Troubled Waters Of Sikkim

Shako Chho—a glacial lake the size of approximately 70 standard football fields—is located at an altitude of 4,982 metres in the eastern Himalayas. If its embankments breach, the millions of cubic litres of water that it holds would gush out—taking glacial debris along—and can hit the nearest human habitation, Thangu valley, in just seven minutes. The flood’s fury can flatten everything in its way.

Thangu valley stands about 10 km downstream of Shako Chho at an altitude of 3,960 metres in north Sikkim. It has a population of a few hundred people and serves as a popular acclimatisation place for trekkers to higher altitude destinations close to the Indo-China border, such as the Gurudongmar lake and Cho Lhamu—the source of the River Teesta.

The people of Thangu valley, known as the Lachenpa, are ethnic Bhutias. They practise a semi-nomadic life, living in Thangu, where all their farmlands are, during the summer and monsoon months. But in winter, Thangu is covered in thick layers of snow. The Lachenpas then descend to their second home, Lachen, itself a popular tourist destination at an altitude of 2,750 metres.

Over the past few years, Thangu has also been drawing many scientists working with different government institutions. Shako Chho has been identified as one of the lakes in the Indian Himalayas that pose the highest risk from glacial lake outburst floods (GLOFs) for the people living downstream.

According to the March 2023 report that estimated a seven minute duration for the flood’s fury to reach Thangu from Shako Chho, due to its location next to the river and a short warning time, Thangu is in “a very critical high-risk situation, and would be heavily affected, even under a moderate GLOF scenario.” Besides, options for evacuation routes and safe zones are “very limited.”

Glacial lakes are a product of glacial melt, a trend that has accelerated in recent decades due to global warming. As glaciers melt and recede, the voids they leave behind are filled with meltwater. The lakes thus formed, in many cases, have natural moraine embankments, formed from accumulated debris and sediment.

GLOFs can cause catastrophic devastation, as exemplified by the 2013 Kedarnath disaster, in which the GLOF from Chorabari Lake, triggered by a cloudburst, claimed over 6,000 lives; or the October 2023 Sikkim floods that claimed over a hundred lives.

A range of factors, including cloudbursts, seismic activities and avalanches, can lead to a breach of the dam. Shako Chho, too, is vulnerable to breaching due to multiple factors.

Its end moraine has a low width-to-height ratio, is steep and composed of loose granular material. A 1000-metre-high mountain slope rises just above the lake, increasing chances of avalanches. The dam is susceptible to erosion. There is no clear lake outlet. It is surrounded by potential avalanche sites. The lake has a maximum depth of about 96 metres, which can potentially accelerate GLOF events in chain reaction that snow avalanches generate.

Shako Chho, though, is not the sole threat to Thangu. The valley stands approximately 47 km downstream of Khanchung Lake, a glacial lake located at an elevation of 5,294 metres that has been identified as a high risk one.

Floods from Khanchung Lake can hit Thangu in about two-and-a-half hours, according to a February 2025 report by the National Remote Sensing Centre (NRSC) of the Indian Space Research Organisation (ISRO).

Thangu also faces a GLOF threat from the lakes around Gurudongmar, often called together as the Gurudongmar Lake Complex. In any large-scale GLOF scenario, a GLOF from the Gurudongmar Lake Complex “could exceed the magnitude of the October 2023 South Lhonak Lake flood event,” says a November 2024 paper by scientists working with institutions in India, the UK and the Czech Republic.

Janam Lachenpa, a former Pipon (headman) of the Lachen Dzumsa (traditional village body), says that some locals have grown concerned over the conditions of the glacial lakes in the vicinity of Thangu valley since the October 2023 GLOF in South Lhonak, another glacial lake located at about 5,200 metres, caused one of Sikkim’s worst disasters.

“There’s no question of relocation. This has been our land for centuries. The government must fix the lakes,” he says.

The ‘Fixing’

The governments are, indeed, trying to ‘fix’ the lakes. However, going by past experiences, the governments may have to race against time to get it done before it’s too late. In the case of South Lhonak, the measures turned out to be too late.

In March 2023, a report by the Swiss Agency for Development and Cooperation (SDC)—with which Indian government institutes are collaborating on GLOF risk assessment and management—said that peak discharge from South Lhonak was “considered extremely unlikely to lead to catastrophic failure of the hydropower dam at Chungthang.” Five months later, the October 2023 GLOF at South Lhonak completely swept away the 1,200 megawatt (MW) Teesta Stage III hydroelectric dam, Sikkim’s largest hydropower project.

That the town of Chungthang, roughly 60 km south of Thangu at an altitude of about 1,790 metres, was at risk from GLOF had been well-understood among the scientific community by 2022. It faces GLOF risks from South Lhonak, Shako Chho and the Gurdongmar Lake complex.

