Barrages, Dams and Embankments on Nepal's Rivers: A Directory

A barrage is a low, gated diversion structure built across a river to raise its level just enough to push water into canals; unlike a high storage dam it holds back little water and passes floods through its gates. An embankment (also called a bund or levee) is an earthen wall built along a river to confine floodwaters to a defined channel. Nepal's largest such structures stand where the great snow-fed rivers — the Koshi (Saptakoshi), the Narayani (Gandak) and the Mahakali — leave the hills and enter the Tarai plains near the Indian border.

Because these rivers flow on into densely populated Bihar and Uttar Pradesh, every major barrage on them was built under a bilateral arrangement with India: the Koshi Agreement of 25 April 1954 (2011 BS, revised 1966), the Gandak Agreement of 4 December 1959 (2016 BS, amended 1964) and the Mahakali Treaty of 12 February 1996 (2052 BS). Alongside these transboundary works, Nepal has built its own barrages for irrigation on the Kamala, Bagmati, Babai and West Rapti rivers. This directory covers each structure's year, purpose, design flow and legal basis; power-generation dams are treated separately.

The Koshi Barrage — also called the Bhimnagar Barrage or Koshi Bandh — spans the Saptakoshi River at Bhimnagar, linking Sunsari district on the east bank with Saptari on the west, about five kilometres upstream of Hanuman Nagar. It was built under the Koshi Agreement signed by Nepal and India on 25 April 1954, during the premiership of Matrika Prasad Koirala; construction ran from 1959 to 1962, and the barrage has been in operation since 1962 (2019 BS). The structure is about 1,150 metres long with 56 gates and a design flood capacity of roughly 950,000 cusecs (about 26,900 cubic metres per second), making it the largest engineered structure on any Nepali river.

Its stated purposes are flood control, irrigation, hydropower generation and prevention of bank erosion. The Eastern Koshi Main Canal and Western Koshi Main Canal take off from the barrage and irrigate large areas of Bihar; on the Nepali side, the Koshi Distributary System (about 11,300 hectares) and the Koshi Pump Irrigation System (about 13,180 hectares) serve Saptari district and were handed over to Nepal in 1989. The barrage also carries the highway crossing that for decades was the only road link between Nepal's eastern Tarai districts on either side of the Koshi.

The 1954 agreement was revised on 19 December 1966 to address Nepali concerns: the amended text affirms Nepal's right to use water of the Koshi and Sunkoshi basins for irrigation and other purposes, while the land occupied by the project was leased to India for 199 years. The barrage is operated and maintained by India's Koshi Project, and the lease term, sedimentation of the barrage pond and inundation upstream remain recurring points of debate in Nepal–India water diplomacy.

The barrage works as a system with the Koshi embankments — long earthen afflux bunds upstream and flood embankments downstream on both banks, built in the 1950s to pin down a river infamous for swinging tens of kilometres across its own alluvial fan (the Koshi is often called the 'Sorrow of Bihar'). Downstream embankments run for roughly 246 kilometres in Nepali and Indian territory, kept about 12 to 16 kilometres apart so the channel can spread and drop its enormous silt load between them.

On 18 August 2008 (2 Bhadra 2065 BS) the eastern afflux embankment failed at Kusaha in Sunsari district, about 12 kilometres upstream of the barrage. The river was carrying only around 144,000 cusecs at the time — a small fraction of the barrage's design flood — which investigators attributed to poor embankment maintenance rather than an extreme flood. The Koshi avulsed into an old channel far to the east, inundating some 2,700 square kilometres; tens of thousands of people were displaced in Sunsari and more than three million were affected in Bihar. The breach was closed in early 2009, and embankment strengthening on the Nepali side remains a standing agenda item of the Nepal–India Joint Committee on Water Resources.

The Gandak Barrage — known in India as the Valmikinagar Barrage and locally associated with Triveni, the confluence where the Narayani leaves Nepal — was built under the Gandak Irrigation and Power Project Agreement signed on 4 December 1959 (2016 BS) and amended on 30 April 1964 to protect Nepal's riparian rights. The barrage, completed in 1968-70 at Bhainsalotan/Valmikinagar, is about 747 metres long with 36 sluice gates and a maximum discharge capacity of roughly 850,000 cusecs; part of the structure lies in Nepali territory in Nawalparasi (West).

Under the agreement India built two canal systems for Nepal at its own cost. The Nepal Eastern Canal, taking off through the Don Branch Canal, was designed for a gross command of about 103,500 acres (roughly 41,900 hectares) in Bara, Parsa and Rautahat districts, while the Nepal Western Canal was designed for about 40,000 acres (roughly 16,200 hectares) in Nawalparasi — though the area actually served has long been well below design. The agreement also gave Nepal the 15 MW Gandak (Surajpura) hydropower station on the Western Canal about 18 kilometres downstream of the barrage, which began generating in April 1979 and is operated by the Nepal Electricity Authority. Gate operation during monsoon floods and inundation of Nepali villages near Susta and Triveni remain periodic sources of local grievance.

