Upstream damming greatly altered the flow and sediment regime entering downstream reaches in the Middle Yangtze River, and the bed material in a sand gravel bed reach coarsened continuously, which had a significant influence on the sediment transport and bed evolution. In order to study the riverbed armoring, the sediment exchange process (SEP) among bed material, bed load and suspended load in a sand gravel bed river is firstly clarified, and then, the three-state transition probability model (Markov chain) is proposed in this study, with the hiding-exposure effect of non-uniform sediment being considered. Finally, the equilibrium equation of sediment quantity in an active layer is presented to calculate the grain size distribution of bed material. In this model, the influences of flow and sediment conditions, riverbed erosion and deposition on the SEP are discussed. The results show that the composition of surface bed material at the Zhicheng station became obviously coarse, and the median grain size (d50) of surface bed material increased from 0.230 to 0.424 mm in 2003 2017, with an upward increasing trend. The proposed probabilistic model was validated against field measurements of bed material, and calculated results show reasonable agreement with the measured data at Zhicheng. Accordingly, the probabilistic model can be used to predict the riverbed armoring and to investigate the non-equilibrium transport of non-uniform sediment in a sand gravel bed river.
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