Reqiang Glacier (R) and Jicongpu Glacier (J) in 1993 and 2019 Landsat images.  M=Moraine, red arrow is the 1993 terminus location, yellow arrow the 2019 terminus location and purple dots the snowline.

Requiang Glacier, Tibet is just east of Shishapangma Mountain one of planets 14 peaks that exceed 8000 m and terminates in the rapidly expanding proglacial lake Gangxico at 5200 m. Jicongpu Glacier drains south from Shishapangma terminating in the proglacial lake Galongco at 5100 m. Both glaciers are fed by avalanching from the high slopes of  Shishapangma. Reqiang Glacier has been undergoing a rapid retreat since 1976, Li et al (2011) noted the retreat of 65.7 m/year from 1976-2006.  The retreat of this glacier fit the pattern of all 32 reported and was due to that increasing temperature. Zhang et al (2019)  observed that from 1974-2014 Galongco and Gangxico lakes expanded by ~500% (0.45 km² /year) and ~107% (0.34 km/year.  As the lakes have expanded the wide moraines impounding the lakes have not experienced visible change. Here we examine the retreat of Reqiang and Jicongpu Glacier from 1993-2019 using Landsat imagery and the GLOF risk of Galongco and Gangxico.

Glacier lake outburst floods (GLOF) are a significant hazard in glaciated mountain ranges. The principal causes of GLOF are ice dam failure, moraine dam failure and/or avalanching into a lake.  Harrison et al (2018) noted there has been a decline in recent decades of GLOF events globally and in the Himalaya due to moraine dam failure.  In the Himalaya the main cause of moraine dam failure is ice avalanches into the lake. This decline has occurred during a period of rapid glacier retreat and the formation of many more alpine lakes. Hence, the number of locations where a potential GLOF could occur has increased, but the actual risk of any particular location generating a GLOF has declined even more.  Carrivick and Tweed (2016) observed that the number of GLOF’s due to all causes globally has declined since the mid 1990’s, and that this decline is not a reporting issue, since reporting has gotten better.  The main cause of the 1348 GLOF’s that they archived had been ice dam failure at 70%.  How has the retreat of Reqiang and Jicongpu Glacier impacted the risk of a GLOF?

In 1993 Reqiang Glacier terminated in a 3.1 km long Gangxico, which had an area of 2.9 km².  The lowest 2.5 km of the glacier had a low slope and the snowline was above this at 5500 m.  Jicongpu Glacier terminated in a 2.8 km long Galongco with an area of 2.6 km² and had a 3.5 km low slope debris covered terminus zone. By 2000 Reqiang Glacier had retreated 400 m and the low slope terminus tongue had a significant expansion of debris cover. Jicongpu Glacier had retreated 300-400 m.  By 2018 Reqiang Glacier had retreated 1900 m, the glacier snowline is only 1 km from the calving front at ~5500 m.  Jicongpu Glacier has retreated 2100 m on the east side and 1400 m on the western margin of the lake.  The debris covered area has been reduced to ~1 km². From 1993-2019 Reqiang Glacier has retreated at a rate of ~95 m/year.  Gangxico  has expanded to an area of 4.6 km² and is 5.0 km long. The snowline on Reqiang Glacier has been consistent in location in each of the years.  Jicongpu Glacier has retreated at an average rate of ~70 m/year.  Galongco has expanded to an area of 5.5 km².

At Reqiang Glacier the moraine band impounding Gangxico is 1950 m wide and does not have visible signs of change.  With time since emplacement and retreat of the glacier into the lake the moraine will stabilize more.  Given the continued even if slow increased  moraine stability and the large moraines width the risk of dam failure is limited. At Jicongpu Glacier the moraine band is 1200 m wide impounding Galongco, again considerable.  These two glacier indicate the competing factors for GLOF risk, the size and stability of the moraine, versus the expanding volume of the lake.  Similarly a retreating glacier can reduce the ice avalanche hazard as the lake expands and ice slope diminish or the retreating glacier can provide access to steeper ice slope depending on the specific topography.  Zhang et al (2019) suggest both lakes have limited room to expand as they near a glacier surface slope increase.The retreat of these two glaciers follows that of many alpine glaciers in the region where lakes exist at the terminus which has enhanced retreat such as at Yanong Glacier and Drogpa Nagtsang Glacier.

Reqiang Glacier (R) and Jicongpu Glacier (J) in 2000 and 2018 Landsat images.  M=Moraine, red arrow is the 1993 terminus location, yellow arrow the 2019 terminus location and purple dots the snowline.

Gangxico Lake fed by Reqiang Glacier in Digital Globe image from 2015 indicating the moraine that impounds the lake with yellow arrows.

Galongco Lake fed by Jicongpu Glacier in Digital Globe image from 2015 indicating the moraine that impounds the lake with yellow arrows.