Mo Chhu and Pho Chhu Rivers

The Pho Chhu (Male River) and Mo Chhu (Female River) are paired rivers in western-central Bhutan that converge at Punakha Dzong to form the Punatsangchhu, known downstream in India as the Sankosh.^[1][2] The pairing is gendered: in Dzongkha pho means male and mo means female, and the rivers are conventionally distinguished by current and temperament — the Pho Chhu fast-flowing and turbid with glacial sediment, the Mo Chhu calmer and clearer.

The two rivers drain the bulk of Punakha, Gasa and parts of Wangdue Phodrang, and together they form the principal upper basin of one of the country's larger river systems. Their confluence at Punakha — at the foot of the dzong, on a triangular promontory traditionally described as having been chosen by Zhabdrung Ngawang Namgyal — is the centre of one of Bhutan's most politically and religiously charged landscapes.

The combined Pho Chhu–Mo Chhu basin carries Bhutan's most heavily documented glacial lake outburst flood (GLOF) risk and supports two of the country's largest hydropower projects: Punatsangchhu I and Punatsangchhu II.

Pho Chhu

The Pho Chhu rises in the glaciers of Lunana in northern Gasa dzongkhag, on the border with Tibet. Its main headwaters drain a cluster of supraglacial and moraine-dammed lakes including Lugge Tsho, Thorthormi Tsho, Raphstreng Tsho and Bechung Tsho.^[1] The river flows roughly south-east through Lunana, then south through Gasa and into Punakha valley, picking up tributaries from the Tang Chhu and the Khoma Chhu.

Because of its glacial source, the Pho Chhu carries a high seasonal discharge and a heavy sediment load. It is also the most GLOF-prone river in Bhutan: the Lugge Tsho GLOF of 7 October 1994 released approximately 18 million cubic metres of water down the Pho Chhu, killed an estimated 21 people, damaged Punakha Dzong and destroyed about 90 houses downstream.^[3] The Thorthormi Tsho lake-lowering project of 2008–2011, undertaken by the Royal Government with UNDP and GEF support, drained around 17 million cubic metres of water from another upstream lake on the same drainage in a precautionary mitigation operation.^[4]

Mo Chhu

The Mo Chhu rises in the high pastures of Lingshi and Laya in Gasa dzongkhag, with smaller headwater contributions from across the Tibetan border. It flows south through the Laya–Gasa corridor, past Gasa Dzong and the Gasa hot springs, and continues through Punakha valley to its confluence with the Pho Chhu just below Punakha Dzong.^[2]

Compared to the Pho Chhu, the Mo Chhu has a more snow-and-rain-fed regime with less glacial influence, lower sediment, and a calmer flow profile through its lower reaches. It is the river typically associated with the calmer "female" character in popular and religious accounts of the river pairing. The Mo Chhu basin supports the white-bellied heron (Ardea insignis), one of the world's rarest birds, with the species' Bhutanese stronghold concentrated along the river's middle reaches between Tashithang and Damji.

The Mo Chhu also carries documented GLOF risk: the smaller Lemthang Tsho GLOF of 28 July 2015 on a tributary in the upper Mo Chhu released approximately 0.37 million cubic metres of water, killing four horses and destroying trekking infrastructure on the Laya route, although there were no human deaths.^[5]

The confluence at Punakha

The two rivers meet directly below Punakha Dzong on a triangular spit of land that, by tradition, Zhabdrung Ngawang Namgyal identified as fulfilling a vision of an "elephant lying on its back" before establishing the dzong in 1637–38. The confluence has anchored Punakha as a religious capital ever since: every Druk Gyalpo from Ugyen Wangchuck in 1907 to Jigme Khesar Namgyel Wangchuck in 2008 has been crowned at Punakha Dzong, with the rivers framing the ceremony.^[6]

The confluence is crossed by a traditional cantilever footbridge linking the dzong to the eastern bank, and is one of the most photographed riverine sites in Bhutan. The annual Punakha Drubchen and Tshechu, religious festivals that re-enact a 1639 victory over a Tibetan invasion, conclude with a procession to the riverbank.

Hydropower

The Punatsangchhu, formed by the confluence of the two rivers, hosts two large hydropower projects developed under the Bhutan–India bilateral framework:

• Punatsangchhu I (1,200 MW) — under construction at Wangdue Phodrang since 2008, with repeated delays caused by geotechnical instability at the dam site.
• Punatsangchhu II (1,020 MW) — fully commissioned in August 2025.

Both projects route flows from the combined Pho Chhu–Mo Chhu system. Their planning has had to contend with elevated GLOF risk on the Pho Chhu and with sediment loading from the basin's heavily glaciated upper reaches.

Recreation and biodiversity

The lower Mo Chhu and Pho Chhu sections downstream of Punakha support commercial whitewater rafting and kayaking operations, with the lower Mo Chhu typically rated easier (Class II–III) than the more demanding lower Pho Chhu (Class III–IV).^[7] The combined river corridor falls within the buffer zone of Jigme Dorji National Park in its upper reaches and supports populations of common merganser, ibisbill and the white-bellied heron, alongside golden mahseer and snow trout fisheries.

References

1. Pho Chhu — Wikipedia
2. Mo Chhu — Wikipedia
3. The 1994 Lugge Tsho Glacial Lake Outburst Flood, Bhutan Himalaya — ICIMOD
4. Thorthormi Glacier Lake — NASA Earth Observatory
5. Cause and Impact: The 2015 Lemthang Tsho GLOF in Bhutan — ICIMOD
6. Punakha Dzong — Wikipedia
7. Lower Pho Chhu Rafting and Kayaking — Whitewater Guidebook