Complex fluvial response to Lateglacial and Holocene allogenic forcing in the Lower Rhine Valley (Germany)

Abstract

The Rhine catchment experienced strong changes in upstream allogenic forcing during the last 20,000 years. Climatic changes of the glacialeinterglacial transition and steadily growing human impact during the second half of the Holocene forced the Rhine to adapt, resulting in changes in the fluvial morphology. The lower Rhine left two late Weichselian terraces and many Holocene palaeo-meanders in the Lower Rhine Valley (western Germany). This well-preserved terrace sequence is used to investigate the exact course of events of the lower Rhine response to changes in allogenic forcing. Five detailed cross-sections that integrate new and existing borehole data were constructed, and the deposits were analysed with regards to abandonment of terraces, changes in number of active channels, fluvial style, terrace gradients, and overbank sedimentation during the Lateglacial and Holocene. We improved and expanded the chronology of the Lower Rhine Valley deposits by dating new samples (14C, OSL), and integrated these with existing dating evidence (archaeological and historical data, cross-cutting relationships). Twice during the glacialeinterglacial transition, the lower Rhine changed from braided to meandering fluvial style. During both transitional episodes (meandering) secondary channels existed alongside the main channel, with a life span up to 2500 years. The findings imply that the lower Rhine was inherently slow to complete the full morphological transition to a single thread meandering system. On specific aspects of response, the morphological response (point bar/terrace formation, contraction to a single thread) extended over relatively long periods of time, whereas discharge-related response (e.g. fluvial style change, abandonment of braidplains, channel bed lowering/incision) seems to have been near instantaneous. Reach-specific conditions determine the degree to which the geomorphic response is delayed and the complexity of the resultant morphology. Increased human-induced sediment delivery (last 2000e3000 years) is expressed as relative thicker and coarser overbank deposits in the entire study area. In the downstream part of the Lower Rhine Valley it accelerated high-stand deltaic backfilling and decreased incision. The response to human activities occurred relatively quickly in contrast to the long-term fluvial response to the glacialeinterglacial transition, because the human impact mainly involved change in delivery of the suspended load.