Sunkosh River

The Sunkosh River (also spelled Sankosh) is the downstream Indian name for the Punatsang Chhu, a major river of west-central Bhutan. The river system begins at the confluence of the Mo Chhu ("Female River") and Pho Chhu ("Male River") at Punakha Dzong, at an elevation of approximately 1,212 metres. From this point, the combined river flows southward through Wangdue Phodrang, Dagana, and Tsirang districts before crossing into India, where it traverses the Duars plains of West Bengal and Assam and joins the Brahmaputra system. The river's total course from Punakha to the Brahmaputra covers approximately 250 kilometres within Bhutan and continues for a further stretch through India.^[1]

Course

At Wangdue Phodrang (elevation 1,364 metres), the Punatsang Chhu is joined by the Tang Chhu before entering a steep gorge. The highway running south from Wangdue Phodrang toward Tsirang and Dagana follows the river for much of this stretch. The terrain is precipitous — the river drops rapidly through narrow valleys flanked by forested hillsides. In its lower reaches, the river widens as it approaches the Indian border, entering the flat alluvial plains of the Duars.

In India, the Sunkosh flows on the boundary between Assam and West Bengal before merging with the Brahmaputra. The river is sometimes also called the Raidak in its lower Indian reaches, though this name is more properly applied to a separate stream. On the Indian side, the Sunkosh is prone to annual monsoon flooding, particularly near Srirampur in Assam, where the river widens but communities along its banks face recurrent inundation.

Hydropower

The Punatsang Chhu/Sunkosh basin is the site of Bhutan's largest and most troubled hydropower undertakings. Two major projects straddle the river in Wangdue Phodrang District:

Punatsangchhu-I (1,200 MW): Construction began in November 2008 with an original completion target of 2015. In July 2013, a major right-bank slope failure halted dam construction. Satellite InSAR analysis (published in Nature Scientific Reports, 2020) revealed a pre-existing active landslide visible from 2007 — a geological problem that was underestimated at the planning stage. Stabilisation work has been ongoing since 2013. As of 2026, the project remains under construction, with most components (headrace tunnel, powerhouse, transmission lines) complete but the main dam still unfinished. The delays and cost overruns have contributed to Bhutan's hydropower debt burden.^[2]

Punatsangchhu-II (1,020 MW): Located downstream of PHPA-I, this project also faced geological challenges — unstable rock formations slowed excavation and flash floods disrupted construction. However, it reached completion ahead of its sister project: all six 170 MW units were synchronised between December 2024 and August 2025, with full commissioning on 27 August 2025. The joint inauguration by Indian Prime Minister Narendra Modi and King Jigme Khesar Namgyel Wangchuck took place in November 2025. PHPA-II raised Bhutan's national hydropower capacity by approximately 40 per cent.^[3]

A third proposed project, the Sunkosh (Sankosh) Reservoir scheme at approximately 2,585 MW, has been under discussion since the 1990s but has never progressed beyond feasibility studies. The estimated cost of US$2.8 billion and the environmental implications of a large reservoir in the geologically unstable Himalayan foothills have kept the project in planning limbo.

Debt and Economic Impact

As of 31 March 2024, hydropower-related debt accounted for Nu 167.5 billion — 64.1 per cent of Bhutan's total external debt and 62.8 per cent of GDP. The IMF characterises this debt as "FDI-like" because it is rupee-denominated and matched by rupee revenues from electricity exports to India, which provide a reliable revenue stream. Nevertheless, the prolonged delays on Punatsangchhu-I have drawn criticism for their contribution to fiscal pressure, with total public debt reaching 105.9 per cent of GDP by September 2024.^[4]

Ecology and Flooding

The upper Punatsang Chhu basin lies within the buffer zone of Jigme Dorji National Park and supports temperate broadleaf and conifer forest ecosystems. The river is susceptible to glacial lake outburst floods (GLOFs) — the 1994 Luggye Tsho GLOF in the Lunana region sent a surge down the Pho Chhu and into the Punatsang Chhu corridor, killing at least 21 people and damaging infrastructure including sections of Punakha Dzong. Climate change has doubled the number of glacial lakes in Bhutan since the 1990s, from 213 in 1990 to 436 in 2017, increasing future GLOF risk along the Punatsang Chhu/Sunkosh system.^[5]

References

1. Sankosh River — Wikipedia
2. Punatsangchhu-I resumes main dam construction — BBS
3. Overview of the Project — PHPA-II
4. What Bhutan’s failed hydropower goal means for energy geopolitics — Dialogue Earth
5. Unstoppable Rivers: Bhutan’s quest for energy security — Mongabay India