Human‐induced changes in bed shear stress and bed grain size in the River Waal (The Netherlands) during the past 900 years

Human impact has changed the flow characteristics and bed‐sediment characteristics of lowland rivers, affecting channel stability, flood risk, navigability and biodiversity. We analysed the effect of human activities on the flow and bed‐sediment characteristics of the River Waal (The Netherlands). The objectives were: (a) to reconstruct the historical change in bed shear stress during the past 900 years; (b) to reconstruct the contemporary change in bed grain size; and (c) to identify the main causes of these changes. Various data types were used, such as borehole descriptions, historical river maps and modern hydraulic data. It was found that the bed shear stress in the River Waal strongly increased during the past 900 years. In the same period, the gravel content of the sandy river bed increased, causing a coarsening of the river bed. Before ad 1870 the shear stress increase and bed coarsening were mainly due to embankment (artificial levee construction). After ad 1870, the shear stress increase and bed coarsening were mainly due to river narrowing and dredging. The systematic grain‐size difference between present‐day and historical rivers should be taken into account in river restoration projects.