The Perito Moreno Glacier is one of only three Patagonian glaciers that are not retreating. Periodically the glacier advances over the L-shaped "Lago Argentino" ("Argentine Lake") forming a natural dam which separates the two halves of the lake when it reaches the opposite shore. With no escape route, the water-level on the Brazo Rico side of the lake can rise by up to 30 meters above the level of the main lake. The enormous pressure produced by this mass of waters finally breaks the ice barrier holding it back, in a spectacular rupture event. This dam/rupture cycle is not regular and it naturally recurs at any frequency between once a year to less than once a decade.
To see the falling of the natural dam wait for the end of the video. It's well worth it.
From: Wikipedia
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