Bhutan's geography: three steps from the Duars to the Himalayan crest

How the land falls in three steps

Britannica divides Bhutan into three physical regions from north to south: the Great Himalayas, the Lesser Himalayas and the Duars Plain. The divisions are not administrative, but they explain the distribution of almost everything else in this article, from rainfall to rice to where the roads run.

The northern step is the main range. Its peaks pass 7,000 metres, with the highest at Gangkhar Puensum, 7,570 metres, and high valleys run down from the glaciers between about 3,700 and 5,500 metres. Alpine pastures there are used for grazing yaks in summer. The climate is dry, and the high passes carry wind all year.

The middle step is a set of ridges that run southward off the main range like the teeth of a comb. These north to south ranges form the watersheds between the rivers, and between them lie the broad, flat, cultivated valleys of Paro, Punakha, Thimphu and Ha. The Black Mountains in the centre separate the western valleys from the central ones, and the Donga Range separates the central valleys from the east.

The southern step is narrow. Britannica gives the Duars Plain as a strip 12 to 16 kilometres wide along the border; Wikipedia's account of Bhutan gives 10 to 15 kilometres, and notes that most of the Duars lies in India. The name means "doors", after the passes into the hills that the strip controlled, and those doors are why Bhutan fought Britain for it in the 19th century.

The rainfall figures come from two sources and are kept apart in the table: Britannica gives the Duars range and the valley range, and Wikipedia's geography article gives the 40 millimetres of the north and the 7,800 recorded in the south.

Where Bhutan's borders run

Bhutan is landlocked between two neighbours. Wikipedia's geography article gives the border as about 477 kilometres with the Tibet Autonomous Region of China and about 659 kilometres with India, for a total of about 1,139 kilometres. On the Indian side the neighbours, in order, are Arunachal Pradesh, Assam, West Bengal and Sikkim. The country lies between latitudes 26° and 29° north.

The southern line follows the edge of the Duars and is agreed. The northern line is not fully settled. Britannica describes the boundary with Tibet as undefined and generally following the crest of the Great Himalayas, and Wikipedia's article on Bhutan records that about 269 square kilometres remain under discussion with China. In 2017 Indian and Chinese troops faced each other for two months on a small plateau, and in 2021 China and Bhutan signed a memorandum to speed up border talks then more than 35 years old. This catalogue records that the boundary is disputed and takes no position on where it runs.

The present southern border is itself the result of loss. Bhutan once held territory south into present-day Assam and controlled Cooch Behar, and from 1772 the East India Company pushed the line back by war and treaty until the Treaty of Sinchula of 1865.

Why every river runs south to the Brahmaputra

Bhutan has four main river systems, and all four do the same thing: they rise among the snows, cut south through the middle step in deep gorges, cross the Duars and join the Brahmaputra in India, which carries their water on through Bangladesh to the Bay of Bengal.

• The Drangme Chhu, the largest, drains most of eastern Bhutan through three branches, the Drangme, the Mangde and the Bumthang Chhu, and is called the Manas where eight tributaries join it in the Duars. Where it crosses into India, at 98 metres, is Bhutan's lowest point.
• The Puna Tsang Chhu, or Sankosh, 320 kilometres long, forms at Punakha where the Mo Chhu and the Pho Chhu meet.
• The Wang Chhu, 370 kilometres long, has tributaries rising in Tibet, drains the Ha, Paro and Thimphu valleys and enters West Bengal as the Raigye Chhu.
• The Amo Chhu, called the Torsa further south, flows out of Tibet through the Chumbi Valley and widens near Phuntsholing.

Because the ridges run north to south, the rivers do too, and travel from east to west means crossing one ridge after another. The valleys are linked by passes, called la in Dzongkha. The Chele La between the Ha and Paro valleys, at 3,780 metres, is the highest pass any Bhutanese highway crosses. The road from Thimphu to Punakha crosses the Dochu La at 3,116 metres, and the Pele La east of Wangdue Phodrang stands at 3,390 metres. The main east to west highway, the Lateral Road, runs from Phuentsholing to Trashigang over a chain of these passes.

The same rivers are the source of the country's electricity, which the economy article follows from Chhukha to the plants under construction now.

How the monsoon divides the year

Elevation sets the temperature and the monsoon sets the calendar. Wikipedia's geography article describes five seasons. Spring is dry, from early March to mid-April. Summer brings occasional showers until the pre-monsoon rains of late June. The monsoon runs from late June to late September with heavy rain from the southwest; the Himalayas block it from moving north, so it brings flash floods and landslides to the slopes that face it. Autumn, from late September or early October to late November, is bright and dry. Winter, from late November to March, brings frost to much of the country and snow above 3,000 metres.

Western Bhutan receives between 60% and 90% of its rainfall from the monsoon. Southern Bhutan has hot humid summers and cool winters, while the centre and east are temperate and drier than the west.

