Which glacier in Los Glaciares National Park periodically advances and dams a lake?

The glacier that periodically advances and dams a lake is the Perito Moreno Glacier.

Located in Argentina’s Los Glaciares National Park, Perito Moreno flows into Lake Argentino. Its advancing ice front can reach the opposite shore of the Magallanes Peninsula, separating the lake’s Brazo Rico arm from the main lake and forming a temporary ice dam.

Water trapped in Brazo Rico may rise dramatically—by as much as about 30 metres above the main lake. Pressure and erosion eventually open a tunnel through the ice, followed by a spectacular rupture as the barrier collapses and the water drains toward Lake Argentino and the Santa Cruz River.

A common mix-up is assuming the event happens on a fixed schedule. Ruptures have occurred at irregular intervals, from roughly annual occurrences to gaps of nearly a decade. Perito Moreno is also unusual because its dramatic advances and collapses have made it one of Patagonia’s best-known natural attractions, even though glacier fronts elsewhere in the region may be retreating.