The Sharda (Sarada) Barrage at Banbasa is the oldest major structure on any Nepal border river. British India built it between 1918 and 1928 under an exchange of letters dated 23 August and 12 October 1920, in which Nepal transferred about 4,000 acres on the east bank of the Mahakali in return for land elsewhere, Rs 50,000 and a guaranteed water supply — originally 150 cusecs (4.25 cubic metres per second) in the dry season and 460 cusecs (13 cubic metres per second) in the wet season. The barrage feeds the giant Sarda canal system of Uttar Pradesh, while Nepal's share supplies the Mahakali Irrigation Project in Kanchanpur, developed in stages since 1975.

The Tanakpur Barrage, a short distance downstream, was built by India between 1981 and 1992 as part of the 120 MW Tanakpur hydropower project. Because its left afflux bund needed to be tied to high ground inside Nepal, Nepal agreed in a 1991 understanding — revised by a 1992 memorandum — to provide about 2.9 hectares of land at Jimuwa village, a step that triggered a constitutional controversy in Kathmandu over whether parliamentary ratification was required.

The dispute was resolved by the Mahakali Treaty, signed on 12 February 1996 by Prime Ministers Sher Bahadur Deuba and P. V. Narasimha Rao and ratified by Nepal's parliament with a two-thirds majority on 20 September 1996. The treaty confirms Nepal's entitlement from the Sharda Barrage at 1,000 cusecs (28.35 cubic metres per second) in the wet season (15 May–15 October) and 150 cusecs in the dry season, and from the Tanakpur Barrage at 1,000 cusecs wet-season and 300 cusecs dry-season supply plus 70 million kilowatt-hours of free energy every year. It also envisages 350 cusecs (10 cubic metres per second) for irrigating Nepal's Dodhara–Chandani enclave and frames the still-unbuilt Pancheshwar Multipurpose Project, whose detailed project report Nepal and India have yet to finalise.

Nepal has also built wholly domestic barrages for irrigation. The Kamala Barrage, headworks of the Kamala Irrigation Project implemented in 1975-85 (2032-2042 BS), diverts the Kamala River where it forms the Dhanusha–Siraha border, with a design command of about 25,000 hectares; heavy sediment from the Chure hills means dry-season coverage has been reported at only around 10,000 hectares, and rehabilitation is ongoing. Downstream in Bihar, India operates a separate Kamala weir near Jainagar. The Bagmati Barrage at Karmaiya, Sarlahi — a 403.5-metre structure just upstream of the East-West Highway crossing — was begun in 1978 and became operational by 1993 (2050 BS) as the headworks of the Bagmati Irrigation Project, designed for about 37,000 hectares in Sarlahi and Rautahat.

In the western Tarai, the Babai Irrigation Project, started in 1988-89 (2045/46 BS), diverts the Babai River in Bardiya through barrage headworks near Gulariya to command 36,000 hectares — 21,000 hectares east and 15,000 hectares west of the river. By mid-2025 it was watering just over 27,300 hectares with physical progress of about 82 percent. Because the Babai alone cannot supply the required 60 cubic metres per second year-round, the national-pride Bheri Babai Diversion Multipurpose Project is boring water from the Bheri River through a 12.2-kilometre tunnel (holed through in April 2019, Nepal's first tunnel-boring-machine project) to add 40 cubic metres per second and generate 46.8 MW.

The Sikta Irrigation Project on the West Rapti River is the newest large headworks: a 317-metre barrage at Agaiya in Rapti Sonari Rural Municipality, Banke, designed to divert around 60 cubic metres per second and irrigate 42,766 hectares (33,766 hectares west and 9,000 hectares east of the river). The barrage is complete, but canal construction has been slow and troubled — test releases collapsed sections of the western main canal in 2016-2018, prompting corruption investigations — and after two decades the project had reached only about half of its total works.

Three instruments govern the transboundary structures. The Koshi Agreement (1954, revised 1966) covers the Koshi Barrage and its embankments, leasing the project lands to India for 199 years while affirming Nepal's water-use rights in the basin. The Gandak Agreement (1959, amended 1964) covers the Gandak Barrage and obliges India to supply Nepal's eastern and western canal commands and the Surajpura power station, while barring neither country's reasonable upstream uses. The Mahakali Treaty (1996) is the only instrument of full treaty rank ratified by parliament, integrating the Sharda Barrage, the Tanakpur Barrage and the future Pancheshwar project into one package.

For quick comparison of design flows: the Koshi Barrage passes about 950,000 cusecs, the Gandak Barrage about 850,000 cusecs, while the domestic irrigation barrages are an order of magnitude smaller — Sikta diverts about 60 cubic metres per second and Babai requires about 60 cubic metres per second at full development. Proposals to move beyond barrages to true storage dams — the Saptakoshi High Dam, Pancheshwar and the Kamala and Sunkoshi-Marin schemes — have been studied for decades; only the Sunkoshi-Marin diversion is currently under construction, and none of the high dams has broken ground.