Natural levee evolution in the Rhine-Meuse delta, the Netherlands, during the first millennium CE

This paper presents reconstructions on natural levee development in the Rhine-Meuse delta, the Netherlands, during the first millennium CE, covering the full delta plain. It is the first study that performs this on a delta scale, which allows seeing the delta-wide trends on levee-forming controls and their feedbacks. We mapped the levee morphology and elevation by combining LiDAR imagery, lithological borehole data, soil mapping, radiocarbon dates, archaeological data, and GIS-reconstruction techniques. From the detailed levee reconstructions we quantified natural levee dimensions and evaluated the temporal changes therein. The dimensions and the changes therein were then linked to external forcings (increasing suspended sediment load, variable flooding intensity) and to natural preconditions (e.g., delta plain width, flood basin configuration). We show that natural preconditions are an important control on levee shape. This is demonstrated for the upper delta where the relatively narrow delta plain combined with strong compartmentation (i.e., the occurrence of many alluvial ridges and enclosed flood basins) caused the flood levels to be amplified allowing the natural levees to grow relatively high. Compartmentation also seems to have stimulated trapping of coarse grained overbank sediments, explaining the clear downstream trend in levee width. This effect was probably further aided by the clearance of the riparian forests, mainly in the upstream and central delta, which caused the coarser fraction of the suspended load to be further dispersed into the flood basin leading to wider levees. In the first millennium CE several new river courses formed that avoided the areas of natural levee relief of abandoned alluvial ridges. On these fossil alluvial ridges, the topographical expression gradually reduced because of widespread flood basin trapping of overbank sediment, which led to topographic levelling. The natural levees that formed during this period along the new courses appear to be relatively high compared to precursor generations in the upper and central delta. This is most likely related to the increased suspended sediment supply and intense flooding regime during their formation. The hypotheses generated with this new delta-wide overview help to better understand the controls in the development of levees, which are important elements in river landscapes and in fluvial sedimentary records.