The Baltoro Glacier is a valley glacier about 62 km long in the Karakoram range, in the Shigar District of Gilgit-Baltistan, the Pakistan-administered part of the wider Kashmir region. It flows west from the Conway Saddle, at about 6,000 m on the watershed with the Siachen Glacier, past Concordia at roughly 4,600 m, to a debris-covered snout near Paiju at about 3,400 m, where meltwater leaves it as the Braldu River. The Braldu feeds the Shigar, which joins the Indus at Skardu. Four of the world's fourteen peaks above 8,000 m rise within about 20 km of the ice: K2 (8,611 m), Gasherbrum I, Broad Peak and Gasherbrum II. In Balti its name is usually rendered as "bone breaker". The glacier lies inside Central Karakoram National Park, created in 1993, and is the standard approach route to K2 base camp.
Expeditions travel it from the last road at the bottom to the saddle at the top, and the sections below keep that order, naming each glacier that joins on the way.
Skardu to Askole: the road up the Shigar and Braldu valleys
Every journey to the Baltoro starts in Skardu, the main town of Baltistan, which sits in a valley about 40 km long and 10 km wide where the Shigar River meets the Indus. The airport lies at 2,230 m. Glaciers from the Indus and Shigar valleys widened this basin between about 3.2 million and 11,700 years ago, and the sediment they left now underlies its fields and orchards.
From Skardu a road follows the Shigar River north and then turns east up the Braldu valley. The Braldu is about 78 km long. It rises at the snout of the Baltoro, flows west for about 25 km, and there receives meltwater from the Biafo Glacier, another of the Karakoram's giants at about 67 km long. Villages such as Korphe and Tingstun line its banks. The road ends at Askole, at 3,040 m, a settlement of 370 people in 1996 and the last permanent village before the ice. Better road access in recent years has brought more trekkers, and with them problems: overburdened mules, waste left along the route and limited sanitation have prompted clean-up schemes, first by the Mountain and Glacier Protection Organisation in the 1990s and 2000s and more recently through the UN Development Programme's GLOF II project.
Paiju and the snout at 3,400 metres
Beyond Askole the path follows the Braldu to Paiju, a campsite below Paiju Peak (6,610 m), whose summit stands about 3 km north-west of the glacier's end. The snout itself lies a little above the camp at about 3,400 m. It does not look like the white tongue of an Alpine glacier. The lower Baltoro is buried under rock debris, so for the first days walkers cross a jumble of grey moraine ridges, meltwater pools and ice cliffs, with the actual ice showing only in exposed walls. Across the glacier's width the surface can rise and fall by 25 m over 140 m of horizontal distance, which is why the walk from Paiju upward is slow even where the gradient is slight.
Debris cover and a 65-metre retreat since 1904
That debris is the most important fact about the Baltoro's behaviour. Measurements made in 2004 found that its snout had retreated only about 65 m since 1904, a tiny change compared with Alpine glaciers over the same century. The rock layer probably insulates the ice from the sun, and the glacier's stability fits what some researchers call the Karakoram anomaly, the tendency of glaciers in the central and western Karakoram to hold steady or gain slightly while many Himalayan glaciers shrink. Observations between 2001 and 2011 recorded surges and shifting flow near Concordia rather than steady retreat.
The snout is also a poor guide to the health of the whole glacier, because so much of the ice arriving there comes from a single late tributary rather than from the high source basins. Even above Concordia, on the clean ice of the Upper Baltoro, the mass balance was found to be close to even, probably because avalanches and wind keep loading the glacier with snow from the surrounding walls. During the last ice age the picture was very different: Karakoram glaciers then joined into networks hundreds of kilometres long, and the snowline lay about 1,300 m lower than today.
The Trango Glacier and the granite towers of the north wall
About 5 km above the snout the Trango Glacier comes in from the north. It is short but supplies roughly 45 per cent of the ice reaching the snout, while the two big source glaciers far upstream contribute about 40 per cent together. Above it stand the Trango Towers, a ridge of granite spires whose main summit, Great Trango, reaches 6,286 m. Their walls are among the tallest cliffs in the world and have drawn big-wall climbers since the first ascent of Great Trango in 1977 by Galen Rowell, John Roskelley, Kim Schmitz, Jim Morrissey and Dennis Hennek. In 1992 two Australians BASE-jumped from 5,955 m on its north-east face, then the highest starting point on record.
