Drangme Chhu

The Drangme Chhu (Dzongkha: brang me chu) is the largest river system of eastern Bhutan and, downstream of its confluence with the Mangdechhu in southern Zhemgang, the principal source stream of the Manas Chhu. It flows through the dzongkhags of Trashi Yangtse, Trashigang, Mongar, Pema Gatshel and Zhemgang, drawing its headwaters from a network of tributaries that drains effectively the whole of eastern Bhutan, before crossing the southern border at the edge of Royal Manas National Park into Indian Assam.^[1][2]

Across its course in Bhutan the river runs for approximately 272 kilometres, making it the longest river system entirely within Bhutanese territory. Its eastern Bhutanese watershed is the basis for major planned hydropower projects including the proposed Kuri-Gongri (2,640 MW reservoir scheme) and the partly built Kholongchhu (600 MW) project on a tributary; the area is also the most biodiverse part of Bhutan and includes Royal Manas National Park, Bhutan's oldest protected area.^[3][4]

This article covers the river's hydrology and tributaries, its hydropower programme, and its biodiversity and cultural significance.

Hydrology and tributaries

The Drangme Chhu is conventionally identified by the confluence of two main headwater streams in eastern Bhutan: the Kuri Chhu from the north and the Gongri Chhu from the east. The Kuri Chhu rises in southern Tibet and is the only major Bhutanese river whose source lies north of the Great Himalaya; it flows south through Lhuentse and Mongar before joining the Gongri at Gongri Zomsa, between Mongar and Pema Gatshel.^[1][5]

The Gongri Chhu itself is fed from the eastern dzongkhags by tributaries including the Kholong Chhu from Trashi Yangtse, the Gamri Chhu from Sakteng in eastern Trashigang, and a tributary often called the Drangme proper that descends from northeastern Trashigang. After the Kuri-Gongri confluence the combined river flows southwest through Zhemgang and joins the Mangdechhu at Tingtinbi, after which it is generally referred to as the Manas. The combined Drangme-Manas system has a total length of approximately 400 kilometres from headwater to its junction with the Brahmaputra at Jogighopa in Assam.^[1][2][6]

Hydropower programme

The Drangme system carries a large share of Bhutan's undeveloped hydropower potential. The Kuri-Gongri project, with a planned capacity of 2,640 MW, is the largest scheme in the national pipeline. Its detailed project report has been finalised and the scheme has been redesigned from a run-of-river to a reservoir-based dam, but as of 2026 no construction agreement has been signed and the project remains in the planning pipeline.^[3][7]

On the Kholongchhu tributary, the 600 MW Kholongchhu Hydroelectric Project was begun as a joint venture between Bhutan's Druk Green Power Corporation and India's Satluj Jal Vidyut Nigam (SJVN). After prolonged disputes between the partners, SJVN withdrew in early 2023 and the project was restructured for completion by Druk Green alone. Costs had escalated from the original Nu 33.05 billion budget to over Nu 54.82 billion, and commissioning has been repeatedly deferred.^[3][7] Smaller schemes on the Gamri and other tributaries have also been studied.

Biodiversity

The Drangme catchment includes the largest contiguous tract of subtropical and temperate broadleaf forest in Bhutan and is the wettest part of the country, with annual rainfall exceeding 5,000 mm in some southern reaches. Its lower course passes through Royal Manas National Park, established in 1966, which together with the contiguous Indian Manas Tiger Reserve forms one of the most significant transboundary biodiversity sites in the eastern Himalaya. The park hosts populations of the Bengal tiger, golden langur, Asian elephant, water buffalo, hispid hare and gangetic dolphin in its lower reaches.^[4][8]

The river itself supports populations of the golden mahseer, a large game fish that has become an icon of riverine conservation in Bhutan, and a number of endemic and migratory fish species. Hydropower development in the catchment has raised concerns about fish migration, sediment flow and downstream effects on Manas, and these issues have shaped the design of recent projects.

Cultural geography

The Drangme system has structured the cultural geography of eastern Bhutan. The Kuri Chhu valley is the traditional heartland of the Kurtoep people in Lhuentse, while the Gongri tributaries define the territories of the Tshangla-speaking communities of Trashigang and Mongar. The Pema Gatshel hill country between the Kuri and Gongri valleys was historically a buffer between Tibetan-influenced northern dzongkhags and the Indian-facing southern lowlands. The river network served, in earlier centuries, as the principal axis of communication and trade in a region where overland travel was slow and vertical.^[5][8]

The river also features in the regional folklore of eastern Bhutan, with localised river deities recognised at major confluences and crossings. The Manas crossing — where the Drangme as Manas leaves Bhutanese territory — is among the most ritually significant sites on the southern boundary.

References

1. Manas River — Wikipedia
2. List of rivers of Bhutan — Wikipedia
3. What Bhutan's failed hydropower goal means for energy geopolitics — Dialogue Earth
4. Wangchuck Centennial Park (and Bhutan protected areas) — Wild Nature Quest
5. Bhutan — River Systems — US Library of Congress Country Studies
6. Drangme Chhu — Whitewater Guidebook
7. Why Bhutan failed its hydropower goal — The Wire
8. Bhutan Rivers Watch — International Rivers Resource Hub