A large region of population surrounding the central and southern Andes of Argentina and Chile reside in arid areas that are dependent on water supplies from melting glaciers. The water from the glaciers also supplies rivers that have in some cases been dammed for hydroelectric power. Some researchers believe that by 2030, many of the large ice caps on the highest Andes will be gone if current climate trends continue. In Patagonia on the southern tip of the continent, the large ice caps have retreated a since the early 1990s and since the late 19th century. It has also been observed that Patagonian glaciers are receding at a faster rate than in any other world region. The Northern Patagonian Ice Field lost of glacier area during the years between 1945 and 1975, and from 1975 to 1996, which indicates that the rate of retreat is increasing. This represents a loss of 8% of the ice field, with all glaciers experiencing significant retreat. The Southern Patagonian Ice Field has exhibited a general trend of retreat on 42 glaciers, while four glaciers were in equilibrium and two advanced during the years between 1944 and 1986. The largest retreat was on O'Higgins Glacier, which during the period 1896–1995 retreated . The Perito Moreno Glacier is long and is a major outflow glacier of the Patagonian ice sheet, as well as the most visited glacier in Patagonia. Perito Moreno Glacier is in equilibrium, but has undergone frequent oscillations in the period 1947–96, with a net gain of . This glacier has advanced since 1947, and has been essentially stable since 1992. Perito Moreno Glacier is one of three glaciers in Patagonia known to have advanced, compared to several hundred others in retreat. The two major glaciers of the Southern Patagonia Icefield to the north of Moreno, Upsala and Viedma Glacier have retreated in 21 years and in 13 years respectively. In the Aconcagua River Basin, glacier retreat has resulted in a 20% loss in glacier area, declining from to . The Marinelli Glacier in Tierra del Fuego has been in retreat since at least 1960 through 2008.