Mixing of Rhône River water in Lake Geneva: Seasonal tracing using stable isotope composition of water

Abstract

Determining the path of river intrusions into lakes is essential, both for a better understanding of the lake circulation as well as the nutrient transport and the distribution of pollutants introduced by the rivers. The objective of this study is to understand the mixing of Rhône River water within Lake Geneva. The stable H- and O-isotope composition of water for this alpine lake has been shown to be a powerful tool to trace the Rhône River intrusion within the lake, but the details of this interflow and how it changes in space and time have not been well established yet. The present study focusses on using the isotopic tracer method in detailed cross-sections sampled at different times during the year as a tool to determine how the interflow changes with time. Different sampled cross-sections present large spatiotemporal heterogeneities of the Rhône River water dispersion. During summer and early autumn, when the lake is thermally stratified, the Rhône River is intruding into the metalimnion as an interflow, and it is directed by the currents in the top layer. The stronger the thermal stratification, the more concentrated and vertically constrained will also be the Rhône interflow. Vertical and horizontal displacements of the interflow are controlled by wind-induced internal waves and the gyres within the lake established as a function of wind strengths and directions.