A young, still-rising mountain range
Nepal sits directly on the collision zone between the Indian and Eurasian tectonic plates, a process that began roughly 40–50 million years ago and continues today at around two centimetres a year. This ongoing collision has produced a dense network of active faults and thrusts — including the Main Frontal Thrust, Main Boundary Thrust and Main Central Thrust — making Nepal one of the most seismically active countries on Earth; research suggests a magnitude 8-plus earthquake strikes somewhere along the Main Frontal Thrust roughly every 750 to 870 years in eastern Nepal alone.
Extreme elevation change over a short distance
Nepal rises from around 60 metres above sea level in the Terai to 8,848 metres at the summit of Sagarmatha (Everest) within a horizontal distance of little more than 200 kilometres. This compresses the country into three sharply defined ecological belts:
- The Himalaya/Mountain region (c. 35–42% of land area): home to only around 6–8% of the population, but the source of virtually every major river and thousands of glaciers and glacial lakes.
- The Hill region (c. 35% of land area): home to roughly 40–45% of the population, including the Kathmandu and Pokhara valleys, and characterised by steep, often deforested and landslide-prone slopes.
- The Terai plains (c. 23% of land area): now home to over half of Nepal’s population (53.66% at the 2021 census), following large-scale migration after malaria was eradicated in the 1960s. This is the most fertile and most flood-exposed part of the country, as it absorbs the combined discharge of every Himalayan river system before those rivers cross into India.
A river system built for flash floods
Because rivers such as the Bhote Koshi, Trishuli and Narayani drop from Himalayan peaks to the plains in a very short horizontal distance, they run at exceptionally steep gradients through narrow gorges — ideal for hydropower generation and white-water rafting in normal conditions, but equally challenging for channelling a sudden glacial or landslide-triggered surge downstream at speed, with minimal warning time for communities living along the banks.
Population growth in exposed corridors
Nepal’s total population stood at 29,164,578 at the 2021 census, with an urban population that has grown from 17% in 2011 to over 66% in 2021 — much of that growth concentrated in river valleys, the Kathmandu Valley, and Terai towns along exactly the kind of river corridors implicated in this week’s disaster.
Though it still called ‘paper urbanisation’, 66% of Nepal’s population now lives in municipalities and thus is called urban. Many of these municipalities are in these disaster-prone exposed corridors.
Rasuwa, the district worst affected this time, is sparsely populated (fewer than 47,000 residents, a density of just 30 people per square kilometre) precisely because it is high, remote and mountainous — yet it sits astride a major pilgrimage and trade route to Tibet, which is why so many transient tourists and workers, rather than long-term residents, were exposed to the flood.
Climate change as a multiplier
Layered on top of this geography is accelerating glacier melt across the Hindu Kush Himalaya, doubling in rate since 2000. Nepal is now ranked among the most climate-vulnerable countries in the world, and the increasing frequency of GLOFs and glacier-collapse events — three major incidents in the Rasuwa corridor alone since July 2025 — suggests this week’s tragedy is unlikely to be the last of its kind.