Fluvial responses to environmental perturbations in the Northern Mediterranean since the Last Glacial Maximum

Abstract

The numerical model HydroTrend, that produces daily time series of water discharge and sediment load to the ocean, is applied to three Mediterranean drainage basins to: (i) simulate the water and sediment flux changes to sedimentary basins through time and (ii) determine the impact of potential forcing factors on sediment and water fluxes to sedimentary basins and how this impact varied through time. Climate (precipitation, temperature and glacier equilibrium line) and drainage basin (basin elevation, drainage area and reservoir) reconstructions of the Po, Rhône and Têt river basins over the last 21 000 Cal. years B.P. are used as input to the model.Simulated sediment fluxes at the river mouth for the Po, Rhône and Têt River systems during the late Pleistocene were considerably higher compared to Holocene pristine sediment flux, with a factor 3.5, 2.4 and 2.4 respectively. For the Po River system and the Rhône River, deglaciation in the late Pleistocene is the main factor, responsible for these higher sediment fluxes. Drainage basin area change due to sea-level rise is the main cause of decrease in sediment flux for the Têt River system and to a certain extent for the Po River system. Man-made reservoirs reduced sediment flux to the ocean for the Po, Rhône and Têt rivers over the last 3–6 decades with a factor of 1.3, 3.8 and 2.4 respectively.Fluvial responses to climate and basin variations are reflected in the peak flood discharge and sediment concentration curves, where present-day water peak flood curves for all three rivers are the highest in the last 21 000Cal. years B.P. However, their associated sediment concentration curves show opposite results because of diminishing glacial area and the impact of reservoirs. The Têt River system has the ability to generate hyperpycnal plumes although the occurrence frequency changes over time. Prior to 15 500 Cal. years B.P. the river system area was extended due to the low sea level, causing a less favorable regime to generate hyperpycnal events. Presently sediment trapping due to man-made reservoirs alters the river ability to generate hyperpycnal events.