Thimphu shows what the middle step is like. Britannica gives typical January highs of about 12 °C and lows of about 2 °C, and July highs of about 19 °C with lows of about 13 °C. Wikipedia's figures for the capital run from about 15 to 26 °C in the monsoon months and from about −4 to 16 °C in January. The city receives almost no rain from December to February and close to 650 millimetres over a year, with the wettest month August at about 220. In the south the range is narrower, about 15 to 30 °C through the year, with valley temperatures sometimes reaching 40 °C in summer.

What the glaciers and glacial lakes mean downstream

The glaciers of the north feed the rivers through the dry months, and the same ice is the country's main natural hazard. Where a glacier dams a lake and the dam fails, the result is a glacial lake outburst flood that runs down the valleys where people live.

The measurements disagree in a way worth recording. Wikipedia's geography article states that glaciers covered about 10% of the country's surface in the 1980s, while the Bhutan Glacial Inventory 2018 counted 700 glaciers covering 629.55 square kilometres, 1.64% of the land. The two figures may not measure the same thing, and the sources do not reconcile them. A United Nations report of 2008, as the same article cites it, found Bhutan's glaciers retreating at 30 to 40 metres a year.

The country holds at least 59 natural mountain lakes and about 2,674 glacial lakes, of which about 25 are counted as potential outburst hazards. The non-glacial lakes together cover about 4,250 hectares, mostly above 3,500 metres and mostly used, if at all, for grazing yaks. Japan has helped develop an early warning system for glacial floods.

Why most Bhutanese live in the middle valleys

The top and bottom steps repel settlement for opposite reasons. Britannica names the thinnest-peopled parts of the country as the cold and rugged Great Himalaya and the malarial tracts along the Duars, which confine most people to the farmed valleys of central and western Bhutan and to the southwest near the Indian border.

Settlement is scattered. Until the late 1960s, in Britannica's account, Bhutan had no urban settlements at all. Larger villages then grew into towns along the new roads, and by the mid-2010s the places the government counts as urban centres held nearly two-fifths of the population. In almost every populated valley the focal building is a dzong, a fortress-monastery on a rise above the river with thick white walls sloping inward, which houses the district administration and a monastic community together.

Thimphu stands on the Wang Chhu, which Britannica calls the Raidak, at about 2,000 metres in Britannica's figure and 2,200 metres in Wikipedia's. Britannica dates its designation as the seat of government to 1962, when the seat had previously moved with the king; Wikipedia's history of Bhutan dates its adoption as the year-round capital to 1966. Britannica recorded its population at 79,185 in 2005. The southern gateway is Phuntsholing in the Duars, the end of the highway from Thimphu, and Paro holds the only international airport.

Administratively the country is divided into 20 dzongkhags, or districts, and 205 gewogs, groups of villages. The World Bank's figures show how much the steps still matter: the poverty rate ran from 1.5% in Thimphu to 41.4% in Zhemgang, against 12.4% for the country as a whole in 2022.

How much of Bhutan is kept as forest

Forest is the land's default state. The Ministry of Agriculture reported forest cover of 64% in October 2005; the figures Wikipedia's geography article cites put it at about 71% in 2020, or 2,725,080 hectares, up from 2,506,720 hectares in 1990. Only about 15% of the naturally regenerating forest counted as primary forest, and in 2015 all forest was in public ownership. Arable land is about 2% of the country, most of it in the south.

The forest changes with altitude. Britannica records pine with some oak between about 900 and 1,800 metres; mixed forests of pine, oak, walnut, rhododendron and magnolia above that; the most valuable stands, of cypress, fir, spruce and juniper, between 1,800 and 2,700 metres; and birch up to the timberline at about 4,200 metres.

Conservation is written into policy. The government has committed to keep at least 60% of the land under forest, has designated more than 40% of the territory as national parks, reserves and other protected areas, and has identified a further 9% as corridors linking them. Royal Manas National Park, created in 1966 along the Manas River on the Indian border, shelters the golden langur; Jigme Dorji National Park, created in 1974 in the northwest, spans all three of the country's climate zones. Both have been on Bhutan's UNESCO tentative list since 2012, with Bumdeling and Sakteng wildlife sanctuaries.

More than 770 species of bird and more than 5,400 species of plant have been recorded. The snow leopard, blue sheep and the takin, the national animal, live in the north; the red panda and Himalayan black bear in the temperate forests; the Bengal tiger, clouded leopard and Asian elephant in the south. The country produces about 2.2 million tonnes of carbon dioxide a year, which its forests more than absorb, and the World Bank describes Bhutan as one of very few carbon-negative countries. The cost falls on farmers, whose crops and livestock are lost to protected wildlife, and the government has responded with an insurance scheme and solar-powered alarm fences.