Nepal Has 21 Glacial Lakes Scientists Call Time Bombs, and One Already Burst This Year

Every trekker who has stood in Dingboche and looked up the side valley toward Island Peak has walked within a few kilometres of Imja Tsho. It barely existed a century ago — it was a scatter of meltwater ponds on the Imja Glacier. Today it is over a kilometre long and among 21 lakes inside Nepal that scientists have formally classified as potentially dangerous glacial lakes, capable of bursting their moraine walls and sending a wall of water, rock and ice down the valleys below.

The mechanism is simple and the physics are not forgiving. As glaciers retreat, they leave lakes dammed only by loose piles of rock and ice debris called moraine — not bedrock. An avalanche, a landslide, or even a chunk of hanging glacier calving into the water can send a wave over the dam. Once the moraine breaches, the lake empties in minutes, not hours. The 1985 collapse of Dig Tsho lake, triggered by an ice avalanche, released an estimated 5 million cubic metres of water in under an hour and destroyed the almost-finished Namche hydropower plant six kilometres downstream.

2026 has already put the theory back on front pages. In July 2025, ICIMOD's own analysis attributed a destructive flood on the Bhote Koshi river to the sudden collapse of a superglacial lake on a glacier across the border in Tibet — a textbook GLOF. Then, on 26 August 2026, a catastrophic flash flood tore through the Rasuwa–Tibet border area, and while investigators have not yet confirmed a single cause, the pattern — a mountain river turning from a trickle to a wall of debris with almost no warning — is exactly what a glacial lake failure looks like.

The lakes on the watch list

Nepal has not ignored the problem. In 2000, engineers cut a 70-metre canal into Tsho Rolpa's moraine dam and lowered the lake by three metres — an operation that, at the time, was one of the highest-altitude engineering projects on Earth. In 2016, a joint Nepal government, UNDP and Global Environment Facility team spent months siphoning Imja Tsho down by another 3.4 metres and installed a community early-warning system in the villages below. Both projects bought time rather than solved the problem — Tsho Rolpa has already re-expanded past its drained level, because the glacier feeding it keeps retreating.

What this means for trekkers

The honest long-term picture is that this list is not shrinking. Warmer decades mean more meltwater and more lakes, faster than engineers can drain them one at a time. For now, the system that actually saves lives is the boring one: known hazard zones, community sirens, and trekking operators and guides who treat a distant flood report as a reason to check the trail ahead, not just a headline.

Sources: ICIMOD and UNDP joint assessment of potentially dangerous glacial lakes in the Nepal, Tibet and India river basins; The Himalayan Times reporting on the Imja Tsho water-level lowering project.

Cover photo: Jesse Allen via Wikimedia Commons (Public domain).