Brief Description of Glacial Lake Inventory The inventory of glacial lakes is based on topographic maps and satellite images. There are 148 sheets topographic maps with a scale of 1:50,000 in total, which were published before 1987. The 17 scenes TM images had been used to obtain the changes of glacial lakes, while several decades ASTER images as supplement information when TM images are impacted by snow cover or clouds. Glacial Lakes—Their Numbering, Type and Characteristics A glacial lake is defined as a water mass existing in a sufficient amount and extending with a free surface in, under, beside and/or in front of a glacier and originated by glacier activities and/or retreating processes of a glacier. The numbering of the lakes started from the mouth of the major stream and proceeded clockwise round the basin. For the inventory of glacial lakes, it is obvious to note that the lakes associated with perennial snow and ice originate from glaciers. But the isolated lakes found in the mountains and valleys far away from the glaciers may not have a glacial origin. Due to the faster rate of ice and snow melting, possibly caused by global warming noticed during the last half of the twentieth century, accumulation of water in these lakes has been increasing rapidly. The isolated lakes above 3,500 masl are considered to be the remnants of the glacial lakes left due to the retreat of the glaciers. The lakes are classified into erosion lakes, valley trough lakes, cirque lakes, blocked lakes, moraine-dammed lakes (lateral and end moraine-dammed lakes), and supraglacial lakes. Erosion lakes Glacial erosion lakes are the water bodies formed in a depression after the glacier has retreated. They may be cirque type and trough valley type lakes and are stable lakes. Supraglacial lakes The supraglacial lakes develop within the ice mass away from the moraine with dimensions of from 50 to 100m. These lakes may develop in any position of the glacier but the extension of the lake is less than half the diameter of the valley glacier. Shifting, merging, and draining of the lakes characterise the supraglacial lakes. The merging of lakes results in expansion of the lake area and storage of a huge volume of water with a high potential energy. The tendency of a glacial lake towards merging and expanding indicates the danger level of the GLOF. Moraine-dammed lakes A typical example of a moraine-dammed lake is one formed on the tongue of the Cuolangma Glacier in the Pumqu basin (Figure 8.1). In the retreating process of a glacier, glacier ice tends to melt in the lowest part of the glacier surrounded by lateral and end moraines. As a result, many supraglacial ponds are formed on the glacier tongue. These ponds sometimes enlarge to become a large lake by interconnecting with each other and have a tendency to deepen further. A moraine-dammed lake is thus born. The lake is filled with melt water and rainwater from the drainage area behind the lake and starts flowing from the outlet of the lake even in the winter season when the flow is minimum.
There are two kinds of moraine: an ice-cored moraine and an ice-free moraine. Before the ice body of the glacier completely melts away, glacier ice exists in the moraine and beneath the lake bottom. The ice bodies cored in the moraine and beneath the lake are sometimes called dead ice or fossil ice. As glacier ice continues to melt, the lake becomes deeper and wider. Finally when ice contained in the moraines and beneath the lake completely melts away, the container of lake water consists of only the bedrock and the moraines. Blocking lakes Blocking lakes formed through glacier and other factors, including the main glacier blocking the branch valley, the glacier branch blocking the main valley, and the lakes through snow avalanche, collapse and debris flow blockade. Ice-dammed lakes An ice-dammed lake is produced on the side(s) of a glacier, when an advancing glacier happens to intercept a tributary/tributaries pouring into a main glacier valley. The typical ice core-dammed lakes are shown in Figure 8.2. Three lakes are seen on the right bank of the debris covered glacier tongue of the Ngozumpa Glacier in the Dudh Koshi Basin, which is one of the largest glaciers in the Nepal Himalayas and flows from top to bottom in the figure. The lakes were still frozen and covered by snow when the image was captured. Since the glaciers in the Nepal Himalayas produce relatively rich debris, thick lateral moraines are deposited on both sides of the glacier tongue. As such an ice core-dammed lake is usually small in size and does not come into contact with glacier ice. This type of lake is less susceptible to GLOF than a moraine-dammed lake. A glacial lake is formed and maintained only up to a certain stage of glacier fluctuation. If one follows the lifespan of an individual glacier, it is found that the moraine-dammed glacial lakes build up and disappear with a lapse of time. The moraine-dammed lakes disappear once they are fully destroyed or when debris fills the lakes completely or the mother glacier advances again to lower altitudes beyond the moraine-dam position. Such glacial lakes are essentially ephemeral and are not stable from the point of view of the life of glaciers.
In generally, only moraine-dammed lakes pose a threat in the Pumqu Basin. The description hereafter is, thus, mainly concentrated on moraine-dammed lakes and associated outburst floods. Glacial Lakes of Himalaya-China Region There are 824 lakes in the Himalayan-China Regions covering an area of around 85.191 sq.km, including erosion lakes, cirque lakes, end moraine dammed lakes, valley trough lakes, and blocking lakes. Among them the largest number and area are associated with end moraine-dammed lakes. This kind of lakes normally develops in the inner side of moraine ridges of the Little Ice Age, not far from their originating glaciers or connects directly to the originating glaciers. Because water level and stability of the dam are directly affected by the glacier variation, and the Little ice Age moraine ridges developed rather recently, the moraine materials have not cemented hard enough to become a rock. The dam is very easy to burst and form an extraordinary serious flood or debris flow (Table 8.1).
Jiazhagangge basin The Jiazhagangge basin is the one of westernmost branch of the Himalayan-China basin. There are 10 moraine-dammed lakes in total, and only one lake is dangerous. And the erosion lakes and valley lakes are not potentially dangerous as they are isolated and not associated with the hanging glaciers. (Table 8.2)
Daoliqu basin Though there are only 7 lakes in the Daoliqu basin, all of them are moraine-dammed lakes but one erosion lake. After analysis of these lakes change in two different periods, we found no lakes belong to the potentially danger lakes. (Table 8.3)
Majiacangbu basin The Majiacangbu basin is one of the most dangerous basins, and these dangerous lakes locate the eastern of basin. In total, there are 11 danger lakes in this basin (Table 8.4)
Jilongcangbu basin In the Jilongcangbu basin, the number of glacial lakes is large, and it is one of dangerous sub-basins. (Table 8.5)
Poiqu basin The Poiqu basin also is one of dangerous sub-basins, and there are 91 lakes in total. In final, 9 lakes are identified as potentially danger lakes. (Table 8.6)
Pumqu basin The Pumqu basin is the largest basin in this research region, and there are 383 lakes distributed in 9 sub-basins. Pumqu basin also is the dangerous basin, 38 lakes are identified as potentially danger lakes. (Table 8.7)
Ganmazangbo sub-basin
Kadapu sub-basin
Zhagarqu Sub-basin
Zongbuxan sub-basin
Moinqu sub-basin
Loloqu sub-basin
Yarozangbo sub-basin
Bailungpu sub-basin
Natangqu sub-basin
Rongxer basin The Rongxer basin is also one of dangerous basins, and there are 183 lakes in total. (Table 8.17) In final, 16 lakes are identified as potentially danger lakes.
Rongxer Qu_A sub-basin
Rongxer Qu_B sub-basin
Rongxer Qu_C sub-basin
Zangbuqin basin The Zangbuqin basin is a small basin, and there are only 5 lakes. There are no potentially danger lakes. (Table 8.21)
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