Nepal floods: 20 years of satellite images reveal how a Himalayan river corridor became a disaster trap

For two decades, the landscape around Nepal’s Lhende–Trishuli river corridor has steadily filled with roads, bridges, hotels, truck yards, substations and hydropower infrastructure, leaving a violent mountain river with less room to absorb sudden surges. A comparison of satellite imagery over the period helps explain why the latest floods were so devastating, according to a report by TOI’s Chandrima Banerjee.

The geography has always been brutal: a narrow Himalayan gorge, steep slopes and very little level ground. But what has changed dramatically is the scale of development packed into that constrained space.

Click here to read latest developments on Nepal Flash Floods


The consequences were laid bare on August 26, when a flood tore through the Lhende river corridor after apparently being triggered by the collapse of bedrock and part of a glacier high above the valley. The torrent travelled downstream along a roughly 168-km path for about 10 hours, killing at least 469 people across Nepal and Tibet and leaving nearly 1,500 missing, according to the report.

At Rasuwagadhi, on the Nepal-China border, the flood once again demonstrated the danger of building critical infrastructure directly alongside a river with nowhere to go.

The Miteri, or Friendship, Bridge over the Lhende had already been swept away by a flood in July 2025. China subsequently installed a temporary Bailey bridge, allowing the crossing to partially reopen by December.

Then the river came back.

The August 26 flood destroyed the crossing again, along with bridges and substantial stretches of the road connecting the border to Nepal's interior.

The river did not change. The valley did.

Rasuwagadhi has been a practical passage between the Kathmandu Valley and Tibet for centuries, but its importance has grown sharply in recent years. By 2024-25, the border crossing was handling more than $620 million in trade with China.

The 110-metre Miteri Bridge itself had been rebuilt only six years before the latest disaster. Its predecessor was badly damaged during the 2015 Nepal earthquake, after which China dismantled the old structure, financed a replacement and opened the new bridge in 2019.

Yet the repeated destruction of the crossing is more than a story about one unlucky bridge. It is a snapshot of a much bigger problem running through the entire river system.

Twenty years of development, squeezed into a narrow valley

Satellite images spanning roughly two decades show a valley where the human footprint has expanded considerably even though the underlying geography has barely become more forgiving.

These were not newly discovered river settlements. Communities had existed there long before the recent infrastructure boom, and population growth in the upper valley has been relatively modest.

What changed was what was built around them.

Roads, hotels, shops, truck yards, substations and hydropower facilities increasingly occupied the limited usable land around the river and along transport corridors. With little flat terrain available, development naturally gravitated towards river terraces and roads — precisely the areas most exposed when the river suddenly swells.

The July 2025 flood offered a stark demonstration of how quickly the river can redraw the landscape. After that event, the river shifted roughly 80 metres towards a new substation.

The same corridor has also become increasingly important to Nepal-China connectivity. The two countries are working on improving the road to Rasuwagadhi, developing a cross-border power line and potentially extending rail connectivity from Tibet towards Kathmandu. Their 2024 agreement envisages these projects as part of a broader “Trans-Himalayan” transport network.

Further south, the river's steep descent has made the basin highly attractive for hydropower. By 2020, more than 36 hydropower projects were operating, under construction or planned across the Trishuli basin.

The river eventually reaches another crucial piece of infrastructure: the Prithvi Highway, one of Nepal's busiest roads and a major link between Kathmandu, Pokhara, the southern plains and India through Mugling and Narayanghat.

In other words, the flood was not moving through an empty mountain landscape.

It was moving through a corridor that has become increasingly important for trade, tourism, transport, power and cross-border connectivity.

Also read: Nepal Flash Floods: 626 dead, over 2,400 missing as rescue efforts enter fourth day

A river that has been warning Nepal for decades

The vulnerability is not new either.

The Lhende–Trishuli system has experienced glacial-lake floods in 1964 and 1995. Flooding damaged hydropower infrastructure, bridges and the dry port in 2018, 2019 and 2020. In 2023, the river rose suddenly again after water travelled down from Tibet.

The glaciers themselves are also changing. In the Langtang catchment, glaciers have lost about 41% of their area since the early 1800s, with retreat accelerating after 2000.

The July 2025 flood, which killed 11 people and left 19 missing, was caused by water suddenly draining from a glacier, an event researchers believe may also have occurred in 2023.

The river's journey helps explain why a disaster at the border can become a much wider catastrophe.

Streams fed by rainfall, snowfall and glacier melt begin high in southern Tibet before feeding into the Lhende and Gyirong rivers. They converge near Rasuwagadhi at around 1,850 metres, forming the Bhotekoshi.

At that point, the water is already being forced through a steep gorge.

When an enormous volume of water, ice, rock and mud arrives suddenly, there is little space for it to spread.

The river continues south into Nepal. Near Syapru Besi, around 15 km from the border, another mountain river joins it. Downstream, the combined river is known as the Trishuli, carrying the same flood pulse farther south while accumulating water from additional tributaries.

As the landscape eventually opens out, the river has more space to widen, slow and spill over flatter ground.

At Devghat, about 130 km from the border, the Trishuli joins the Kali Gandak to form the Narayani. The river then crosses into India as the Gandak before eventually flowing into the Ganga.

That was the path followed by the August flood.

And at almost every stage, the satellite record shows a corridor that has become more heavily used and more heavily built over the past 20 years.

Little warning at the source, heavy exposure downstream

The physical vulnerability was compounded by a warning system that appears far less developed than the infrastructure now dependent on the river.

Nepal and China cooperate on disaster response and exchange information, and China has provided upstream flood warnings in the past. But there does not appear to be a dedicated mechanism guaranteeing Nepal real-time warnings for floods travelling down from Tibet.

Nepal said it received no advance warning before the July 2025 Rasuwagadhi flood.

On August 26, officials learnt about the latest flood only after it had already entered Nepal. The Syapru Besi river gauge stopped transmitting minutes earlier, leaving little warning time for communities near the border. Authorities were able to send alerts farther downstream, but places around Rasuwagadhi and Timure had almost no lead time.

The timing also mattered.

August is the Kailash-Mansarovar pilgrimage season, and Rasuwagadhi has become one of the principal routes for pilgrims travelling towards the sacred site. It is Nepal's shortest land route and is cheaper than the alternative through Hilsa.

More than 13,500 pilgrims used the route in 2025-26, an increase of nearly 19% in a year.

When the flood struck, groups were travelling in both directions. One group of 77 people was inside the Gyirong immigration building when it was hit. By the following day, hundreds of the missing foreign nationals were identified as Kailash pilgrims.

The repeated destruction of the Miteri Bridge therefore captures something much larger than a bridge repeatedly being washed away.

It shows what happens when a rapidly developing transport and economic corridor meets a river system capable of changing course in minutes.

The satellite images tell one part of that story: over 20 years, more infrastructure has moved into the narrow spaces available beside the river.

The floods tell the other: the river has never agreed to make room.

(With inputs from ToI's Chandrima Banerjee)