaches along three U.K. rivers. This analysis supports a simple power law model between discharge in excess of bankfull and sedimentation rate per unit valley floor length per unit sediment concentration. The power law model is then incorporated within a sediment budget framework and implemented using a Monte Carlo approach to allow an assessment of uncertainty in the model parameterization. The relative likelihood of competing model structures is determined using estimates of mean annual floodplain sediment storage derived by analysis of the 137Cs content of floodplain sediment cores. Comparison of 137Cs-derived sedimentation rates with uncertainty-bounded model predictions highlights the potential for assessing the significance of overbank sedimentation within catchment sediment budgets using the simple power law model.