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The grain size preserved within the stratigraphic record over thousands to millions of years has several important applications. In particular, it can serve as a record of significant climatic, eustatic, or tectonic events. Here we present a new model for grain size fining predictions that combines a landscape evolution model based on the Stream Power Law but modified for sedimentation (Yuan et al., 2019) with an extension of the self-similar grain size model (Fedele and Paola, 2007). The new model, which we called GravelScape, includes the effects on grain size fining of lateral heterogeneities in deposition rate caused by dynamically evolving channels. We show that, when multi-channel dynamics (i.e. avulsions) are prevented, by reducing the planform model to a single downstream dimension, our new model can reproduce results obtained by other methods that assume that fining is controlled by subsidence only. We demonstrate that including across-basin (two-dimensional) effects can lead to deviations from previous subsidence predictions for grain size fining. The magnitude of these deviations correlates with the extent of sediment bypass and the configuration of surface topography, both of which influence the amplitude of across-basin variability within the sedimentary system.