Why Nepal floods should worry India

Nepal's recent devastating floods, which have claimed over 1,000 lives near the Tibetan border, highlight a critical transboundary risk for India. While both nations share Himalayan rivers, experts emphasize that the connection between a flood in Nepal and a disaster in India is more intricate than a simple downstream flow.

Some floods can descend from the mountains with immense volumes of mud, rock, and debris. Others may lose much of their intensity upon reaching the plains, resulting in a more typical flood. The extent to which this water becomes a disaster in India depends not only on rainfall in Nepal but also on precipitation, river levels, embankments, drainage, and development within India.

Nepal's floods serve as a warning for India, not merely due to water flowing south across the border, but because the two countries share an expansive and increasingly volatile Himalayan river system.

Nepal boasts over 6,000 rivers and streams, with the majority draining south into India and eventually the Ganges. Three primary systems dominate: the Kosi in the east, the Gandaki (known as Gandak in India) in the center, and the Karnali (Ghaghara in India) in the west, alongside the Mahakali (Sharda) along the western border. Additionally, a dozen smaller rivers originating in the Siwaliks and Chure ranges—including the Bagmati, Kamala, Rapti, and Babai—are less renowned but can generate swifter, more unpredictable floods due to their smaller, steeper catchments.

According to Manish Shrestha, a hydrologist at the International Centre for Integrated Mountain Development (ICIMOD), seven to nine major rivers flow from Nepal into the Indian plains and are responsible for flooding in India. India's Central Water Commission identifies the Kosi, Gandak, Bagmati, and Ghaghara as key transboundary rivers.

Bihar stands as India's most flood-prone state, with approximately three-quarters of north Bihar officially designated as such. Nearly all major rivers that inundate the state originate in Nepal, with the Kosi—dubbed the "sorrow of Bihar"—being the most infamous, followed by the Gandak, Bagmati, and Kamala.

However, the risk extends beyond Bihar. Eastern Uttar Pradesh is vulnerable to the Ghaghara, Rapti, and Gandak rivers, regularly affecting districts like Gorakhpur, Bahraich, Lakhimpur Kheri, and Shravasti. Uttarakhand state shares the Mahakali-Sharda rivers, while North Bengal receives the Mechi and Mahananda. Pradeep Man Dangol, an ICIMOD hydrologist, notes that "Bihar dominates in scale. Uttar Pradesh is second and often under-reported," emphasizing that while Bihar bears the greatest burden, Uttar Pradesh represents a significant and frequently overlooked aspect of the risk.

Hydrologists describe the flood risk as a three-part chain. First, rainfall: its location and intensity. Flood peaks reaching India often originate not in the high Himalayas but lower down in the Siwaliks, Chure, and Terai, where intense monsoon rains can fall on steep, small catchments. Second, the river itself. Nepal's rivers encounter a sharp change in slope at the mountain front, depositing sediment and fanning out across the plains as braided, shifting channels. Third, the interaction when the water reaches India. The severity of flooding depends on what the water encounters—the volume of rain on the Indian side, the Ganges' water level, the presence of embankments and barrages, and blocked drainage from roads, railways, and settlements on the floodplain.

Thus, while rainfall in Nepal largely determines the peak flow at the border, the severity of flooding depends at least as much on conditions within India. The 2008 Kosi disaster exemplifies this: the river carried far less water than the embankment was designed to handle, with the catastrophe stemming from the embankment's failure, not an exceptional flood.

Saswata Sanyal, ICIMOD's intervention manager for disaster risk reduction, states, "For the peak flow arriving at the border, largely Nepal's rainfall; for the severity of flooding once it arrives, at least as much on Indian conditions." The 2008 Kosi disaster serves as a powerful illustration. Nearly 400 people died in Bihar after the Kosi breached an embankment in Nepal. Remarkably, this was not an extraordinarily high-flow event. According to Rajiv Sinha, a professor of earth sciences at the Indian Institute of Technology (IIT), Kanpur, the flow was only about a tenth of the river's carrying capacity. The catastrophe was caused by a breach in an inadequately maintained embankment built decades earlier. Sinha explains, "The embankments constructed in the 1950s and 1960s had not been properly maintained. They had become eroded and eventually developed a weak point. The river breached its embankment through these vulnerable points and entered parts of India that had not seen flooding for perhaps 100 years." Sanyal adds that the failure was "structural, not hydrological."

