Lateglacial changes in river morphologies of northwestern Europe: An example of a smooth response to climate forcing (Cher River, France)

摘要

Climatic oscillations occurring during the Pleniglacial and Lateglacial periods are generally associated with several changes in fluvial morphology and processes. This study investigated two reaches of the Cher River basin (Noyers-sur-Cher and Thenioux) regarding morphological adjustments during the Lateglacial period and the early Holocene. Transitions from the Pleniglacial to the Lateglacial and from the Lateglacial to the early Holocene (ca. 14,700-11,500 cal. BP) are considered as interesting study cases because it consists of rapid and abrupt climate oscillations generally clearly recorded in current floodplains. The sites studied are located in the Cher River floodplain, which is one of the main tributaries of the Loire River. Several investigations have been conducted in the main valley, but few of them specifically focused on geomorphological evolutions of medium-sized catchments such as tributaries of the Loire River. Therefore, we examined this specific period through investigations recently conducted in the middle valley of the Cher River. This study aimed to (1) provide a geomorphological approach to fluvial adjustments during the Lateglacial from cases located in lowland areas of a mediumsized basin and (2) analyse the spatio-temporal variability of fluvial responses. Assessment of geomorphological trajectories of the two reaches studied highlights that fluvial metamorphosis occurring during the Lateglacial and early Holocene seems to be characterised by lateral readjustments and a low signal of morphological instability (shallow incision and aggradation processes, slow and progressive change in channel morphology). This observation is enhanced by the comparison with known geomorphological evolution patterns of adjacent basins. The intrinsic components of the Cher basin such as slope and valley width were considered as explanatory factors for the lack of morphogenetic processes. In addition, the geographical location of the reaches studied may also have led to a moderated expression of climate forcing. Indeed, the elevation as well as the north-south and east-west gradients may contribute to the low level of geomorphological adjustments in response to these climatic oscillations. This gradient could also have support this the fluvial response because of the weakly developed loess cover, which may have limited the fine sediment supply and the aggradation of the valley bottom. (C) 2019 Elsevier B.V. All rights reserved.