A recent flash flood in Nepal’s Rasuwa district on August 26, 2026, sent a powerful surge of water, sediment, and boulders through the Bhote Koshi and Trishuli river systems. This event has brought into sharp focus the significant cross-border flood risks posed by Himalayan rivers to India, particularly the plains of Bihar and Uttar Pradesh.
Nepal Flood Incident and Immediate Aftermath
According to the International Centre for Integrated Mountain Development (ICIMOD), water levels in the Bhote Koshi basin began rising sharply around 9 AM local time, with the flood wave rapidly moving downstream into the Trishuli river. At Galchchi, water levels in the Trishuli surged by an alarming nine meters within just 30 minutes, indicating the extreme speed and volume of the flood.
A preliminary assessment suggested that a large volume of ice and rock debris may have entered Lende Khola, a tributary of the Bhote Koshi, triggering the sudden surge. However, ICIMOD noted that the exact cause of this specific flash flood had not yet been conclusively established.
While the immediate threat from this particular flood in Nepal's Rasuwa district has eased, with discharge at the Valmikinagar Gandak Barrage falling to around 81,000 cusecs, the incident serves as a stark reminder of the broader hydrological connectivity between the two nations.
Four Himalayan Rivers: Pathways to India's Flood Crisis
Several major rivers that flow through Bihar and Uttar Pradesh originate in or draw substantial water from Himalayan catchments across the border in Nepal. The Gandak, Kosi, Bagmati, and Ghaghara rivers form four distinct pathways through which extreme rainfall and sudden flood surges in Nepal can rapidly impact the Indian plains.
The Gandak (Narayani)
Known as the Narayani in Nepal, the Gandak was central to the latest flood alert. The surge originated near the Nepal-Tibet border in the Bhote Koshi/Lhende river system, flowing into the Trishuli river system. The Trishuli forms part of the Narayani basin, which eventually flows south towards India, where it becomes the Gandak. It enters India near Valmikinagar in West Champaran, Bihar, affecting districts such as East Champaran, Gopalganj, Muzaffarpur, Saran, and Vaishali before joining the Ganga near Hajipur.
The Kosi (Saptakoshi)
The Kosi represents another critical Nepal-to-Bihar flood pathway. Its principal upper tributaries—the Sun Kosi, Arun, and Tamur—converge to form the powerful Saptakoshi system. The river enters Bihar near Bhimnagar and impacts districts including Supaul, Saharsa, Madhepura, Araria, Purnia, Katihar, Madhubani, Darbhanga, and Khagaria. The Kosi is particularly notorious for its enormous sediment load from the Himalayas, which, combined with the reduced gradient in the plains, contributes significantly to flooding and frequent changes in its course.
The Bagmati
Originating in Nepal’s Shivapuri range north of Kathmandu, the Bagmati flows through the Kathmandu Valley and southwards into the Terai region. It enters India in Bihar’s Sitamarhi district near Dheng and then traverses the northern plains. Heavy rainfall in its mountainous upper catchment can quickly send large volumes of water towards the relatively flat areas of north Bihar, where drainage can be slow and flood risks are high.
The Ghaghara (Karnali)
In Nepal, the Ghaghara is known as the Karnali. It originates in the Himalayan region near Tibet, gathering water from several mountain tributaries before entering India. The Ghaghara is a major flood pathway for Uttar Pradesh, impacting districts like Kushinagar, Deoria, and Maharajganj, particularly Khadda tehsil due to its proximity to the river. It eventually flows into Bihar before joining the Ganga near Chhapra.
Preparedness and Monitoring in India
Following the recent Nepal flood, all 36 gates of the Gandak barrage were opened, releasing approximately 150,000 cusecs by 11 PM on August 26. Authorities and National Disaster Response Force (NDRF) teams were placed on high alert in vulnerable Indian districts. Officials are closely monitoring water levels and barrage releases, while district administrations have been advised to remain prepared for potential evacuation and rescue operations if conditions worsen. The interconnected nature of these river systems means that vigilance and coordinated efforts are crucial to mitigate the impact of cross-border flooding.