Mangde Chu Glacier Retreat, Bhutan

The Mangde Chu Basin in Bhutan is home to many alpine lakes, and the number is increasing as glacier retreat leads to both new lakes and expanding lakes. Many of these alpine lakes are impounding by glacial moraines of varying stability. Some of the glacial lakes are capable of outburst floods (GLOF), the resulting hazard has led to an inventory of these lakes in Bhutan and this watershed in a joint effort between Bhutan and EROC in Japan. Here we examine the development of a new lake not shown in the USGS map of the region from a 1993 SPOT image. This glacier is marked by an X and is just south of the Methatshota Tsho (M).
mande chu map
USGS Map

In a pair of Landsat images from October and December of 2000 the lake (C) has formed and the glacier terminates at the yellow arrow, red arrow is 2013 terminus. The lake is 550 m long. By 2006 in the Google Earth image the lake is 850 m long, terminus red dots. In 2013 Landsat images from October and December indicate the lake has expanded to a length of 1400 m. The glacier has retreated 1400-1500 m since 1993 and 850 m since 2000. Just above the red arrow the lake to the west of the glacier is not as close to the glacier, this indicates thinning of the upper sections of this glacier and marginal retreat in the accumulation zone. This is typically a sign that a glacier cannot survive current climate (Pelto, 2010). The lake is not impounded by a substantial terminal moraine, and does not appear to be prone to a substantial GLOF. This is what the joint Bhutan-Japan study indicates as not requiring urgent counter measures as the moraine dam is thick and the slope is not too high either of the moraine or the glacier leading into the lake, note last figure. The channel leading out of the lake is immature and will likely downcut through the glacial sediments, reducing the lake area somewhat. The glacier surface rises quickly just above the current terminus, indicating that the lake will not grow much longer, and glacier retreat will then slow as the terminus pulls out of the lake. This glacier is retreating as are the nearly all glaciers in the region such as Lugge Glacier, Thorhormi Glacier and Theri Kang Glacier all a short distance north. mangde chu 2000a
2000 Landsat image
mangde chu2000
2000 Landsat image
bhutan cho ge
2006 Google Earth Image
mangde chu 2013
2013 Landsat image
mangde chu 2013a
2013 Landsat image
mangde chu moraine
2006 Google Earth Image
mangde chu glof
Image from Kumori

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Lugge and Thorthormi Glacier Retreat, Bhutan

Luge and Thorthormi Glacier drain south from the border with China into the Pho Chu River in Bhutan. Both glaciers end in expanding glacier lakes that are prone to outburst floods, which sweep down the Pho Chu. Osti et al (2012) reported in detail on the nature of these floods, noting there are eight dangerous lakes including the two at the terminus of Lugge and Thorthormi Glacier, Thorthormi Cho and Lugge Cho. In 1994 Lugge Cho experienced a glacier lake outburst flood GLOF which incurred huge damage in the Pho Chu basin. The 1994 GLOF event had a peak discharge of about 2539 cubic meters/s and extended 200 km downstream as a flood wave Osti et al (2012) . The GLOF occurred after rapid retreat of Lugge Glacier from 1988-1993 of 160 m/year. pho chu ge
Google Earth Image
Here we examine the retreat of both glaciers in Landsat imagery from 2000-2014. In each image the red arrow indicates the 2000 terminus, the yellow arrow the 2014 terminus, the blue arrow the snowline on Lugge Glacier and the green arrow the center of the Thorthormi Glacier terminus in 2013. In 2000 the Lugge Glacier ends in a 2 km long Lugge Cho. The Thorthormi Glacier has pockets of proglacial lake in 2000, but also a debris covered terminus that extends across the lake basin to the Little Ice Age moraine (M). The two images from 2000 are from the start of October and late December. Note the snowline remains near 5100-5200 m in both images. The glaciers of Bhutan are summer accumulation type glaciers, in which the main accumulation season is during the summer monsoon. The snowline tends to rise from October into December with limited snowfall. By 2013 Lugge Glacier has retreated 1 km from the 2000 position, and Lugge Cho is now 3 km long. The terminus of Lugge Cho is not stagnant and it is not clear how much longer the deep basin extends under the glacier. If the basin does not extend much further retreat will soon be reduced. Thorthormi Glacier debris covered terminus connection to the moraine (M) has melted away and a lake extends across the full width of the glacier basin. The contiguous lake now has an area of over 1 square kilometer. The retreat of Thorthormi has been 700 m since 2000. The lowest 1 km of the glacier is stagnant and melt should be enhanced by calving into the lake, hence the retreat should remain quick in the next decade. The snowline in the Late November 2013 and early February 2014 image indicate the snowline at close to 5300 m in both. The retreat of these glaciers is leading to expansion of proglacial lakes much like the nearby Theri Kang and many other across the region Changsang Glacier, Sikkim, Lumding Glacier, Nepal Matsang Tsanpo, Tibet .
lugge 2000
2000 Landsat image
lugge 2000a
2000 Landsat image

lugge 2013
2013 Landsat image

lugge 2014
2014 Landsat image

Theri Kang Glacier Retreat Bhutan

One of the glaciers draining north from Theri Kang in Bhutan terminates in proglacial lake, that is a lake at the glacier front. This is an unnamed glaciers like its neighbors, we will refer to it as the Theri Kang Glacier. The glacier has retreated 1700 meters from the moraine complex that impound the lake. The terminus is at 5200 meters, the snowline is at 5800 meters and the summit of the glacier is near 6700 meters. The vegetation adjacent to the glacier is indicative of the overall dry climate in the region. This is also a cold climate with permafrost in the areas adjacent to the glacier. The lake is not in evidence in maps of the region from the 1960’s. Kääb (2005) in Figure 7b examined the velocity of this glacier using ASTER imagery he found that most of the lower glacier extending from where the tributaries join to the terminus moved at a velocity of 50 m per year. A comparison of this 2001 image overlain in Google Earth and the more recent 2009 imagery indicate this glacier has retreated 300 meters in eight years. The 2001 image is from NASA.The red line in the 2001 image indicates the 2009 terminus. The lake has expanded from 1500 meters in length to 1800 meters and now encompasses nearly 2 square kilometers. Often there is concern about glacier lake outburst floods from lakes dammed either by a glacier or impounded, as in this case, by its moraine. For the Theri Kang the moraine complex appears quite wide, and stable and is not of great concern. The 2013 Landsat image does not show significant retreat after 2009. theri kang 2013
2013 Landsat Image
The lower portion of the glacier is debris covered along the lateral margins, but the clean ice portion has an interesting pattern, that is typical of a glacier in a drier climate where sublimation is an important element of the ablation process, dominating over melting. Sublimation is the direct transfer of solid ice to water vapor without melting. That is the crevasse pattern becomes a series of prominent crests almost waves that persists in the absence of the crevasses that formed them. This persistence would not occur in a zone dominated by melting. Though the setting is much different the response to climate is the same as almost all Himalayan glaciers from the Zemu Glacier in Sikkim, to Lugge Glacier, Bhutan, to Imja Glacier in Nepal