8 dec. 2009

Sediment dynamics in the Rhine catchment :

Quantification of fluvial response to climate change and human impact
Dissertation
Publisher: www.knag.nl
Author: Erkens, Gilles

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Fluvial systems are strongly respon- sive to changes in climate and land use — but take their time to show it. Accurate prediction of the timing and degree of future fluvial response requires comprehensive understanding of fluvial response in the past. This PhD-thesis studied the response of the river Rhine over the last 20,000 years, as recorded in the morphology, composition and volumes of its sediment. Borehole data and various dating techniques were combined to time-slice cross-sections and to construct a series of palaeogeographic maps. A method to calculate amounts of sediment carried both in suspension and as bed load during millennia-long time slices was developed. This research shows that only climate changes during the glacial-interglacial transition and the strong rise of human land use during the late Holocene were large enough to trigger response and overwhelm ongoing autogenic behaviour of the river and the buffering capacities of the Rhine catchment. The climatic changes at the glacial-interglacial transition triggered incision and abandonment of the Pleniglacial braidplain terrace along the entire Rhine valley. This was accompanied by a shift from a braided to a meandering fluvial style, but it took several millennia to complete the transition to a single meandering channel. As a result, the Rhine was a multi-channel river during the Late Glacial and most of early Holocene. The last phase of fluvial response to the glacial-interglacial transition consists of increased delivery of fine sediment during the first half of the Holocene (until ~ 6000 years ago). This sediment had been stored in the upstream tributary valleys since the full glacial, and was released by incision of the Rhine and its tributaries when they adapted to the interglacial climate conditions.



Overall, this study shows that sizable fluvial catchments adapted slowly to large scale climatic changes, in both spatially and temporally complex ways. During the Holocene other external factors than climate change became important for the development of the Rhine system. In the Rhine delta, the combination of eustatic sea level rise and subsidence provided in large accommodation space for sediment delivered from upstream. Release of bed load by the meandering and incising Rhine in the trunk valley forms the source for bed load delivery to the delta. Both the Rhine trunk valley and delta are sinks for suspended sediment, and the amount of stored sediment in these areas was used to reconstruct fine sediment transfer through the lower Rhine. Following the aforementioned increased delivery of fine sediment during the first part of the Holocene, sediment transfer stabilised during the middle Holocene. During the last 2000 years the delivery of fine sediments to the sinks is up to 150 % higher compared to the directly preceding millennia. This can only be explained as the result of progressive deforestations for agricultural land use since the late-Neolithic (6300 years ago). The timing of the increased delivery of fine sediments suggests that pre-Roman land use was already extensive. Hence, prehistoric land use should be regarded as a drainage basin-wide forcing factor for fluvial systems.

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