Further up the north side, the Dunge, Muztagh and Younghusband glaciers join, each draining part of the Baltoro Muztagh, the subrange whose crest marks the border with Xinjiang, China. Muztagh Tower (7,273 m) is the best known of the peaks on this side. The southern wall belongs to the Masherbrum Mountains, from which the Mandu, Yermanendu and Biarchedi glaciers descend. Masherbrum itself, 7,821 m, was labelled K1 by the surveyor Thomas Montgomerie in 1856 and first climbed in 1960 by George Bell and Willi Unsoeld of an American-Pakistani expedition.
Gore and the widest ice, 3.1 kilometres across
The middle Baltoro is broad and gentle. Its average width is about 2.1 km, and it reaches its maximum of about 3.1 km near the Gore camps, a little above the point where the Yermanendu Glacier enters from Masherbrum. Over its whole length the main trunk falls at an average gradient of only 3.9 per cent, low for the Karakoram, and in one stretch just south of Concordia it drops a mere 21 m in 2 km. Small tributaries meet this wide trough in icefalls, and the steep side walls carry striations scratched by the moving ice. Lakes collect in hollows on the surface.
From Gore the view opens ahead to the Gasherbrum group. Gasherbrum IV, 7,932 m, fills the head of the valley; Montgomerie listed it as K3 in 1856, when he first sighted the Karakoram peaks from more than 200 km away during the Great Trigonometrical Survey. Gasherbrum I was his K5 and was renamed Hidden Peak by Martin Conway in 1892 because it stays out of sight behind the front peaks for most of the walk up the glacier.
Concordia, where the Godwin-Austen Glacier arrives from K2
Concordia is the hinge of the system. Here the Godwin-Austen Glacier flows down from K2 in the north and the Upper Baltoro arrives from the south-east, and the merged ice turns west. Mitre Peak (6,010 m) marks the corner. Elevations given for the junction range from about 4,600 m to 4,691 m. The name was applied by Aleister Crowley during the 1902 Eckenstein–Crowley attempt on K2, after a glacier confluence of the same name in the Bernese Alps. Robert Lerco reported reaching the spot in 1890 and Conway followed in 1892, the year he named Broad Peak after the Breithorn in the Alps.
From Concordia, K2 appears at the end of the Godwin-Austen valley. It is not visible from Askole, and only fleetingly from lower on the Baltoro, which may be why it never acquired a widely used local name and kept Montgomerie's survey label. The glacier takes its own name from Henry Haversham Godwin-Austen, who explored the area in 1861. K2 was first climbed on 31 July 1954 by Lino Lacedelli and Achille Compagnoni of an Italian expedition led by Ardito Desio, and Broad Peak (8,051 m) in June 1957 by an Austrian team.
Upper Baltoro, the Abruzzi Glacier and the Conway Saddle
South-east of Concordia the Upper Baltoro climbs toward the Gasherbrums and Chogolisa. Just before the junction the Vigne Glacier enters from the south, offering an alternative exit over the Gondogoro La into the Hushe valley. Higher up, the ice is clean rather than rock-covered, and the uppermost section is called the Abruzzi Glacier, after Prince Luigi Amedeo, Duke of the Abruzzi, whose 1909 expedition, with the photographer Vittorio Sella, tried K2 by its south-east ridge, now called the Abruzzi Spur. On Chogolisa that party reached 7,498 m, a world altitude record that stood until the 1922 British Everest expedition.
The glacier system's highest point is the Conway Saddle at about 6,044 m, with the longest flow line starting near 6,200 m. Beyond the saddle the ice drains the other way, into the Siachen Glacier, which lies in Indian-administered Ladakh and is claimed by Pakistan. The snowline is high: the equilibrium line sits above 5,300 m on the Godwin-Austen and above 5,500 m on the Upper Baltoro, so only about 29 per cent of the glacier itself is accumulation area. Avalanches and wind-blown snow from the firn fields on the peaks, a further 351 km², make up much of the difference. Including tributaries in contact with the main ice, the glacier covered about 524 km² in surveys from 1999 to 2001, within a catchment of about 1,500 km² above Paiju.
Read from top to bottom, the Baltoro is a conveyor: snow and avalanche debris collected beneath K2 and the Gasherbrums move down a shallow, rock-armoured trough for 62 km and leave as the Braldu, part of the Indus system. For the peaks that feed it, see the guide to K2 and the overview of the Karakoram range, and for the physics of how snow becomes moving ice, the explainer on how glaciers form.