Changing river styles in response to climate change :

Vandenberghe, J.F. [Promotor]Kasse, C. [Copromotor

Pleistocene geochronologies for fluvial sedimentary sequences: an archaeologist's perspective

Current models of Pleistocene fluvial system development and dynamics are assessed from the perspective of European Lower and Middle Palaeolithic stone tool assemblages recovered from fluvial secondary contexts. Fluvial activity is reviewed both in terms of Milankovitch-scale processes across the glacial/interglacial cycles of the Middle and Late Pleistocene, and in response to sub-Milankovitch scale, high-frequency, low-magnitude climatic oscillations. The chronological magnitude of individual phases of fluvial activity is explored in terms of radiocarbon-dated sequences from the Late Glacial and early Holocene periods. It is apparent that fluvial activity is associated with periods of climatic transition, both high and low magnitude, although system response is far more universal in the case of the high magnitude glacial/ interglacial transitions. Current geochronological tools do not permit the development of high-resolution sequences for Middle Pleistocene sediments, while localised erosion and variable system responses do not facilitate direct comparison with the ice core records. However, Late Glacial and early Holocene sequences indicate that individual fluvial activity phases are relatively brief in duration (e.g. 10(2) and 10(3) yr). From an archaeological perspective, secondary context assemblages can only be interpreted in terms of a floating geochronology, although the data also permit a reinvestigation of the problems of artefact reworking. Copyright (c) 2005 John Wiley I Sons, Ltd

Changing river styles in response to Weichselian climate changes in the eastern Netherlands.

In the Vecht valley, eastern Netherlands, distinct changes in fluvial style and erosional phases took place from the middle part (Pleniglacial) of the last glacial to the Holocene. A low energetic, laterally inactive river changed around 41 ka into a higher energetic river, accompanied by a distinct erosional phase. Even higher energetic conditions occurred during the Late Pleniglacial, when a braided river was active. Fluvial activity diminished in the Vecht valley towards the end of the Late Pleniglacial as aeolian processes became much more dominant. The river became confined to a much more narrow flood plain where it incised at the onset of the Late Glacial. Deposits of two kinds of river types can be recognized in the Late Glacial: low-energetic, probably meandering and braided. The Vecht became and remained meandering during the Holocene. The observed fluvial changes and erosional phases are most likely the result of climatically induced changes in the water-sediment discharge ratio. Several distinct climate changes occurred since the Middle Pleniglacial. Some of them, not all, are reflected by changes in fluvial styles. The response of vegetation to climate changes appears to be an important factor as it determined bank and soil stability and therefore sediment supply but also evapotranspiration rates, which influenced discharge characteristics. A model explaining the fluvial response to climate change is presented and discussed by the example from the Vecht river. Incision took place at the onset of a cooler period when the discharge increased before the vegetation cover was destroyed. Incision also occurred as a response to warmer climate conditions but with a time-lag, depending on the time needed to establish a vegetation cover dense enough to decrease sediment supply and increase evapotranspiration rates. (C) 2000 Elsevier Science B.V. All rights reserved