The affected area is around 160 kilometres from the Indian border, but its effects can extend into India because of the interconnected river systems across the region. Several major Himalayan rivers originate in or pass through Nepal before entering India and flowing across the Gangetic plains. A sudden increase in upstream water levels can therefore affect downstream areas even after the flood wave crosses an international border.
On Wednesday, the Centre said it recorded a sudden rise in the Trishuli river at Furke Khola in Nepal following a suspected flash flood or glacial lake outburst flood (GLOF) near the Nepal-China border. The Central Water Commission (CWC) said the resulting water surge could move downstream through the Gandak river in India, prompting Bihar and Uttar Pradesh to take precautionary measures.
However, a flood in one part of Nepal does not pose the same risk to every part of northern India. The impact depends on the river basin involved. In the current case, the important river link is from the Bhote Koshi and Trishuli rivers to the Narayani and then the Gandak in India.
According to the International Centre for Integrated Mountain Development (ICIMOD), a flash flood hit Rasuwa district on the morning of August 26. A large surge of water, sediment and boulders moved through the Bhote Koshi and Trishuli river systems.
Water levels in the Bhote Koshi basin started rising sharply around 9 am local time. The flood then moved downstream into the Trishuli.
ICIMOD said the flood wave was exceptionally fast. At Galchchi, water levels in the Trishuli rose by as much as nine metres in just 30 minutes. A preliminary assessment indicated that a large amount of ice and rock debris may have entered Lende Khola, a tributary of the Bhote Koshi, causing the sudden surge. However, ICIMOD said the cause had not yet been conclusively established.
The floodwater moved downstream through the Bhote Koshi and Trishuli rivers. From there, the Narayani-Gandak river system provides the route towards India.
The connection between the Rasuwa flood and India becomes clearer when the river’s downstream route is mapped. The Trishuli flows south through Nepal and meets the Kali Gandaki at Devghat. At Devghat, the two rivers combine to form the Narayani, one of the major tributaries of the Ganga. The Narayani flows into India, where it is called the Gandak, before eventually joining the Ganga in Bihar.
This river route explains why a flood in Rasuwa can lead to alerts in Bihar. The same flash flood conditions seen in the narrow Himalayan valleys do not necessarily continue all the way into India. Instead, additional water entering the river system can move downstream and raise river levels.
The CWC has a long-running flood forecasting and warning arrangement with Nepal for rivers shared by the two countries. The system relies on hydro-meteorological observations from monitoring stations in both Nepal and India.
Nepal’s Department of Hydrology and Meteorology also operates monitoring stations on the Trishuli and Narayani, including at Betrawati and Devghat. These stations are important because both rivers form part of the same Narayani drainage system.
The CWC’s August 26 advisory said the flood wave observed in the Trishuli could move downstream towards the Gandak in India.
The areas most directly exposed are along the Gandak corridor in eastern Uttar Pradesh and Bihar. The Uttar Pradesh State Disaster Management Authority (UPSDMA) identifies Kushinagar, Deoria and Maharajganj among districts affected by flooding in the Gandak basin. Its flood-planning document also identifies overflow from Nepal’s Narayani as one of the possible causes of inundation.
Khadda tehsil in Kushinagar, according to the document, is particularly exposed because it is closest to the river and could be among the first areas to face flooding.
In Bihar, the state’s Flood Management Information System says the Gandak flows through or along West Champaran, East Champaran, Gopalganj, Muzaffarpur, Siwan, Saran and Vaishali before joining the Ganga.
However, the risk is not spread evenly across these districts. Areas closest to the Gandak, its embankments and its floodplain face the greatest exposure.
In Bihar, all 36 gates of the barrage were opened following the Nepal flood, while a technical team was stationed at the site. Officials cited by news agency PTI said around 150,000 cusecs of water were being released from the barrage by 11 pm on August 26.
The Centre has also put local administrations, rescue teams and the National Disaster Response Force (NDRF) on high alert in potentially affected border areas.
A flash flood in a narrow Himalayan valley can be very different from flooding hundreds of kilometres downstream. When a sudden event adds a large amount of water to a river, it creates a flood wave that moves downstream. As the river widens and the land becomes flatter, the wave usually spreads out and loses some of its intensity.
The flood’s impact may weaken with distance, but the additional water still contributes to downstream river levels and discharge. This means the powerful surge seen in a narrow Himalayan valley is unlikely to reach India at the same speed or with the same height. Its downstream impact depends on several factors, including the width and capacity of the river channel, water flowing in from tributaries, the amount of water that can spread into floodplains and the river’s water level before the flood wave arrives.
