Perito Moreno Glacier is a 250 km² (97 sq mi) tongue of ice, about 30 km (19 mi) long, that flows east from the Southern Patagonian Ice Field into Lago Argentino in Santa Cruz Province, Argentina, 78 km from the town of El Calafate. Its front is 5 km wide and stands on average 74 m (240 ft) above the lake. Almost all of it lies inside Los Glaciares National Park, created in 1937 and listed as a UNESCO World Heritage Site in 1981; a small part of its source lies across the border in Chile’s Bernardo O’Higgins National Park. For decades it was known as a glacier that did not shrink, but since about 2019 it has been thinning fast, and since 2020 its northern front has been retreating.
Its fame rests on a cycle of damming and rupture. When the ice front reaches the Magallanes Peninsula it cuts off the Brazo Rico arm of the lake, the water behind it rises, and eventually it bursts through. That spectacle, and the glacier’s long stability, have made it one of the most important tourist attractions in Argentine Patagonia. This guide sets out what threatens it and what protects it, in order of date.
1937 and 1981: Los Glaciares National Park and World Heritage Listing
The first protection came early. Los Glaciares National Park was established on 11 May 1937 and now covers 7,269 km² (2,807 sq mi), the largest national park in Argentina. About 30 percent of it is ice. The park takes in two long glacial lakes: Lago Argentino in the south, the largest lake in the country, and Lago Viedma in the north. Perito Moreno is one of three great glaciers that flow into Lago Argentino, alongside the Upsala and the Spegazzini. UNESCO added the park to its World Heritage List in 1981.
Protection also covers the life on and around the ice. The glacier is home to Andiperla willinki, a stonefly about 1.5 cm long that spends its whole life on Patagonian glaciers and feeds on wind-blown bacteria. Below the ice, Magellanic subpolar forest of lenga, guindo and ñire gives way to the dry Patagonian steppe, home to guanaco, rheas, cougars and the South American grey fox.
The park’s creation also set off growth just outside it. El Calafate, founded in 1927 for wool traders, grew once the park opened and roads improved. Its population has risen steeply with tourism, and the town now holds more than 89 percent of the people in its department.
Where the Ice Begins: The 1881 Treaty, the 1902 Award and the 1998 Ice Field Agreement
A glacier that starts in one country and ends in another needs a border, and settling it took more than a century. The area was claimed by both Argentina and Chile until the Boundary Treaty of 1881 drew the line along the Andes as far south as the 52nd parallel. Francisco Moreno, the explorer later honoured in the glacier’s name, argued Argentina’s case in the border arbitration that ended with the 1902 award by the British Crown. The glacier itself went through three names: Chilean naval captain Juan Tomás Rogers called it Francisco Gormaz in 1879, Rudolph Hauthal of Argentina’s boundary commission called it Bismarck, and in 1899 Lieutenant Iglesias of the Argentine Hydrographic Institute named it Perito Moreno.
The ice field above remained in dispute for most of the 20th century. The 1998 agreement between the two countries finally set a line that crosses the head of the glacier, leaving a small part of its source in Chile and the rest in Argentina. The Southern Patagonian Ice Field that feeds it covers about 16,480 km², of which about 14,200 km² lies in Chile and 2,600 km² in Argentina. It is the second-largest continuous ice field outside the polar regions and one of the world’s largest stores of fresh water.
Brazo Rico Floods and the Navy’s Explosives of 1939
For the people who farmed the shores of Lago Argentino, the glacier’s advance was itself a threat. When the ice blocks the narrow channel against the Magallanes Peninsula, the Brazo Rico arm loses its outlet, and its level can rise by as much as 30 m above the rest of the lake. Tree rings on the shore suggest no major damming between about 1635 and the 20th century. In 1899 the ice front was 750 m from the peninsula, and in 1917 it closed the channel for the first time.
Flooding of farms and ranches once led the state to try to break the dam by force. At the end of 1939 Argentina’s Navy Ministry dropped explosives on the ice to open a channel. It failed; the dam broke naturally in February 1940. In 1952 the rising water flooded 6,670 hectares of cropland and grazing land, though no buildings. When a dam bursts suddenly, the outflow can reach 8,000 m³ per second, as it did in 2008, and in 1966 the surge pushed the Santa Cruz River above 2,000 m³ per second at the Charles Fuhr gauging station.
Ruptures have been recorded in 1917, 1935, 1940, 1942, 1947, 1952, 1953, 1956, 1960, 1963, 1966, 1970, 1972, 1975, 1977, 1980, 1984, 1988, 2004, 2006, 2008, 2012, twice in 2013, 2016, 2018 and 2019. The 16-year gap after 1988 led some to fear the cycle had stopped. Today the ruptures are a tourist draw rather than a danger; an estimated 10,000 people watched the 2004 event.
