The Kosi River, or Koshi, is a major transboundary river system that flows through China, Nepal, and India, and is the third-largest tributary of the Ganges by water discharge. It is also known as the Saptakoshi (Seven Rivers) because it is formed by the confluence of seven major Himalayan tributaries, including the Arun from Tibet and the Sun Kosi and Tamur from Nepal. The Kosi is an antecedent river, meaning its course predates the rise of the Himalayas, and it is notorious for its high sediment load and catastrophic lateral channel migration, having shifted its course westward by over 100 kilometers in the last 250 years. This dynamic behavior earned it the historical nickname, the "Sorrow of Bihar".
The problem of devastating annual floods led to the Kosi Project, which was conceptualized following the disastrous floods of 1954. The project's mechanism is governed by the Kosi Agreement between India and Nepal, first signed on April 25, 1954, and revised on December 19, 1966. The agreement authorized the construction of the Kosi Barrage (also called Bhimnagar Barrage) between 1959 and 1963 on the Indo-Nepal border, with the entire cost borne by India. The project's key provisions include flood control, irrigation, and hydropower generation, with the mechanism involving the barrage to anchor the river and the construction of embankments downstream to confine the flow. The revised 1966 agreement superseded the original and granted Nepal the right to withdraw water for irrigation and other purposes, while India retained the right to regulate the balance of supplies at the barrage site. The agreement also leased the project area to the Government of India for 199 years for operation and management.
The Kosi Project connects to the broader framework of transboundary water governance, alongside other agreements like the Gandak Agreement and the Mahakali Treaty between India and Nepal. A significant recent change occurred in 2008 when a breach in the man-made embankments caused a massive flood, displacing millions and highlighting the ongoing challenges of sedimentation and flood management. The core mechanism of the barrage and embankment system remains, but the focus has shifted towards improved flood forecasting and disaster preparedness, supported by projects like the World Bank-supported Bihar Kosi Basin Development project.