As a result, the nature of the flooding can change by the time it reaches the plains. The force of boulders and debris may reduce, but higher river discharge can still push up water levels and threaten settlements and farmland along the river.
Bihar’s geography makes it vulnerable to floods from Nepal. According to Bihar’s Water Resources Department, the Gandak has a catchment area of 40,553 sq km up to its meeting point with the Ganga. Of this, 35,470 sq km is in Nepal, while 4,188 sq km is in Bihar and 895 sq km is in Uttar Pradesh.
This means a very large part of the Gandak’s catchment lies upstream in Nepal. Changes in the upper catchment can consequently have a significant effect on the volume of water entering India.
The terrain also adds to the flood risk. Bihar’s Water Resources Department says the Gandak has a steep slope in the mountains, but the gradient becomes much flatter after the river enters the plains.
The river also carries substantial quantities of sediment during the monsoon. Some of this material gets deposited on the riverbed in the plains. Over time, such deposits can raise the riverbed and make it easier for water to spill over the banks.
The broader flood risk is not limited to the Gandak. Bihar’s flood-management authorities identify rivers such as the Kosi, Gandak, Burhi Gandak, Bagmati, Kamla Balan, Mahananda and the Adhwara group as major rivers draining the plains adjoining Nepal. Their upper catchments are located in the geologically young Himalayan region. These rivers can carry large amounts of water and sediment into the plains.
High water levels in the Ganga can also worsen Gandak flooding. According to Bihar’s Water Resources Department, high Ganga levels can slow the Gandak’s outflow and create a backwater effect, worsening flooding.
The impact of a flood wave from Nepal therefore depends on more than the water entering the river upstream. Existing water levels in the Gandak, rainfall in Bihar and eastern Uttar Pradesh, tributary inflows and Ganga levels can all affect the extent of flooding.
Research by ICIMOD has also highlighted the heavy sediment load carried into West Champaran. Sediment can alter the river’s course, meaning flood risk depends on both the volume of water and the sediment and debris carried downstream.
Bihar’s official flood-monitoring system showed that the Gandak was already running at high levels at several locations on August 27. At Dumariaghat in Gopalganj, the river stood at 62.53 metres, above the danger level of 62.22 metres. At Lalganj in Vaishali, the level was 50.63 metres, compared with a danger level of 50.50 metres.
At Bagaha in West Champaran, the Gandak was at 89.10 metres. This was above the warning level of 88.30 metres but below the danger level of 89.30 metres. Thakraha in West Champaran was also above its warning level, while Khadda on the Uttar Pradesh side was at its warning level.
These readings show that parts of the Gandak system were already at elevated levels as authorities watched for the possible downstream effects of the Nepal flood.
However, the readings should not by themselves be treated as proof that the August 26 flash flood was responsible for these levels. The Home Ministry said on Wednesday night that, based on the CWC’s assessment, water levels on the Nepal side were falling and the situation was under control.
Not all Nepal floods pose the same risk to India. The downstream impact depends on which river basin carries the floodwater towards India.
The CWC identifies the Ghaghra, Sharda, Rapti, Gandak, Burhi Gandak, Bagmati, Kamla, Kosi and Mahananda among rivers originating in or flowing from Nepal that can contribute to flood situations in Uttar Pradesh and Bihar. Because these rivers enter India at different points, a flood in one part of Nepal can have a very different impact from a flood in another basin.
For example, flooding in the Kosi system mainly threatens parts of north and eastern Bihar. The Bagmati affects areas including Sitamarhi, Muzaffarpur and Darbhanga, while the Kamla-Balan basin covers large parts of Madhubani and Darbhanga.
The Rasuwa flood is different because the affected rivers are part of the Trishuli-Narayani-Gandak system. Its most direct hydrological link is therefore with the Gandak corridor in eastern Uttar Pradesh and Bihar.
India and Nepal have a long-standing system for sharing hydrological and weather data on rivers common to both countries. The CWC says its flood forecasting system uses observation stations in both countries during the flood season.
According to the CWC’s 2023-24 annual report, the system covers 42 meteorological or hydro-meteorological sites in Nepal and 18 hydrological sites in India. The data helps authorities forecast floods and issue warnings downstream.
While flash floods can develop quickly in Himalayan valleys, the resulting flood wave takes time to move downstream. This gives Indian authorities some time to track river levels and prepare.
In the current case, the CWC’s assessment of the sharp rise in the Trishuli led to alerts and preparedness measures in Bihar and Uttar Pradesh ahead of the possible arrival of the flood wave in the Gandak system.
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