Glaciologist Karma Töeb is inspecting a vulnerable glacial lake at an altitude of 4,500 meters in Bhutan, despite communication issues. He notes the rapid environmental changes and the associated risks and dangers for downstream areas. Töeb emphasizes that Bhutan does not contribute to climate change but is forced to bear its burden.
Töeb is at the Törtormi glacial lake in Lunana, one of the most remote settlements in northwestern Bhutan. As part of his thirty-second visit to the lake since 1997, he is leading a group of experts testing technologies to determine the feasibility of draining the lake to reduce the risk of Glacial Lake Outburst Floods (GLOF).
A GLOF is a sudden release of water from a lake formed by melting mountain glaciers and held back by ice or moraine. Such floods can trigger earthquakes, avalanches, or excessive accumulation of meltwater and are often extremely destructive, posing a growing threat in the Himalayan watershed.
A GLOF from the lake complex including Törtormi and located below Rangsteng could jeopardize two major hydropower projects. The Punatsangchu-II project, with a capacity of 1 GW, began operations in December 2024, while the Punatsangchu-I project, with a capacity of 1.2 GW, is still under construction.
For many years, Bhutan has taken steps to protect its population and key hydropower facilities from potential GLOFs in Lunana. In addition to attempts to drain glacial lakes, the country is modernizing its early warning system, shifting from flood detection to forecasting, which could provide more time for response.
These efforts have gained new urgency and public attention following the devastating floods in neighboring Nepal and China on August 26. This disaster resulted in the deaths of nearly 1,500 people, damaged at least 15 hydroelectric power plants and power lines, disrupting generation and exports, and prompting Nepal to request additional electricity from India.
A potential GLOF in Törtormi could have a similar impact on Bhutan, causing concern among the residents of Lunana and the government.
Hydropower accounts for a significant 12–15% of Bhutan's GDP, with electricity revenues making up about 40% of exports and 36% of government revenue. This energy source is also vital for diplomatic relations between India and Bhutan, as India is the sole importer of Bhutanese hydropower. Both countries have built dams, transmission lines, and financial agreements around this trade for decades; much of Bhutan's hydropower-related debt obligations are historically linked to projects financed by the Indian government. Both Punatsangchu projects were partially funded by India.
In 2025, electricity exports to India reached 24.3 billion BTN (285 million USD) after the launch of Punatsangchu-II, contributing to an almost 9% growth in Bhutan's economy.
Michael Williamson, Head of the Energy and Sustainable Development Section at the UN Economic and Social Commission for Asia and the Pacific, noted that the recent floods in Nepal underscore the need to 'consider a broader range of cascading and complex hazards [in the Himalayas],' rather than just traditional risks of precipitation and flooding. These hazards include ice and rock avalanches, landslides, and GLOFs.
He also pointed out that 'more systematic screening of cascading and complex hazards' should be conducted for new hydropower projects before making major investment decisions.
Nearly 99% of Bhutan's territory is mountainous, and its glaciers are becoming increasingly unstable due to rising temperatures. Bhutan's 2026 Ice Inventory Report recorded an increase in the number of lakes from 567 to 620 over the last five years. The largest growth was observed in the Pho Chu sub-basin, which covers part of northwestern Bhutan and is home to Törtormi and Rangsteng, where the number of lakes increased from 157 to 187.
A 2024 study of this basin estimated that a GLOF from the Törtormi and Rangsteng lakes could cause a peak discharge of nearly 16,400 cubic meters of water per second for four hours. This would flood up to 245 hectares of agricultural land and nearly 1,300 buildings, seriously endangering both Punatsangchu-I and Punatsangchu-II.
Trashi Namgyal, Head of the Hydrology and Water Resources Division of the National Centre for Hydrology and Meteorology of Bhutan (NCHM), reported that the Rangsteng moraine dam has stabilized, while the Törtormi dam has not.
Official assessments have also documented risks of flooding, riverbank erosion, and landslides around other hydropower projects in Bhutan, such as the 336 MW Chukha and 60 MW Kurichhu.
Dasho Chewang Rinzing, Managing Director of Druk Green Power Corporation, the state-owned power company and primary developer and operator of Bhutan's hydropower, stated that a GLOF should not affect the energy cooperation between Bhutan and India. He added that during a real event, generation might temporarily decrease in affected areas to reduce infrastructure risk, but otherwise, the grid is very tightly connected.
However, the risk is not limited to floodwaters. A former official of the state company Bhutan Power Corporation, who wished to remain anonymous, believes that having sufficient sluice gate capacity on dams is not enough. Serious problems could arise if the flood carries heavy debris, sediment, boulders, and trees that could clog tunnels, gates, and intakes.
Recent regional floods illustrate this risk. The Rasuwagadhi hydropower project in Nepal was completely washed away during the August floods. Last year, a GLOF in the same region deposited over two meters of sediment, washing away structures designed to prevent debris ingress. In Bhutan, flash floods and landslides in northeastern Lhuentse in July 2023 caused the deaths of at least six people and damaged the 32 MW Yungichhu hydropower project.
