Manas River

The Manas River (Dzongkha: Manas Chhu; in its Bhutanese upper reaches the Drangme Chhu) is the largest river system of eastern Bhutan and one of the principal northern tributaries of the Brahmaputra. The river is formed in central-eastern Bhutan by the confluence of three major sub-basins — the Drangme Chhu, the Mangde Chhu and the Bumthang Chhu — and continues south through the foothills into the Indian state of Assam, where it joins the Brahmaputra near Jogighopa.^[1]

From source to mouth the Manas runs roughly 376 kilometres, of which about 272 kilometres lie within Bhutan. The combined length of all its tributaries inside Bhutan is estimated at around 3,200 kilometres, and the basin drains the bulk of the country east of the Black Mountains.^[1] The river gives its name to a contiguous transboundary protected landscape, made up of Royal Manas National Park in Bhutan and Manas National Park (a UNESCO World Heritage Site) in India.^[2]

The Manas system is central to eastern Bhutan's hydrology, biodiversity and long-term hydropower planning. It also defines a politically important stretch of the Bhutan–India border in the Sarpang–Pemagatshel sector and is the subject of formal bilateral cooperation on conservation and water resources.

Course and tributaries

The Drangme Chhu, the principal upper branch, rises in the eastern districts of Trashi Yangtse and Lhuentse, with a small headwater section in Tibet (Cona County) where it is known as the Kuru Chhu. It flows south through Mongar dzongkhag, where it is joined by the Kuri Chhu draining Lhuentse, and continues into Pemagatshel and Zhemgang. In the Zhemgang–Sarpang corridor it meets the Mangde Chhu, which drains Trongsa and the eastern flank of the Black Mountains, and the Bumthang Chhu (also called Chamkhar Chhu), which drains Bumthang. From this confluence onwards the river is referred to as the Manas.^[3]

The Manas crosses into Assam at Mathanguri, runs south past the town of Barpeta Road, and joins the Brahmaputra after a further 100 kilometres. In Assam the river is joined on its left bank by the Aie River, which itself drains a smaller corridor of southern Bhutan. Annual discharge is highly seasonal, with the bulk of the flow concentrated in the south-west monsoon period from June to September.^[1]

Royal Manas and transboundary conservation

Royal Manas National Park covers the Bhutanese portion of the river corridor in southern Zhemgang, Sarpang and Pemagatshel. Originally gazetted as a wildlife sanctuary in 1966 and re-designated as a national park in 1993, it is contiguous with India's Manas National Park, which was inscribed on the UNESCO World Heritage List in 1985.^[2]

In 2011 the two governments formalised the Transboundary Manas Conservation Area (TraMCA), a coordinated landscape covering Royal Manas, Manas National Park and adjoining reserve forests on both sides of the border. TraMCA addresses anti-poaching, wildlife corridors and habitat fragmentation, and is supported by IUCN and WWF.^[4] Tiger numbers across the joint landscape more than doubled between 2010 and 2018, according to surveys cited by IUCN.^[4]

Biodiversity

The Manas basin is one of the richest biological corridors in the eastern Himalayas. Royal Manas alone records more than 900 plant species, 365 bird species and over 60 mammals.^[2] Flagship species include the Bengal tiger, Asian elephant, greater one-horned rhinoceros (recolonised on the Bhutanese side after reintroduction efforts in Assam), gaur, clouded leopard, and the endangered golden langur (Trachypithecus geei), whose distribution is largely confined to the Manas basin between the Sankosh and the Manas itself.^[5]

Riverine and floodplain species include the Asiatic water buffalo, hog deer, smooth-coated otter and pygmy hog. The river itself supports the endangered golden mahseer, a target of catch-and-release sport fisheries managed under permit by Bhutan's Department of Forests and Park Services.

Hydropower potential

The Manas system holds the largest unrealised hydropower potential of any Bhutanese basin. The Mangde Chhu sub-basin already hosts the 720 MW Mangdechhu Hydroelectric Project, commissioned in 2019. The Kuri Chhu has hosted the 60 MW Kurichhu plant since 2002. A much larger scheme on the main Drangme Chhu — the proposed Kuri-Gongri (or Kuri-Gongri Reservoir) Project in the 1,800–2,560 MW range — has been identified in successive India–Bhutan hydropower cooperation agreements but has not progressed to construction, with funding and environmental review cited as the main obstacles.^[6]

The presence of Royal Manas National Park downstream constrains dam siting on the main stem. Environmental assessments for proposed schemes have repeatedly flagged risks to elephant and tiger movement, sediment regimes feeding the Assam floodplain, and the world heritage status of Manas National Park immediately south of the border.^[6]

Cultural significance

The Manas basin contains some of the oldest centres of Bhutanese settlement, including the Bumthang valleys associated with Guru Padmasambhava and the treasure-revealer Pema Lingpa, and the old eastern principalities of Khyeng (Zhemgang) and Kurtoe (Lhuentse). The river is referred to in Indian mythological literature as the daughter of Manasa, the serpent goddess, from which its modern name is generally derived. In contemporary Bhutan it functions as a border feature, a conservation icon and a planning unit for water and energy policy.

References

1. Manas River — Wikipedia
2. Manas Wildlife Sanctuary — UNESCO World Heritage Centre
3. River basin management context — Ministry of Energy and Natural Resources
4. Beyond borders: How Bhutan and India are saving the tiger — IUCN
5. Community protection of the Manas Biosphere Reserve and the Endangered golden langur — Oryx
6. Status of hydropower dams in Bhutan — International Rivers