The definition of the initial spatially varying grain size distributions is one of the key challenges in river morphodynamic modelling, which affects morphological changes such as bed elevation modifications. The grain size distribution is generally not accurately known. It is subject to variation during transport, and is sensitive to the history of flow and sediment supply. This study presents multiple approaches to determine the sensitivity of the grain size distribution in the river bed of the Iffezheim reservoir of the Upper Rhine River in Germany, based on discharge period with dominant deposition. The quantitative comparison of predicted and measured bed elevation changes indicated the model, with adequate spatially varying grain size distribution of bottom sediment in the Iffezheim Reservoir is generally, in good agreement with the measured data.