"Bihar dominates in scale. Uttar Pradesh is second and often under-reported," says Pradeep Man Dangol, hydrologist at ICIMOD. Bihar bears the greatest burden, but Uttar Pradesh is a significant and often overlooked part of the risk.

The Three-Part Chain of Disaster

Hydrologists describe the process as a three-part chain. First is the rain: 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 rain can fall on steep, small catchments. Second 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. Third is when the water meets India.

How bad the flooding becomes depends on what the water encounters - the volume of rain on the Indian side, how full the Ganges is, embankments and barrages, and blocked drainage from roads, railways and settlements on the floodplain. So the peak flow at the border is largely determined by rainfall in Nepal. The severity of flooding depends at least as much on conditions in India.

The Nepal floods of recent times echo the lessons of 2008. In that year, 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. Nearly 400 people died in Bihar after the Kosi breached an embankment in Nepal. But, remarkably, it was not an extraordinary high-flow event on the river. According to Rajiv Sinha, a professor of earth sciences at 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 embankment built decades earlier and inadequately maintained.

"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," says Sinha. The failure, in other words, was "structural, not hydrological", adds Sanyal.

Warning Times and Data Gaps

For ordinary monsoon floods, India may have "roughly 12 hours to two days of warning", according to Saswata Sanyal, ICIMOD's intervention manager for disaster risk reduction. 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 do share real-time rainfall and river-level data from a network of stations. Nepal's Department of Hydrology and Meteorology feeds rainfall and river levels to India's Central Water Commission, which uses them for flood forecasts. But important gaps remain, says Sanyal. They include too few real-time monitoring stations in the fastest-flowing Chure catchments. There's also no single shared forecasting model between the two countries; and limited routine sharing on embankment conditions, river-channel changes and sediment. And then there's, crucially, the last mile. A warning reaching a district office is not the same as reaching a family in danger.

Debunking Myths

A common misconception is that Nepal "releases" water into India, say scientists. 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", says Sanyal. 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.

Nepal supplies around 40% of the average annual flow of the Ganges - and a much larger share in 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," says Sanyal. Whether a given flow becomes a disaster depends on conditions in India as much as in Nepal: the Ganges' level, Indian rainfall, embankment integrity, drainage and sediment. "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," says Sanyal.

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

Bihar Floods Face Escalating Climatic Threats

The biggest misconception is that Nepal "releases" water; in reality, extreme rainfall intensifies due to rapid regional warming. As the region warms faster than the global average, warmer atmospheres hold more moisture, increasing the frequency of intense rains. This climatic shift exacerbates the risk of both conventional floods and high-energy slurry events mixing water with sediment. Future disaster resilience will depend heavily on improved real-time monitoring of glacial lakes and sediment outflows to mitigate the growing threat of debris flows and flash floods across the vulnerable Terai plains.