Abstract: Since the Xiaolangdi Reservoir (XLD) operation, bed materials coarsening and changes in flow resistance have been occurring substantially in the Lower Yellow River (LYR).Thus, to quantitatively describe the effects of XLD on flow resistance in the LYR, the varying characteristics of flow velocity, river width, bed material particle diameter and Manning's coefficient were analyzed using measured data from the hydrological stations.A movable bed resistance formula incorporating the bedform index was developed based on control factor theory and measured bedform data.The accuracy of the formula was verified using 1 508 data sets measured at the hydrological stations.The results highlight that, first, the bed material is coarser in the upper reaches and finer in the lower reaches of LYR.More developed dunes and higher flow resistance occurred in the wandering reach.Second, in reaches downstream of Gaocun, the bed coarsening and flow resistance reduction are less pronounced than in the wandering reach.The cross-section of the main channel is narrower and deeper, increasing wall resistance obviously during flood periods.XLD operation has facilitated bedforms development and increased flow resistance in the LYR.