River Systems of Bhutan

Bhutan's river systems are the defining feature of the kingdom's geography, economy, and settlement patterns. Rising from glaciers and snowfields along the Great Himalayan range at elevations above 7,000 metres, the rivers carve steep, densely forested gorges through the inner Himalayas before emerging onto the subtropical Duars plain at the Indian border. All of Bhutan's rivers are tributaries of the Brahmaputra (known as the Tsangpo in Tibet), and they collectively drain an area of approximately 38,394 square kilometres. The rivers provide water for agriculture, generate the hydroelectric power that is Bhutan's largest export commodity, and sustain some of the richest biodiversity in the eastern Himalayas. They also present significant natural hazards, including monsoon flooding and glacial lake outburst floods (GLOFs), which are increasing in frequency as climate change accelerates glacial retreat.^[1]

Bhutan's rivers are organised into four major drainage basins from west to east: the Amo Chhu (Torsa), the Wang Chhu, the Punatsang Chhu (Sunkosh), and the Manas system (also known as the Drangme Chhu). A fifth, smaller basin — the Nyera Ama Chhu in the far south-east — is sometimes identified separately. Each basin has distinct characteristics in terms of flow volume, gradient, hydropower potential, and the agricultural valleys it sustains. The rivers are fed by a combination of glacial meltwater, monsoon rainfall (which accounts for 60 to 90 per cent of annual precipitation between June and September), and groundwater seepage, producing highly seasonal flow patterns with peak discharge during the summer monsoon.^[2]

Major River Basins

Amo Chhu (Torsa)

The Amo Chhu, known as the Torsa in India, is the westernmost of Bhutan's major rivers. It originates in the Chumbi Valley of Tibet, enters Bhutan through the Haa valley, and flows south-westward through Samtse dzongkhag before crossing into West Bengal, India. The river drains approximately 3,300 square kilometres within Bhutan. The Amo Chhu basin is relatively less developed for hydropower compared to other basins, though a 540 MW hydroelectric project has been planned for decades as a bilateral India-Bhutan venture. The project has faced repeated delays due to geological challenges and environmental concerns. The Amo Chhu basin receives less monsoon precipitation than the eastern basins and has a smaller glacial component in its flow.^[3]

Wang Chhu

The Wang Chhu is Bhutan's most economically important river basin, draining the Thimphu, Paro, and Haa valleys — the political and cultural heartland of the kingdom. The basin is formed by the convergence of three major tributaries: the Thim Chhu (flowing through the capital, Thimphu), the Pa Chhu (flowing through Paro, past the iconic Rinpung Dzong), and the Haa Chhu. These tributaries merge to form the Wang Chhu, which flows approximately 370 kilometres southward through Chhukha dzongkhag before entering India, where it is known as the Raidak. The Wang Chhu basin hosts two of Bhutan's largest operational hydropower plants: the Chhukha Hydroelectric Plant (336 MW, commissioned in 1986) and the Tala Hydroelectric Power Station (1,020 MW, commissioned in 2007). The Tala plant, fed by a 22-kilometre headrace tunnel with a net drop of 860 metres, is the single largest power station in Bhutan and one of the largest underground powerhouses in the region.^[4]

Punatsang Chhu (Sunkosh)

The Punatsang Chhu, known in India as the Sunkosh, is formed by the confluence of the Pho Chhu (Father River) and Mo Chhu (Mother River) at Punakha, the former winter capital of Bhutan. This confluence — directly below the magnificent Punakha Dzong — is one of the most sacred and visually striking sites in the kingdom. The Mo Chhu rises from the glacial lakes of Lunana in northern Gasa district, while the Pho Chhu originates in the mountains north of Punakha. After their confluence, the combined river flows south through Wangdue Phodrang, Dagana, and Tsirang before entering India. The Punatsang Chhu basin has the greatest hydropower development in Bhutan, hosting the Punatsangchhu-I (1,200 MW) and Punatsangchhu-II (1,020 MW) projects. The completion of Punatsangchhu-II boosted Bhutan's total installed hydropower capacity by approximately 40 per cent.^[5]

Manas System (Drangme Chhu)

The Manas river system is the largest of Bhutan's four major basins, draining most of central and eastern Bhutan. Known within Bhutan as the Drangme Chhu, it has three major tributary branches: the Mangde Chhu (draining Trongsa and Zhemgang), the Chamkhar Chhu (draining Bumthang), and the Kuri Chhu (draining Lhuntse and flowing south through Mongar). The Drangme Chhu proper drains the far eastern dzongkhags of Trashigang, Trashiyangtse, and Pemagatshel. These tributaries converge in south-eastern Bhutan before the combined Manas flows into the Indian state of Assam, where it feeds the Manas National Park — a UNESCO World Heritage Site. The Manas basin hosts the Kurichhu Hydroelectric Plant (60 MW, on the Kuri Chhu) and the Mangdechhu Hydroelectric Plant (720 MW, on the Mangde Chhu in Trongsa), commissioned in 2019 as a landmark India-Bhutan cooperation project.^[6]

Seasonal Flow Patterns and Flood Risks

Bhutan's rivers exhibit extreme seasonal variability. During the summer monsoon (June to September), river discharges can increase by a factor of ten or more, as monsoon rains combine with accelerated glacial melt to produce torrential flows. Flash floods and landslides are common during this period, particularly in the narrow gorges of the inner Himalayas, where road infrastructure is regularly damaged. During the dry winter months (November to March), flows diminish substantially, sustained primarily by groundwater and residual glacial melt.

Glacial lake outburst floods (GLOFs) represent the most catastrophic flood risk in Bhutan. As Himalayan glaciers retreat due to rising temperatures, meltwater accumulates in unstable moraine-dammed lakes. A sudden breach of these natural dams releases enormous volumes of water downstream with devastating speed. Bhutan has experienced several significant GLOF events, including the 1994 Lugge Tsho outburst in the Punakha valley, which caused extensive damage and loss of life. The National Centre for Hydrology and Meteorology monitors over 2,600 glacial lakes, of which 25 have been classified as potentially dangerous. The Mo Chhu valley downstream from Lunana is considered particularly vulnerable.^[7]

Hydropower and the Economy

Hydropower is the cornerstone of Bhutan's economy, accounting for the largest share of government revenue and the majority of export earnings. Electricity exports to India, conducted under bilateral agreements, generate hundreds of millions of dollars annually. Bhutan's total installed hydropower capacity now exceeds 3,500 MW following the completion of the Punatsangchhu-II project, with an estimated total potential capacity of 30,000 MW — making Bhutan one of the most hydropower-rich countries per capita in the world. However, the seasonal nature of river flows means that generation capacity drops significantly during winter, creating a paradox in which Bhutan must import electricity from India during the dry season despite being a net exporter overall. Climate change adds further uncertainty, as altered precipitation patterns and glacial retreat may fundamentally change the hydrological regime upon which the entire hydropower sector depends.^[8]