Thinning Since 2019: The End of a Stable Glacier
The threat that now matters most is climate. For much of the late 20th and early 21st centuries Perito Moreno was one of the few glaciers in the world in balance, gaining about as much ice as it lost while its neighbours retreated. Why it stayed stable for so long is still debated by glaciologists. That balance has broken. Since 2020 the northern part of the front has been pulling back, and satellite images show open water where ice stood a few years earlier.
A study published in a Nature journal measured how fast the change has come. Using radar and lake-depth surveys flown from a helicopter, together with records of surface height and speed, it found that the surface near the front was lowering by 0.34 m a year between 2000 and 2019, and by 5.5 m a year between 2019 and 2024, roughly sixteen times faster. The study also mapped a ridge in the bed beneath the glacier. Its simple model suggests that if the front retreats behind that ridge, the ice could begin to float and break up quickly, as has happened at other Patagonian glaciers that end in lakes.
Retreat also threatens the glacier’s best-known behaviour. The dam-and-rupture cycle depends on the ice front reaching the Magallanes Peninsula; if the front pulls back far enough that it no longer touches the shore, the Brazo Rico can no longer be sealed off.
Cattle, European Hares and Trout: Invasive Species and Fire
The ice is not the only part of the park under pressure. Tourism, overgrazing and forest fires are among the park’s recorded problems. Cattle, European hares and introduced trout live in the park as alien or feral species. Fire has damaged and in some areas destroyed stretches of forest. Outside the park, large-scale sheep and cattle ranching across Patagonia has cut guanaco numbers sharply, and the South American grey fox has suffered as ranching spread.
Visitor numbers add another kind of pressure. A large visitor centre with a network of walkways looks across at the southern and eastern faces of the glacier, boats carry visitors along the northern face, and guided treks on the ice run from a short walk of about an hour and a half to a five-hour route. The walkways close after 8 pm, which is why the ice bridge collapsed overnight in March 2018 with no one there to see it.
Cóndor Cliff and La Barrancosa: The Santa Cruz River Dams
Everything that melts from Perito Moreno leaves Lago Argentino by one route: the Santa Cruz River, which runs 385 km east to the Atlantic, carries about 790 m³ of water per second on average, and is one of the last large free-flowing rivers in Patagonia. In August 2013 a consortium led by China’s Gezhouba Group won the contract for two hydroelectric dams on it, first named after Néstor Kirchner and Jorge Cepernic and since renamed Cóndor Cliff and La Barrancosa. They were planned at a combined 1,740 MW, at an initially estimated cost of more than US$4.8 billion.
By one estimate the dams would destroy more than half of the river’s ecosystem. On 21 December 2016 Argentina’s Supreme Court suspended work pending an environmental impact assessment, which was approved on 8 May 2017. Construction restarted in March 2018 but ran into repeated delays. On 18 July 2025 Santa Cruz governor Claudio Vidal signed an agreement with the companies to restart one of the dams, and in December 2025 a transfer of US$150 million to Gezhouba was completed.
Law 26.639 and the 2026 Glacier Law Reform
Argentina’s main legal shield for its ice is Law 26.639 on the Preservation of Glaciers and the Periglacial Environment, passed in 2010 and put into force in 2011. An earlier version, Law 26.418, was approved in 2008 but vetoed by President Cristina Fernández de Kirchner on 10 November 2008. The 2010 law treats glaciers as strategic water reserves, orders a national glacier inventory carried out by the IANIGLA institute of CONICET, and bans mining, industry, non-scientific infrastructure and the release of pollutants on glaciers.
That protection has now been cut back. In December 2025 the national government sent Congress a bill to amend the law. The Senate approved it on 27 February 2026, and the Chamber of Deputies passed it by 137 votes to 111 in a session that ended on 9 April 2026. The reform limits protection to ice with a relevant, proven water-supply function and gives provinces more say over which areas to protect and which to open to mining and other development, although ice bodies already in the national inventory keep their protection until an authority rules they have no such function. Environmental groups and lawyers opposed it as unconstitutional. Perito Moreno’s national park status is a separate protection, but the reform shows that the legal ground under Argentina’s ice is shifting too.
To see the glacier alongside the Upsala, Viedma and the peaks of El Chaltén, read our Los Glaciares National Park guide. For the wider ice field and its Chilean glaciers, see our overview of the glaciers of Patagonia, and for the country from the Andes to the Atlantic, our guide to Argentina’s geography.