Human Land Use and Land Abandonment Greatly Impacted Sedimentation Rates and Fluvial Styles Since the Iron Age in the Valdaine (Southern France)

Abstract:

In this study we present the Holocene time differentiated sediment budget for the Valdaine region (334 km²) and its main streams (Roubion and Jabron, totaling 610 km²). Sediment budget studies have been used previously in western and central Europe to quantify the changing sediment dynamics under climatic variations and changes in human land use. This study is one of the first attempts to quantify such Holocene sediment dynamics in southern Europe. We achieved a more detailed chronology than comparable European sediment budgets, by combining traditional geomorphology with geo-archaeological research.

The results demonstrate that total colluvial deposition is high in comparison with catchments in temperate Europe, while alluvial deposition falls within the range of those other studies. High colluvial deposition values are explained by the long intensive land use in this region, combined with a Mediterranean climate. The temporal evolution of sediment deposition shows a pattern of three main deposition phases alternating with three phases of hillslope stability and valley incision. Intensive human land use in the lower fertile parts of the region is responsible for the deposition phases, while land abandonment caused hillslope stability and valley incision. The first major peaks in soil erosion occurred during the Iron Age and Roman Period, with rates about half as high as for the last 800 years. In addition to these variations in sedimentation rates, there occurred also major variations in sediment texture and fluvial style. During most of the Holocene rivers had a meandering pattern and deposited silty and clayey sediments. But major sedimentation peaks during the Roman Period and especially since the 12^th century coincided with the establishment of a braided river and deposition of gravel and sand. These changes in river style demonstrate how the montane headwaters become an important sediment source. It remains an open question whether this is only related to (document) forest clearings in the mountains, or that more torrential rainfall events also played a role. The sediment budget demonstrates how the quantities, timing and nature of sediment deposition varied during the Holocene mainly as a result of human land use, while are unable to show any influence of climatic changes.