Ponkar Lake is a moraine-dammed glacial lake in Bimthang Valley in Manang, Nepal. (Photo: Sudan Bikash Maharjan/ICIMOD)

A new glacial lake inventory report has identified 47 potentially dangerous glacial lakes (PDGLs) within the Koshi, Gandaki, and Karnali river basins of Nepal, the Tibet Autonomous Region of China, and India. For the past two years, we have been working with UNDP Nepal to identify PDGLs in these three basins using high-resolution satellite imagery. These moraine-dammed glacial lakes are at risk of breaching, which would result in glacial lake outburst floods (GLOFs).

GLOFs: A perennial threat

GLOFs are a prominent water-induced hazard in Nepal and other mountainous countries in the HKH and occur when melting glaciers create reservoirs of water that can suddenly burst, leading to floods downstream. Himalayan glaciers are vulnerable to the warming climate and have been melting and retreating at unprecedented rates since the mid-20th century, impacting flow regimes in major river basins. These changes lead to the formation of new glacial lakes as well as the expansion of existing glacial lakes, increasing the risk of GLOFs.

Varying threat levels across countries

The inventory lists 25 PDGLs in China, 21 in Nepal, and 1 in India that need to be closely monitored to reduce the vulnerability of mountain people and settlements downstream. Forty-two of the PDGLs are within the Koshi basin, making it the most at-risk river basin. The Gandaki and Karnali basins have three and two PDGLs, respectively.

We found that a significant increase in annual maximum temperature in Nepal has been resulting in glacier retreat, shrinkage, and fragmentation, thereby leading to an increase in the number and area of glacial lakes. We have mapped 3,624 glacial lakes equal to or larger than 0.003 km² (approximately 2.4 times bigger than a standard Olympic size swimming pool) in the three basins. Of these, 2,070 lakes are in Nepal, 1,509 in the Tibet Autonomous Region of China, and 45 in India.

PDGLs ranked

We grouped the glacial lakes into three categories in order of potential danger: lakes in Rank I have the highest risk of breaching and those in Ranks II and III have the potential to grow and need close and regular monitoring. Thirty-one PDGLs were classed as Rank I, 12 as Rank II, and 4 as Rank III. Lakes such as Tsho Rolpa and Imja Lake continue to fall within Rank I. The water levels of these two lakes were lowered in the past to reduce GLOF risk. Even with the lake level lowering, there is some residual risk and these two lakes are still expanding rapidly.

“The up-to-date information in the report about glacial lakes and their distribution is necessary for understanding the dynamics of their changes and for identifying potential hazards and risk reduction. The report will be useful for those studying water resource management and related hazards and risk reduction,” said Sudan Bikash Maharjan, ICIMOD, one of the authors of the report.

This report builds on the 2018 baseline data of glacial lakes in the HKH. The research was conducted to support the Green Climate Fund project proposal formulation led by UNDP Nepal.

Chimi Seldon

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