Early and Middle Holocene fluvial development near the present-day Rhine Delta apex, Germany. A morphological succession from a terraced to a deltaic lower river valley.

Abstract

This aim of this study is to produce a detailed Holocene palaeogeographical reconstruction for the transitional area between the Rhine delta and the Lower Rhine Embayment several kilometers upstream of the Dutch-German border, where over time and space a landscape with river terraces turns into a landscape dominated by fluvio-deltaic deposition. New data is collected in the field along multiple cross-sections. Pollen analysis of channel fills, a longitudinal profile and an age-height correlation of meander and terrace levels are produced. The results are integrated with data collected in previous research. The new insights concern the valley gradient, changes in fluvial style/river morphology and incision-aggradation rates. Especially knowledge on the incision-aggradation transition, i.e. the upward shifting of the Rhine delta, along the western/left side of the river and the Subboreal fluvial development has grown. Insight into river development in response to climate change and human influences will help in mitigating fluvial responses to future climate change. In total 22 Holocene palaeo-meanders are identified, belonging to the Holocene terrace complex. They can be grouped into 4 meander generations based on fluvial style/river morphology, incision/aggradation rate and palaeo-meander height, besides a transitional terrace zone of the Younger-Dryas/Holocene transition. The latter is a terrace with multiple small and slightly meandering channels that dissect the Younger Dryas braid plain. The oldest Holocene meander generation developed during the Preboreal-middle Boreal (generation I). It was part of a multichannel system with one large and a couple of smaller meandering channels, in modestly incised position. The next meander generation developed during the middle Boreal-Atlantic when flow is concentrated to one main channel and incision increased (generation II). The following meander generation formed during the Subboreal (ca. 5.7-2.6 cal ka BP) while incision turned into aggradation (generation III). Finally, meander generation IV developed during the Subatlantic showing a change in fluvial style/river morphology with coarser floodplain deposits, decreasing size and more central position of the palaeo-meanders. The transitional zone and the first two meander generation formed in response to the climate warming which results in a clear succession of (a) incision and abandonment of the braid plain, (b) a multichannel system with secondary channels, (c) flow concentration into one single channel and (d) delayed continued incision (lowering of top of in-channel deposits). The third meander generation formed under beginning human impact and minor climate variation but is triggered by a downstream control. This control, the upstream shifting of the delta apex, resulted in aggradation since sometime between 5.7 and 4.0 cal ka BP. The fourth meander generation formed in response to increased human impact (and possible climate change) and is also observed in upstream and downstream areas. This research shows that fluvial development is a continuous process controlled by multiple succeeding external forcings that drive a complex sequence of river response.