The March 2023 report estimated that while some bridges could be damaged by a GLOF from South Lhonak, “there is low risk to settlements or other hydropower reservoirs.” In reality, the South Lhonak GLOF damaged or destroyed more than 25,000 buildings; destroyed 31 major bridges; and inundated about 270 sq km of agricultural land.

The March 2023 report had actually estimated a higher risk to the Chungthang town and dam from Shako Chho. It noted that a large magnitude avalanche-triggered outburst from South Lhonak Lake would have an arrival time of around three hours in Chungthang. On the other hand, it was estimated that a breach in Shako Chho would lead to the flood reaching Chungthang hydropower reservoir within an hour.

However, according to a June 2025 publication, titled ‘Reconstruction of the 2023 South Lhonak Lake Outburst Flood and Modelling Future Scenarios in the Sikkim Himalaya’, the South Lhonak GLOF hit Chungthang in just about an hour since the breached dam attained its peak discharge.

The first expedition to Shako Cho and South Lhonak was taken up in September 2023. the National Disaster Management Authority (NDMA) collaborated with the SDC and the Sikkim government for the project.

The South Lhonak Lake was of special concern, as it has been one of the fastest expanding lakes in the Sikkim Himalaya—expanding from 18 ha in 1976 to more than 70 ha in 2000 and over 170 ha in 2023. The expansion was more than ten times longer and over five times wider in 45 years. However, as of then, there was no GLOF early warning system (EWS) in the state.

The September 2023 expedition team installed Automatic Weather Station (AWS) at both the lakes. They conducted risk assessment for future installation of Early Warning Systems (EWS) through the Teesta valley.

But nature did not give governments more time. The next month, South Lhonak Lake breached, causing one the worst disasters that Sikkim has ever faced. The IWS stopped working before the actual breach of the lake and eventually got washed away.

A paper published in the journal, Science, in January 2025, says that the October 2023 GLOF “highlights the need for a shift in modeling and observation methods to improve our understanding and quantification of cascading hazards.”

The Urgency

Sikkim is India’s high-risk glacial lake hub. According to the 2022 report titled ‘Synthesis Report on GLOF Hazard and Risk Across the Indian Himalayan Region’, the greatest number of potentially critical lakes are found in Sikkim and Jammu & Kashmir. Of the 56 glacial lakes identified as ‘high-risk’, 25 are in Sikkim. Khangchung Tso and Yongdi are among other high-risk glacial lakes.

Khangchung Tso, located at an elevation of 5,294 metres and over three-times bigger than Shako Chho, is one of the top five prioritised glacial lakes in the Brahmaputra River basin, according to a February 2025 report by the NRSC of ISRO.

Its nearest settlement Kerang, located around 6.2 km downstream of the lake at an elevation of 5,096, and Dokung, about 27 km downstream, face great risks.

A paper, titled ‘Increasing Risk Of Glacial Lake Outburst Flood In Sikkim, Eastern Himalaya Under Climate Warming’, published in November 2024, highlights the burgeoning GLOF hotspot of the area of Ponggu La, Za La and Gurudongmar Lakes, where their close proximity to each other increases the threat of a cascading, destructive GLOF event.

Overall, their results indicate potential GLOF events in Sikkim directly threaten populations of approximately 10,000 people and critical infrastructure totalling 1,900 settlements, five bridges and two hydropower plants.

In October-November 2024, another expedition was carried out to Shako Cho and South Lhonak Lake. This time, two pressure probes were installed in both lakes. According to the NDMA, the pressure probes can send automated alerts when water levels cross pre-determined thresholds, providing an early indication of potential threats. An Automatic Weather & Water Station (AWWS) was reinstalled at South Lhonak Lake.

“While not yet a full-fledged Early Warning System (EWS), these developments are an essential first step towards a robust monitoring framework for the region,” the NDMA says.

The expedition team proposed building of a check-dam at Dolma Sampa to manage water flow from South Lhonak and installation of solar-power pumping facilities at Shako Chho to reduce its water level. Now, with the October 2023 GLOF releasing nearly half of South Lhonak’s water, Shako Chho poses the greatest risk to its downstream population and infrastructure.

In the last week of August 2025, several multidisciplinary scientific teams set off for expeditions to high-risk glacial lakes, including Shako Chho, Khangchung and South Lhonak, to carry out scientific studies on the lakes’ water level and other risk factors. Monitoring facilities were installed and upgraded and the scope for installing solar-power pumping facilities was explored.

Whether the government can act on time remains a life and death question for the people living downstream of the high-risk lakes.

Snigdhendu Bhattacharya is a journalist, author and researcher