Highaltitude Glaciers The Himalaya

The Himalaya (Figure 2.8) extends for ^2000 km from Afghanistan to Burma. Outside the polar regions it is one of the most extensively glacierised regions of the world, but one that is poorly known from a glaciological perspective. The Himalaya is influenced by two major weather systems, the South Asian monsoon and the mid-latitude westerlies. Probably the most studied glaciers are those in the Everest region, where the dominant influence is the South Asian monsoon from the Indian Ocean, which results in a pronounced summer maximum of precipitation

Figure 2.8 ASTER satellite image of glaciers on the northern slope of the Himalaya in Bhutan. There is evidence that the glaciers are receding in the form of well-developed terminal and lateral moraines, as well as the development of proglacial moraine-dammed lakes [Image courtesy of: Jeffrey Kargel, USGS/NASA JPL/AGU].

Figure 2.8 ASTER satellite image of glaciers on the northern slope of the Himalaya in Bhutan. There is evidence that the glaciers are receding in the form of well-developed terminal and lateral moraines, as well as the development of proglacial moraine-dammed lakes [Image courtesy of: Jeffrey Kargel, USGS/NASA JPL/AGU].

from late-May to mid-September. Maximum precipitation occurs at altitudes between 5000 and 7000 m. Although average daily temperatures at this altitude are below 0°C for much of the year, high winds prevent extensive accumulation of snow at high altitudes. Unlike polar or lower-altitude alpine glaciers, the coincidence of the highest temperatures and maximum precipitation means that the peak of accumulation and ablation occur simultaneously. This glacio-logically unusual regime profoundly influences the manner in which debris is transported and entrained in these glaciers.

There are of course local variations in glacier type within the Himalaya, but it is possible to make the following generalisations.

1. The steep terrain in the high-altitude catchments mean that accumulation is commonly dominated by ice and snow avalanching, both of which incorporate considerable quantities of frost-shattered rock. The glaciers therefore commonly carry a heavy supraglacial debris load derived from this rockfall material. Much of the rock material is incorporated englacially and in the ablation zone it becomes concentrated to form a near-continuous debris mantle. Texturally this material is predominantly a sandy boulder-gravel, but with minor amounts of boulder-gravel and diamict.

2. Many of the glaciers have prominent (up to 100 m high) lateral and frontal moraines, dating from the Little Ice Age or earlier. The moraines often have vegetated outer faces and an unvegetated, loose, collapsing inner face. In extreme cases, for example on the Khumbu Glacier in Nepal, terminal moraines may reach height of 250 m above the surrounding land. Geophysical surveys indicate that these moraines have a substantial core of dead ice within them. Terminal and lateral moraines are composed mainly of sandy boulder-gravel derived from a mixture of rockfall debris and material from the zone of traction along the valley sides.

3. Where they are active, measured surface velocities on the largest Himalayan glaciers are between 10 and 100 m per year. Many Himalayan glaciers, however, have much slower flow rates especially in their terminal areas. Therefore, rather than receding in an active manner, many glaciers are undergoing down-wasting and therefore becoming debris-mantled in their lower reaches. The debris cover is mostly hummocky, and dominated by coarse angular material. Between the debris hummocks are depressions containing meltwater ponds, with steep exposed ice slopes smeared with debris, or ice cliffs several metres high. The thicker cover of debris towards the terminus retards ablation more than the progressively thinner debris cover up-glacier, resulting in a lessening or even reversal of the surface gradient. When the overall average gradient declines to less than 2°, large supraglacial ponds begin to form on the glacier snout. Over time, these lakes coalesce to form large supraglacial lakes, dammed by the terminal and lateral moraines. Where they are impounded by ice-cored moraines, these lakes form a natural hazard because the moraine dams are prone to sudden failure, resulting in glacial lake outburst floods (GLOFs; Box 2.3).

4. Glacial rivers in the Himalaya often lack extensive braided reaches in their proximal zones. In comparison with alpine regions therefore, proximal glacio-fluvial processes are relatively unimportant in these high-altitude environments. Meltwater is largely confined to single channels because of the steepness of the channel gradient and narrowness of the valleys.

5. Below many of the glaciers are breached terminal moraines and well developed alluvial fans representing the tracks of former GLOFs (Box 2.3). Peak GLOF discharges have been calculated as up to 60 times greater than seasonal high flow floods derived from snowmelt runoff, glacier meltwater and monsoonal precipitation. As a result, these floods are tremendously powerful, eroding,

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