The team is testing siphon technology—a controlled reservoir drainage method—to lower the water level in Törtormi. Last year's attempt was unsuccessful, but now the team is testing this technology on a new section near the existing outlet, adjusting the topography, site conditions, and engineering issues identified in the first attempt, according to Töeb.
The siphon project is part of broader efforts to mitigate GLOF risks alongside the use of early warning systems. Currently, Bhutan has 10 hydrometeorological stations and upstream water level monitoring stations, as well as 18 sirens along the Punatsangchu basin. These include monitoring stations in several glacial lakes in the Lunana region, located approximately 130 kilometers upstream from the Punatsangchu hydropower projects.
However, the current system is based on detecting floods as they develop. Since 2017, NCHM has been working on a forecasting-based warning system that will use indicators such as rainfall and temperature to provide earlier warnings.
In the worst-case scenario modeled by NCHM, Törtormi and Rangsteng overflow into each other. Based on the existing detection system and modeling, the model estimates that in this scenario, floodwaters would reach the historical fortress of Punakha Dzong in about six hours. The forecasting system would provide more time than the current system, and although Namgyal says it is impossible to say how much yet, 'we will save many lives' with it.
The first phase of India-Bhutan hydropower cooperation focused on the Chukha, Kurichhu, and Tala projects with a total capacity of 1.4 GW. Since 2019, capacity has been expanded by another 2.5 GW through Mangdechhu and Punatsangchu-II.
Udisha Saklani, a lecturer in human geography and climatology at King's College London, who has researched Indian and Bhutanese hydropower, asserts that hydropower has played a central role in Bhutan's growth. According to her, India's role goes beyond financing and technical expertise; its geographical proximity provides Bhutan with a large market for electricity that cannot be consumed domestically.
She adds that prolonged disruptions in hydropower production would negatively affect Bhutan's economy by hindering the sale of electricity to India. 'There will be a hydropower debt obligation [for the projects], because a hydropower project is a costly long-term undertaking.'
India would face less severe consequences. Bhutanese hydropower constitutes a small fraction of India's 520 GW power system. Meanwhile, according to Debajit Palit, Head of the Climate Change and Energy Transition Center at the non-profit Chintan Research Foundation, Bhutan exports about 65% of its 2.5 GW hydropower capacity to India.
Williamson notes, however, that cross-border connections can also enhance resilience in the region. After the floods in Nepal, 'India was able to redirect electricity to Nepal through existing interconnectors,' helping to manage immediate supply shortages.
Williamson argues that receiving hydropower from both Bhutan and Nepal provides India with 'greater diversification of commercial and counterparty risk compared to physical climate and geological risk.' It also distributes India's exposure across various regulatory systems, generating companies, transmission corridors, and river basins. This means that a natural disaster affecting one power plant or transmission line is less likely to interrupt all of India's hydropower imports.
Saklani also points out that Bhutanese hydropower is not just about the megawatt volume crossing the border: 'it also balances a grid containing a large amount of variable renewable energy sources,' such as solar and wind. Hydropower can complement them, providing power when their output fluctuates.
The resilience of this system depends not only on individual projects but also on how risks are assessed before building new infrastructure. Williamson states that 'hydropower planning has evolved from traditional models to accounting for changing hydrological conditions and the impacts of climate change.' He concludes that the disaster in Nepal demonstrates the need to expand and update current approaches to create increasingly robust planning processes.
Bhutan's National Energy Policy for 2025 sets an ambitious goal of 15 GW of hydropower capacity by 2040, compared to 3.5 GW in the same year. The policy also identifies climate impacts and unpredictable hydrological inflows as energy security challenges and calls for greater grid flexibility, battery energy storage, and diversification, including other renewable sources.
Nevertheless, Bhutan already has significant hydropower debt. The country's national debt, approaching 100%, with nearly 70% directly related to hydropower loans, presents a unique paradox: a prolonged interruption in generation would severely strain national liquidity but does not pose an immediate risk of sovereign default, according to a joint debt sustainability analysis by the World Bank and IMF. This is because most of Bhutan's hydropower-related debt obligations are historically tied to projects financed by the Indian government. Some new projects are being developed through joint ventures with Indian companies.
The analysis notes that a prolonged halt in hydropower generation could put pressure on public finances and the broader economy. However, the structure of Bhutan's hydropower debt obligations, along with its financing mechanisms and power purchase agreements with India, mitigates the risk of debt default.
As hydropower expands in Bhutan, the country is incorporating climate risks into planning, including tightening dam safety standards and considering the risks of building multiple projects in the same river basin, notes Saklani. However, she questions 'how much of this policy and guidance will actually be implemented.'
For people living downstream from Törtormi and Rangsteng, the threat seems inevitable.