For typical monsoon floods, India may have "roughly 12 hours to two days of warning," according to Sanyal, depending on the water's downstream travel speed. Water can take approximately a day to reach the Bihar plains from the Nepal mountain front along the Kosi and Gandak, and slightly 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 do share real-time rainfall and river-level data from a network of stations. Nepal's Department of Hydrology and Meteorology provides this data to India's Central Water Commission for flood forecasts. However, Sanyal points out significant gaps, including too few real-time monitoring stations in the fastest-flowing Chure catchments, the absence of a single shared forecasting model, and limited routine sharing of information on embankment conditions, river-channel changes, and sediment. Crucially, there's also the "last mile" challenge: a warning reaching a district office is not the same as reaching a family in danger.

For these transboundary rivers, Sinha emphasizes that "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."

A common misconception is that Nepal "releases" water into India. Scientists clarify that Nepal has almost no large reservoirs capable of doing this; its main storage dam, Kulekhani, holds less than a tenth of a cubic kilometer, meaning "there's nothing to release," says Sanyal. The Kosi and Gandak barrages are operated by India. When floodwater reaches the border, it is overwhelmingly due to upstream rainfall, not Nepal opening a gate.

However, there's an important truth behind the claim: much of the water that floods north Bihar and eastern UP originates as rain in Nepal, meaning Nepal's rainfall largely dictates the timing and size of flood peaks reaching India. Nepal contributes approximately 40% of the Ganges' average annual flow—and a much larger share during the dry season—making its hydrology critical to India. Sanyal cautions against a simplistic "story of cause and blame," stating, "Rain is not a decision."

Whether a given flow becomes a disaster depends as much on conditions in India—the Ganges' level, Indian rainfall, embankment integrity, drainage, and sediment—as in Nepal. Sanyal adds, "And the effects run both ways: 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." Given that Nepal and India share one river system, with one country upstream and the other downstream, the solution lies not in blame but in improved shared data, forecasting, and river management.

"India should be worried, but not necessarily because of the Kosi river in Nepal," says Rajiv Sinha. Experts suggest the true warning lies in the broader Himalayan context—what happens when water, rock, ice, and vast quantities of sediment suddenly combine. Sinha notes that the recent Nepal disaster resembles catastrophic events India has experienced, such as the 2021 Chamoli disaster and the 2025 Dharali disaster in Uttarakhand, stating the Nepal event was "perhaps five to 10 times larger."

Sinha emphasizes, "The fundamental point is that these are not normal hydrological floods." A conventional flood is primarily water. However, when intense rain, melting ice, or a sudden discharge mixes with enormous amounts of sediment, the result can be a thick, high-energy slurry capable of destroying almost everything in its path. This was evident in the 2021 Chamoli disaster, where a large section of a Himalayan glacier collapsed in northern India's Chamoli valley, unleashing a cascade of debris and water that killed 200 people and damaged hydropower infrastructure. It was also seen at Dharali in Uttarakhand in 2025, when heavy rains triggered a flash flood and debris flow that buried parts of a village under meters of material.

Furthermore, climate change is exacerbating these risks. 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. High-mountain hazards are also increasing. As permafrost degrades, glacial lake outbursts and rock-ice avalanches are expected to rise, producing faster, debris-laden floods that are harder to forecast. More extreme events also lead to more landslides and sediment in rivers, raising riverbeds and reducing channel capacity on both sides of the border. These are all manifestations of the same broad Himalayan hazard system, and India's exposure is considerable.