Even big lakes can form on the surface, inside or at the contact with a glacier. Due to the ice movement these lakes are ephemeral, their life is short and normally their existence comes to a sudden end when the walls supporting them collapse or are deformed, and they rapidly empty out. Thus we experience spectacular yet dramatic flooding that spread over whatever is downstream. Lakes on the surface of glaciers or glacier contact lakes are visible and can be checked, and their evolution can be monitored, and the populations living in the valleys can be warned, and in some cases it is possible to intervene artificially provoking controlled overflows, thus minimizing the damages, but often, as in the case of the lakes that form within the glaciers, there are no premonitory signs. One of the most famous ephemeral lakes is the one that forms every year at the front o f the Perito Moreno Glacier, a glacier descending from the Hielo Continental in Patagonia. The glacier obstructs a part of the Argentinian lake in which it flows, and the arm of the lake that is thus isolated overflows provoking flooding on the banks of the Argentinian lake, spilling about one billion tons of water in 24 hours. In the Alps, the appearance of glacier contact lakes is very frequent. Since 1985 for example, 22 new ones have been recorded, while the formation of lakes on the glacier surface is less frequent. With regard to the former category, in Italy the afore mentioned GLOF of S. Margherita Lake on the Rutor glacier is known, and also the glacial flood of the Locce lake on the homonymous glacier, a tributary of the Belvedere Glacier, already described in 1868. This lake is surrounded on three sides by a moraine ridge and on the other side by the glacier front. The opening of gaps in the moraines surrounding the lake provokes the glacial outburst floods, with sudden drainage (which occurred recently for example in 1970, 1978 and 1979). In order to avoid the continuous repeating of the outburst activity, an artificial outlet channel was built, digging a large trench in the frontal moraine limiting the lake, so as to keep the lake level under control The Belvedere Glacier has recently become famous because between 2001 and 2002 on its surface a large lake, called “Ephemeral lake” formed, which reached an extension of 150,000 m2, with a volume of 3 million m3 and a depth of over 50 m. The danger represented by a similar mass of water contained between the unstable ice walls, (which in the case of the Belvedere, move at a speed of 110 m per year), led to the adoption of particular measures : pumping systems were used to try to lower the level of the lake, decreasing its volume and thus minimizing the risks in case of overflowing. Lake monitoring has also led to the measurements and observations of the lake that will be extremely precious in order to understand such a disastrous phenomenon, so little of which is known.