The fifteenth stage of this year’s Tour takes us from the northeastern end of the Dombes over the Jura mountains and back into the valley of the Rhône. At the finish in Culoz, we got to see the first non-artificial lake we’ve discussed here at Les Eaux du Tour: the Lac du Bourget.
The Lac du Bourget was formed along with nearby lakes Annecy and Geneva after the retreat of the Rhône glacier after the Würm glaciation around 14,000 years ago (Vernet and Favarger 1982).
The lake is mesotrophic, meaning it has a moderate level of biological production and fairly good water quality. It is this water quality that has attracted many to the resort town of Aix-les-Bains on its shores. At the north end of the lake near the outlet that connects it to the Rhône is a set of marshes and bogs called the Marais de Chautagne which, together with the Lac du Bourget, form a Ramsar site. During floods on the Rhône, water actually flows into the lake and the marshes, but hydroelectric developments on the Rhône have limited this flooding to a few days per year. The Marais de Chautagne also forms a kind of wetland complex with the Marais de Lavours on the other side of the Rhône, just south of today’s finishing line in Culoz where the young Colombian Jarlinson Pantano outsprinted the new King of the Mountains Rafał Majka for the win.
Vernet, Jean-Pierre, and P-Y. Favarger. “Climatic and anthropogenic effects on the sedimentation and geochemistry of Lakes Bourget, Annecy and Léman.” Hydrobiologia 91 (1982): 643-650.