How were deep lakes such as Lake Annecy and Lake Constance formed in the Alps?

Deep lakes in the Alps, such as Lake Annecy and Lake Constance, were formed primarily through the processes of glaciation during the Pleistocene Epoch, which began approximately 2.6 million years ago.12

During this period, massive glaciers flowed through mountain valleys that had been established during the earlier uplift of the Alpine chain. These glaciers acted as powerful erosive forces, deepening and widening the valleys into characteristic U-shapes. As the climate warmed and the glaciers retreated at the end of the glacial period, they left behind moraines—deposits of waste material—that acted as natural dams.21

The resulting deep, elongated basins were subsequently filled with water, creating the deep lakes observed today. In some instances, these glaciers extended beyond the mountains into adjacent plains, further shaping the landscape where these lakes are situated.12