Sediment distribution on a current-dominated lake delta (Versoix delta, Lake Geneva, Switzerland)

Abstract

Lake delta morphology is determined by fluvial input, waves, lake currents, lake-level fluctuations and lake-basin topography. The current-dominated Versoix delta (Petit-Lac of Lake Geneva, Switzerland), has been studied in order to determine the role of wind-driven currents on transport and sedimentation of suspensions. Research included the study of sediment distribution in the lake basin, of currents due to wind forcing and modelling of lateral particle transport of suspended matter by lake currents. Sediment input into the Petit-Lac mainly occurs by floods of the Versoix River. These floods are generally linked to precipitation under a regime of SW winds. Suspensions from river input are mainly deposited in an area SE and then S or even SW from the river mouth. Sediment transport is thus generally oriented in the opposite sense of the dominant wind direction during floods. Sediments inside the lake basin originate from reworking on lake platforms, mainly during storms with NE winds. Measurements of currents at the central part of the lake basin indicate wind-parallel lake currents at the surface and strong counter currents in the opposite direction in deep water. This particular current pattern, which is partly explained by the Ekman theory, may be linked to the general basin topography (channelled currents) and the unequal proportion of water masses on either side of the site (coastal set-up and set-down). Modelling shows that, as a consequence of the strong deep currents, net fine sediment transport by deep counter currents occurs over several hundreds of meters to several kilometres. This result is consistent with the sediment distribution observed in the lake basin. It can be postulated that changing wind or precipitation regimes may modify the depositional architecture. Further study of current systems, sediment architecture and sediment composition in lakes may, therefore, provide information about changes in these climatic factors since the last ice age.