KATHMANDU, JULY 9

In a stark warning, scientists at the International Centre for Integrated Mountain Development (ICIMOD) have expressed deep concern over the rapid rise in glacial-origin flood events across the Hindu Kush Himalaya (HKH) region. The back-to-back floods in Rasuwa and Upper Mustang within a 24-hour span this week have shocked hazard experts, who say such frequency is 'completely unprecedented.'

According to ICIMOD, Tuesday morning flood in Rasuwa was triggered by the sudden drainage of a supraglacial lake at the Purepu Glacier, north of Langtang Himal. The lake, first appearing as a small pond in December 2024, expanded significantly by June 2025 before breaching this week, sending debris-laden floodwaters downstream into Nepal from Tibet.

'The acceleration of these types of events is completely unprecedented in the HKH region,' said Saswata Sanyal, Disaster Risk Reduction Lead at ICIMOD. 'We need to delve deeper into the triggers that are resulting in cascading impacts.'

A Worrying Trend

Historically, glacial lake outburst floods (GLOFs) occurred once every 5 to 10 years in the region. But in just the past two months (May and June 2025), three separate glacial-origin floods have struck Nepal, Afghanistan, and Pakistan.

The twin floods in Nepal-first in Rasuwa, then in Upper Mustang-have further highlighted how such disasters are becoming more frequent, unpredictable, and dangerous.

Small Lakes, Big Dangers

ICIMOD experts say size is no longer a reliable indicator of threat. Recent events, including the Rasuwa flood, have been caused by small, newly formed supraglacial lakes, sometimes too small to be detected by freely available satellite imagery like Landsat or Sentinel-2.

Previously, potentially dangerous lakes were defined as those larger than 0.02 km². Based on this criterion, a 2020 ICIMOD assessment identified 21 such lakes in Nepal and 25 in the Tibet Autonomous Region of China. However, the recent events suggest even smaller lakes can pose significant hazards.

'Supraglacial lakes form on the surface of glaciers, particularly in debris-covered areas. They are highly dynamic and ephemeral, often beginning as small meltwater ponds that gradually expand and sometimes merge to form larger supraglacial lakes. The detection of these lakes largely depends on the spatial resolution of satellite imagery,' said Sharad Prashad Joshi, ICIMOD's cryosphere monitoring specialist.

Climate Change Fueling Hazards

Rising temperatures are playing a dual role: enabling the formation and expansion of glacial lakes over time, and triggering sudden events like avalanches or lake breaches during extreme heat days. The floods are also increasingly carrying debris, sediment, and boulders, creating what scientists call non-Newtonian flows-floods with greater erosive force and impact than water alone.

Permafrost thaw, shifting precipitation from snow to rain, and increased glacier retreat are exacerbating these risks.

'These events are signals and symptoms of really rising temperatures, and are more destructive than normal flood due to the debris and steep topography,' Joshi added.

In Rasuwa, the flood has left at least 19 people missing and seven confirmed dead, including Nepali and Chinese nationals. It destroyed a 30MW hydropower plant, the Miteri Bridge linking Nepal and China, and swept away over 100 electric vehicles, disrupting trade and travel.

Call for Enhanced Monitoring and Preparedness

While initiatives are underway-such as the Asian Development Bank's BAR-HKH programme and the UNDP's $36.1 million GCF-funded project to lower high-risk glacial lakes in Nepal-experts say far more needs to be done, ICIMOD noted. At present, only two lakes-Imja and Tsho Rolpa-have early warning systems. The absence of real-time monitoring for hundreds of smaller lakes leaves mountain communities exposed, it added.

'We're talking about a huge expanse of terrain where these sorts of lakes can develop, and the monitoring data and method to keep on top of pace of changes we're now seeing simply does not yet exist,' says Qianggong Zhang, head of Climate and Environmental Risks at ICIMOD.

Researchers say it's crucial to increase investment in high-resolution satellite and ground-based monitoring, frequently update inventories of potentially dangerous lakes, include smaller, short-lived ice-dammed lakes in hazard assessments, and integrate glacier dynamics and temperature extremes into risk models.