Depth profiles were sampled at different locations throughout Lake Geneva on a monthly and seasonal
basis over the course of 2 years and analysed for their stable hydrogen and oxygen isotope compositions.
The isotopic compositions indicate an isotopic stratification in the metalimnion during summer and fall.
This is related to mixing of Rhône River water, which in summer is dominated by snow and glacier melt
waters, and lake water, with the latter having a homogenous isotopic composition. The observed interflow
layer is 7-15 m thick and can be traced by the distinct stable isotope composition of the water
for about 55 km throughout the lake as well as into shallow bay regions. Depth of the interflow layer
close to the Rhône River mouth is similar to those previously described based on echo-soundings and turbidity
profiles of sediment dispersion. In contrast to previous descriptions of the interflow within Lake
Geneva, the stable isotope compositions allow for direct, natural tracing of the Rhône River water even
in cases where the turbidity and conductivity measurements do not indicate such an interflow. In addition,
the method allows for a quantification of the Rhône River and lake water in the interflow with the
fraction of Rhône River water within the interflow estimated to be up to 37% in summer. The isotopic
composition further indicates different vertical mixing processes within the two lake basins of Lake
Geneva, related to the density gradients and local stability within the water column.
The method may be applicable to other lakes in catchments with large differences in the topography as
water that originates from high altitudes or glaciers has a distinct oxygen and hydrogen isotope composition
compared to other sources of water originating at lower altitudes and/or from direct precipitation
over the lake. Stable isotope measurements thus improve the understanding of the circulation of water
within the lake, which is fundamental for an evaluation of the water residence times, dissolved pollutant
and nutrient transport as well as oxygenation.