Palaeohydrologic conditions and geomorphic processes during the Postglacial in the Palatine Upper Rhine river floodplain

Stratigraphic, palynological, sedimentological, geophysical and Mollusc analyses of three peat cores from palaeomeanders in the floodplain of the Southern Palatine Upper Rhine river valley were used to reconstruct interrelations between palaeohydrologic, palaeoclimatic and geomorphodynamic changes throughout the genesis of the floodplain. Pollen analyses indicate that softwood floodplain forests probable were still absent to a large extent until the Early Subboreal. Anthropogenic impact oil the vegetation in the river floodplain was presumably only minor or absent. After the silting up of the oldest meander during the Late Atlanticum, peat formation started at the beginning of the Subboreal. Accumulation rates during this peatification phase were low. Subsequently, peat growth was interrupted during the Subboreal (since 3,450 cal. BP) for a period of almost 1,800 years. The results indicate that during a period of relatively stable river dynamics in the Mid-Holocene, dry and terrestrial site conditions prevailed in the Northern Upper Rhine river valley. The soils were mainly sandy and floods were rare. Since the Younger Iron Ages, the vegetation In the floodplain was influenced by humans. Flood dynamics had already increased considerably during the Late Subboreal, as indicated by a widespread influx of fine grained sediments into the Northern Upper Rhine valley. Due to these changes, a mosaic of different sites developed in the floodplain. This was a prerequisite for the establishment of a softwood floodplain forest, deduced from the new results of pollen analysis. Furthermore, the renewed peat formation within the oldest meander during the Migration Period and the incipient peat accumulation in the younger meanders in the Early Middle Ages suggest a rising groundwater table in the floodplain. The high peat accumulation rates are further evidence for good peat formation conditions in the Younger Holocene. Sediment layers within the upper peat layers of the palaeomeanders show a vast increase of flood activity of the Upper Rhine river since the High- and Late Middle Ages. From that time onwards huge areas of the Rhine river floodplain were flooded periodically.