High frequency variations of water flux and sediment discharge during the Little Ice Age (1586-1725 AD) in the Rhone Delta (Mediterranean France). Relationship to the catchment basin

During the 17th and 18th centuries, the Rhone delta recorded high frequency fluctuations in water and sediment influx. These variations resulted from the drastic climatic changes that took place during the Little Ice Age, which were intensified by dense land settlement in the catchment basin. The use of complementary types of information (iconographic and textual archives, photo-interpretation of traces of fluvial metamorphosis, grain size distribution and mineralogy of alluvial infillings) allows a precise study of the major change that affected the Rhone delta in the second part of the 17th and the beginning of the 18th centuries. This change corresponds to a rapid response by the fluvial system and the occurrence of floods that were higher and more frequent on the lower Rhone. It resulted in a rapid change in fluvial environments, i.e. channel infilling, raising of river banks, appearance of crevasses and secondary channels, and accelerated delta progradation. It is associated with the influx of an abundant and exceptionally coarse-grained bottom load. The provenance of these sediments is discussed, using the heavy mineral assemblages they contain, with reference to reworking of previous alluvial deposits, probably due to changes in the active band and episodic loads in the whole catchment basin. The short response time to hydrologic impulse and the importance of the metamorphosis are related to the sediment influx from the Massif Central, whose steeply inclined rivers are close to the delta. The contribution of the northern Alps and the durancian basin (southern Alps) is not as important because of their distance from the delta and the time lag affecting the bottom load transfer downstream. The aims of this study are to investigate the response time, transfer velocity of sandy deposits, and the contribution of different catchment sub-basins to the supply of the Rhone delta. The role of climatic and anthropic factors are also discussed.