The Gandak River is a major, ancient transboundary river in South Asia, functioning as one of the largest left-bank tributaries of the Ganges River in India. It is also known as the Narayani in Nepal and the Kali Gandaki in its upper reaches. The river originates near the Tibet-Nepal border at the Nhubine Himal Glacier in the Mustang region of Nepal at an elevation of 6,268 meters. After flowing through Nepal, it crosses the Indo-Nepal Border at Triveni Dham and enters India at Valmiki Nagar in Bihar. The Gandak then flows through the states of Uttar Pradesh and Bihar, forming a boundary between them for about 130 km, before merging into the Ganga near Hajipur and Sonepur.
The river's significance in modern India is deeply connected to the Gandak Irrigation and Power Project, which was established to solve the problem of devastating annual floods and to utilize the water for irrigation. The project was created through a bilateral treaty, the Gandak Agreement, signed between India and Nepal on December 4, 1959, and later amended in 1964. The key mechanism of the project is the Gandak Barrage, built in 1968–69 at Valmikinagar (originally Bhainsalotan). The agreement provided for India to construct the barrage and canal systems, including the Western Nepal Canal and Eastern Nepal Canal, to provide flow irrigation to Nepal, as well as to the Indian states of Bihar and Uttar Pradesh. The project also included the construction of a 15,000 KW hydroelectric power house in Nepalese territory on the Main Western Canal. The river is ecologically important, connecting Nepal's Chitwan National Park with India's Valmiki Tiger Reserve and supporting endangered species like the gharials and Gangetic Dolphins. The river's course is not static, as it has undergone lateral erosion and changes in its flow over the years, a process monitored for flood management.