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