Geomorphological records of a ‘flood-dominated regime’ in the rhône delta (france) between the 1st century bc and the 2nd century ad. what correlations with the catchment paleohydrology?

Abstract

In the Rhône Delta, numerous paleoenvironmental indicators coming from geomorphology, sedimentology, paleoecology and archeology provide evidences of remarkable hydrological regime change at the beginning of the Roman Antiquity. A significant reinforcement in the Rhône River's competence is indicated by the increase in grain size of the sediments deposited in the floodplain, within the immediate proximity of the channel. Increase in the fluvial sediment yield is indicated by the increase in rate of sedimentation in the floodplain. Increase in water levels is indicated by the deposition of hydromorphous facies, the development of hydrophytic species, and the decrease in activity of terrestrial lombricoids in the floodplain. Numerous crevasse splays were formed by short-term avulsions of the Rhône River between the first century BC and the AD first century. Their frequency is related to an increase of Rhône River paleodischarge, which makes the channel network incapable of evacuating large volumes of water brought in by the river. The littoral zone experienced a phase of progradation between the AD 1st and the 3rd centuries, in relation to the increase of terrigenous deposits coming from the Rhône of Ulmet. The increase in solid discharge, in the flood-dominated regime (FDR) context, can be related to the overall increase in detrital output from the catchment. The Rhône Delta rather appears in phase with the northern Alps and Massif Central from the 1st century BC to the AD 1st century. The FDR at the beginning of the Christian era contrasts with the drought-dominated regime (DDR) and the reduction of torrential rainfall in the Mediterranean and south Alpine areas. These observations allow to confirm the hydroclimatic limit which could distinguish the southern part of the catchment (Provence, Durancian Alps) from the rest of the Rhône basin. © 2002 Éditions scientifiques et médicales Elsevier SAS.