Natural levee formation along a large and regulated river: The Danube in the National Park Donau-Auen, Austria

We investigated overbank deposition and natural levee formation along the Danube east of Vienna based on airborne laserscan data (ALS) combined with historical information.The analyses of equally spaced cross sections, hypsographic curves and point elevations revealed the significance of the distance from the river bank, the floodplain width and the floodplain flow situation (inflow/outflow sections) for the spatial variability of sediment deposition. Deposition was high near the banks (natural levee formation), in wide floodplain sections and in inflow areas. The overall pattern was modified by the presence of side-channels and the micro-relief of the floodplain.From the derived sedimentation rates (averaged for the past 120 years over the reach) of about 11.0 mm yr− 1 at the natural levee and about 0.3 mm yr− 1 at the flood protection dyke, we can estimate an overbank deposit rate of 416,000 m3 yr− 1 for the entire free-flowing reach (between river-km 1921 and 1880), that is, between 18 and 20% of the annual transported suspended load.An analysis of historical cross sections showed that there was no natural levee along the riverbanks shortly after the regulation, about 100 years ago. We thus assume that natural levee formation is a consequence of the Danube regulation in the late 19th century. Under natural conditions overbank deposition was balanced and limited by side erosion and lateral channel migration. After stabilization of the banks the overbank deposits were not eroded, and distinct ‘natural’ levees formed. These levees will develop and grow further and affect flood protection and floodplain ecology. We discuss the interplay between floodplain deposition and lateral bank migration under the concept of dynamic equilibrium.