Nepal Floods Raise Alarm for India as Himalayan River System Faces Growing Risks

Some floods can race out of the mountains carrying enormous quantities of mud, rock and debris, while others lose much of their force after reaching the plains and become more conventional floods. Whether that water develops into a disaster in India depends not only on rainfall in Nepal, but also on rainfall, river levels, embankments, drainage and development on the Indian side.

Nepal’s floods are therefore a warning for India, not simply because water flows south across the border, but because both countries share a vast and increasingly volatile Himalayan river system.

Three major river systems dominate this network: the Kosi in the east, the Gandaki, known as the Gandak in India, in the centre, and the Karnali, known as the Ghaghara in India, in the west. The Mahakali, known as the Sharda, runs along the western border.

A dozen smaller rivers rising in the Siwaliks and Chure, including the Bagmati, Kamala, Rapti and Babai, are less prominent but can produce faster and more unpredictable floods because their catchments are small and steep.

Seven to nine major rivers flow from Nepal into the Indian plains and are responsible for flooding in India, according to Manish Shrestha, a hydrologist at the International Centre for Integrated Mountain Development (ICIMOD).

India’s Central Water Commission identifies the Kosi, Gandak, Bagmati and Ghaghara as some of the main transboundary rivers.

Bihar is by far the most exposed Indian state, with roughly three-quarters of north Bihar officially classified as flood-prone. Almost all the major rivers that flood the state originate in Nepal, with the Kosi, known as the “sorrow of Bihar”, the most notorious, followed by the Gandak, Bagmati and Kamala.

The risk, however, extends beyond Bihar.

Eastern Uttar Pradesh is vulnerable to the Ghaghara, Rapti and Gandak rivers. Districts including Gorakhpur, Bahraich, Lakhimpur Kheri and Shravasti are regularly affected.

Uttarakhand shares the Mahakali-Sharda rivers, while North Bengal receives the Mechi and Mahananda.

“Bihar dominates in scale. Uttar Pradesh is second and often under-reported,” said Pradeep Man Dangol, hydrologist at ICIMOD.

Bihar bears the greatest burden, but Uttar Pradesh remains a significant and often overlooked part of the risk.

When Nepal floods, hydrologists say the resulting threat can be understood as a three-part chain: rainfall, the river and the conditions encountered when the water reaches India.

The first factor is rainfall, including where it falls and how heavily.

Flood peaks reaching India are often generated not in the high Himalayas but lower down, in the Siwaliks, Chure and Terai, where intense monsoon rainfall can fall on steep, small catchments.

The second factor is the river.

Nepal’s rivers cross a sharp break in slope at the mountain front, drop their sediment and spread across the plains as braided, shifting channels.

The third factor emerges when the water reaches India.

The severity of flooding depends on what the water encounters, including the volume of rainfall on the Indian side, the level of the Ganges, embankments and barrages, and drainage blocked by roads, railways and settlements on the floodplain.

The peak flow at the border is therefore largely determined by rainfall in Nepal, while the severity of flooding depends at least as much on conditions in India.

The 2008 Kosi disaster demonstrates why. The river was carrying far less water than the embankment was designed to handle. The catastrophe was caused by the embankment failing, not by an exceptional flood.

For ordinary monsoon floods, India may have “roughly 12 hours to two days of warning”, according to Sanyal, depending on how quickly the water travels downstream.

Water can take about a day to reach the Bihar plains from the Nepal mountain front along the Kosi and Gandak, and somewhat longer along the Ghaghara. Flash floods from the Chure rivers can arrive within hours.

A landslide-dam burst or debris flow can be even faster. In the current floods, the peak surge from Rasuwa reached Triveni, near the Indian border, in about seven-and-a-half hours.

India and Nepal share real-time rainfall and river-level data through a network of stations. Nepal’s Department of Hydrology and Meteorology provides rainfall and river-level data to India’s Central Water Commission, which uses the information for flood forecasts.

However, important gaps remain, Sanyal said.

There are too few real-time monitoring stations in the fastest-flowing Chure catchments. The two countries also have no single shared forecasting model, while routine sharing of information on embankment conditions, river-channel changes and sediment remains limited.

There is also the crucial last mile.

A warning reaching a district office does not necessarily mean it reaches a family in danger.

For the transboundary rivers, Sinha said, “cooperation on information sharing and early-warning systems is extremely important. We need rapid continuous monitoring of glacial lakes, their outflows and how they are changing over time”.

Scientists also reject a common misconception that Nepal “releases” water into India.

Nepal has almost no large reservoirs capable of doing this. Its main storage dam, Kulekhani, holds less than a tenth of a cubic kilometre, “so there’s nothing to release”, Sanyal said.

The Kosi and Gandak barrages are operated by India. When floodwater reaches the border, it is overwhelmingly because rain has fallen upstream, not because Nepal has opened a gate.

There is, however, an important reality behind the claim. Much of the water that floods north Bihar and eastern Uttar Pradesh has fallen as rain in Nepal, meaning Nepal’s rainfall largely determines the timing and size of flood peaks reaching India.

Nepal supplies around 40% of the average annual flow of the Ganges and a much larger share during the dry season, making its hydrology critical to India.

“What is too simplistic is treating that as a story of cause and blame. Rain is not a decision,” Sanyal said.

Whether a particular flow becomes a disaster depends on conditions in India as much as in Nepal, including the Ganges’ level, Indian rainfall, embankment integrity, drainage and sediment.

The effects also run in both directions. Embankments and barrages built for India’s protection have raised water levels and caused backwater flooding in Nepal’s Terai, which is Nepal’s long-standing concern, Sanyal said.

Nepal and India share one river system, with one country upstream and the other downstream. Experts say the answer is therefore not blame but better shared data, forecasting and river management.

The issue that should worry India most, however, may not be the Kosi river in Nepal.

“India should be worried. but not necessarily because of the Kosi river in Nepal,” Rajiv Sinha said.

The broader warning concerns what is happening across the Himalayas and what occurs when water, rock, ice and enormous quantities of sediment suddenly combine.

Sinha said the latest Nepal disaster resembles a series of catastrophic events that India has experienced in recent years, including the 2021 Chamoli disaster and the 2025 Dharali disaster in Uttarakhand. The Nepal event, he said, was “perhaps five to 10 times larger”.

“The fundamental point is that these are not normal hydrological floods,” Sinha said.

A conventional flood is largely water. When water from intense rainfall, melting ice or a sudden discharge mixes with huge quantities of sediment, however, the result can become a thick, high-energy slurry capable of destroying almost everything in its path.

That was the lesson of the Chamoli disaster in 2021, when a large chunk of a Himalayan glacier collapsed in the valley of Chamoli in northern India, sending a cascade of debris and water downstream that killed 200 people and damaged hydropower infrastructure.

The same danger was evident at Dharali in Uttarakhand in 2025, after heavy rains triggered a flash flood and the resulting debris flow buried parts of a village under metres of material.

Climate change is adding another layer to the risk.

Extreme rainfall is intensifying. The region is warming faster than the global average, a warmer atmosphere holds more moisture, and the monsoon is delivering more rain in fewer, heavier bursts, experts say.

High-mountain hazards are also growing. As permafrost degrades, glacial lake outbursts and rock-ice avalanches are expected to increase, producing faster, debris-laden floods that are harder to forecast.

More extreme events also mean more landslides and sediment in rivers, raising riverbeds and reducing channel capacity on both sides of the border.

These events are manifestations of the same broad Himalayan hazard system, while India’s exposure to its consequences remains considerable.