As recently as 20,000 years ago, the basin of Todos los Santos Lake was filled by a large glacier that did not withdraw until approximately 10,000 years ago. At that point, the area was still a glacial basin with rivers flowing through it. The lake itself did not form until lava flows from the Osorno and Calbuco volcanoes formed a dam across the lower part of the valley, trapping the water from the rivers and gradually forming the lake.