Revisiting Dig Tsho - learning lessons for Imja Tsho

Ang Maya Sherpa of Thamo village in the Khumbu region recalls the day the Dig Tsho glacial lake burst: "I was 14 years old then and studying at Thame School. We were celebrating phangi, a local festival where the Sherpas entertain by dressing up as bride and groom. Suddenly in the afternoon, a huge explosion rang out and people started shouting and running to higher grounds for cover." 

What Ang Maya and the others witnessed was a glacial lake outburst flood (GLOF), a huge black mass of water and debris from the lake came rampaging down on the nearby villages.

The Dig Tsho glacial lake was formed at the bottom of the Langmoche glacier (4,365 meters) from the glacier’s melting snow. Langmoche glacier is situated in the Langmoche Valley sub-basin of the Nangpo-Tsangpo area in the Bhote Koshi Valley in the high altitude regions of Nepal. North of the lake is a Sherpa settlement, popular for its yak sheds, known as ‘Dig’ from which the lake derives its name. On August 4, 1985, the Dig Tsho burst, creating a massive flashflood that swept away 30 houses and 14 bridges, entire farmlands, trails and even a nearly complete Thame Micro Hydropower plant that was being designed to provide power to Thame and the villages downstream. 

Pasang Namgyal of Thameteng, who also witnessed the Dig Tsho outburst, describes the conditions leading up to the GLOF: “There had been heavy snowfall that had caused a massive avalanche from the Langmoche glacier. The avalanche fell on the lake and created a wave powerful enough to break the moraine dam, causing the melt snow and debris to flood out on the surrounding villages.” 

Dig Tsho looked relatively calm in 2008; the lake outlet and the riverbed were on more or less the same level. But evidence of the damage it had caused more than two decades ago remained. Either slope of the Bhote Koshi Valley have been activated since the Dig Tsho GLOF by wind, snowfall and rainfall as a result of which massive landslides and soil erosion have occurred, leaving remote high villages like Thamo, Samde, Thameteng, Hilajug, Chaserwa and Langmoche more vulnerable every day. The landslides have been increasing at an exponential rate, further reducing the limited arable farmland in these mountain areas and confining the villagers’ traditional potato farming to limited space. 

The villages of the Khumbu region near Dig Tsho derive much of their income from mountain ecotourism, largely through trekking, lodging houses and porter services. When the Thame Micro Hydropower Plant at Thamo was destroyed along with bridges and trails by the flashflood, it had an adverse impact on the tourism economy of the region, disrupting road connectivity. Empirical evidence from the Dig Tsho GLOF sounds the alarm for other potentially dangerous glacial lakes like Imja Tsho, Tsho Rolpa and others, not only in Nepal but throughout the Hindu Kush-Himalayan region as well.

Today, Dig Tsho is no longer the dangerous glacial lake it once was; it has been removed from the list of 20 potentially dangerous lakes in Nepal identified by ICIMOD and the UNEP. But there is a lesson to be learnt from the experience. In the contemporary global environment of changing climates and rising earth temperatures, there is a need for disaster preparedness, raising awareness, and developing adaptation mechanisms to build the resilience of local communities who are most at risk from these changing conditions. Appropriate mitigation measures for future GLOF events are essential. 

Snow/ice avalanches and GLOFs are regular phenomena in high mountains such as the Himalayas. But their frequency might just be increasing at a much quicker interval and at a much higher rate today because of climate change. Rising Earth temperatures due to the high emission of ozone gases and global warming have made glaciers retreat at unprecedented rates, forming moraine dam lakes at the glaciers’ tongues.

The growth of glacial lakes is almost directly proportional to glacier retreat: glacial lakes will grow and expand, not just in length but also in depth, the faster the glaciers melt. One salient example is Imja Tsho glacial lake situated at the toe of the Imja and Lhotse Shar glaciers at 5,100 meters. In the 1960 Corona image of the area, the Imja Tsho glacial lake was virtually non-existent and there were only a few small supra glacial ponds. Today, the area of the lake has expanded to almost 1 square kilometers. Since 2001, the Imja Tsho glacial lake has expanded at a rate of 74 meters per year, and it is likely that this growth will further increase as Earth temperatures rise.

From the glacier-melting pattern, the growth rate is forecasted to be little less than 100 meters per year in the coming years. By the sheer stress developed by Imja Tsho, pressure from the lake’s water will weaken its natural dam and ultimately breach it. If and when this happens, we will be witness to another possibly catastrophic GLOF; only this time, the flashflood triggered will be of a far greater scale and magnitude. Hydrodynamic modeling carried out by ICIMOD simulates the potential Imja outburst and its projected flood paths show that it will sweep away entire villages like Dingboche, Syomare and Pangboche, along with Jorsale, Ghat and Phakding downstream of Namche. Most of the important trekking/expedition routes and trails to Everest Base Camp, Ama Dablam and Island Peak will either be destroyed or disconnected from main roads.

The reality of an Imja Tsho outburst appears grim. In addition to human casualties, a lot of infrastructure will be demolished in one sweep, and the environment and ecological catastrophe will have a direct impact on the booming tourism industry of the Khumbu region. The threat of a GLOF in Imja Tsho is immense.

All photography by Paribesh Pradhan.