The Himalayan Kingdom of Bhutan extends about 305 km from east to west and 145 km from north to south, covering an area of 47 000 km2.
Physiographically the Bhutan Himalaya mountains are divisible into three zones from south to north: the sub-Himalayas or Himalayan foothills (up to 1 500 m in altitude); the inner Himalayas or middle ranges with flat valleys (from 1 500 to 4 500 m), and the Great Himalayas of high mountains (about 4 500 m). The Black Mountain Range running north and south divides the country into western and eastern Bhutan.
The narrow foothills rise gently from the Bengal-Assam plains of India to about 610 m and then in steep escarpments up to more than 1 525 m. The Duar plains a northern extension of the Bengal-Assam plains of India extend from 12 to 16 km into Bhutan. Many streams originate in the foothills and flow for a short distance before disappearing underground in the porous gravelly and pebbly region bordering the foothills, The rivers change their courses during floods.
The Bhutan Duar may be divided roughly into northern and southern portions. The northern portion contains deposits of coarse gravel, boulders, pebbles, shingles and sand produced by erosion of the hills. Here the subsoil water is low and the surface dry because of the porosity of the formation. The topography of the southern portion of the Duar is such that the rivers flow more slowly through generally well-defined channels and have less erosive power. This section of the Duar consists of moderately fertile soil. A series of springs mark the junction between the northern and southern sections. These springs occur where the substratum of clay in the south forces the underground water absorbed in the porous gravel formation to the surface again.
These consist of higher mountains which radiate southward from the Great Himalayan range, forming watersheds between the principal rivers. These ranges run from northwest to southeast in western Bhutan and northwest to southwest in eastern Bhutan. The mountains are rugged, with precipitous slopes, level land being limited to eight major but narrow valleys, the Ha, Paro, Thimbu, Punakha, Wangdi Phodrang, Tongsa, Bumthang and Tashigong. The inner Himalayan Ranges have no glaciers at present but some summits and upper slopes are covered with moraines.
The Great Himalayas, with their chain of high peaks lie along the northern frontier. At the westernmost end of the range is the towering snowclad peak of Chomo Lhari (7 272 m). Another group of ice-crowned giant peaks lies farther eastward, of which Kula Kangri (7 497 m) is the principal summit. Glaciers and moraines are a common feature of these peaks.
The rivers (excepting the Manas and Lhobhrak) flow from the side of the Great Himalayas through the narrow defiles at the foot of the mountains, emerging into the Duar and eventually draining into the Brahmaputra River. In southern Bhutan the river system exhibits a dendritic pattern with tree-like branches of smaller streams meeting to form larger rivers. Figure 1 shows the river system of Bhutan.
All the rivers of Bhutan show marked characteristics of mountain streams. They flow between high rocky mountains confining the channel in a narrow valley. As the gradient of the river falls markedly, the streams rush tumultuously over beds of huge boulders and rock masses. None of these streams are navigable in the mountains or even in the plains. There are no flood problems in the mountain regions but serious threats of floods exist in the plains, although the alluvium makes the soil there fertile.
The total length of rivers, with their tributaries, in Bhutan is about 7 200 km. The principal rivers are, from west to east: Amo Chu or Torsa, Wong Chu or Raidak, Mo Chu or Sankosh and Manas.
When they emerge into the Duar, the Manas and Sankosh Rivers carry most of the water. The Raidak carries nearly one-third of the amount carried by the Sankosh, and the Torsa about one-sixth.
The Sankosh drains the entire Great Himalayan range between Chomo Lhari and Kula Kangri. The various upper tributaries of the Sankosh unite near Punakha (altitude 1 515 m). Below Punakha, the river bed is quite wide but below Wangi Phodrang (altitude 1 364 m) the river flows through a precipitous gorge.
The Amo Chu, Wong Chu and Mo Chu drain western Bhutan and the Manas and its tributaries the east.
The Amo Chu, or Torsa, is one of the principal rivers in western Bhutan. It rises in Tibet and, draining the Chumbi Valley, enters Bhutan. The river flows rapidly and follows a confined valley between steep mountains. Even in winter the Torsa is a swift stream, with an average depth of at least one metre. As it leaves the foothills and enters the Duar plain, it widens into a braided channel. The minimum annual flow of the river is 415 ft3/second. The Torsa and its tributaries measure a total of 310 km.
The Wong Chu, or Raidak, rises in the Great Himalayan region. Along with its tributaries, it covers a total length of nearly 610 km in Bhutan. The main river is a rapid stream, running over a bed of large boulders. Between Thimbu and the confluence with the Paro Chu, the course of the river is not severely confined but, after leaving the confluence, it runs through a narrow defile between very steep cliffs. The Paro rises in the snow-capped mountains to the south of Chomo Lhari. It flows southeast through a comparatively open valley, its course strewn with large boulders against which the water foams violently. It is joined by several small tributaries flowing from nearby mountains. Just above Paro Dzong a considerable feeder, the Ta Chu, joins it from the left. To the west, the Ha Chu drains into the Wong Chu. At Tashichho Dzong the bed of the river is about 2 121 m above sea level and at the point of its exit in the Duar its elevation is only 90 m.
The Wong Chu has substantial power potential and a major hydro-electrical project of 252 MW is presently under construction at Chukha, midway between Thimbu and the India border.
The Mo Chu, or Sankosh River, also known as Punakha in its upper reaches, rises in the Great Himalayan region. The main river drains a basin of 9 900 km2 and has a minimum annual flow of 1 900 ft3/second. At Punakha, it is joined by the Pho Chu and 20 km further downstream at Wangdi Phodrang, by the Tang Chu. Several mountain streams join the river on either side. At its exit into the Duar plain, it is a deep river flowing mainly over a bed of boulders. The Sankosh and its tributaries constitute the second biggest river system in Bhutan, with a total length of 1 810 km, including all its tributaries.
The Manas and all its main branches, including all its tributaries, constitutes the largest river system in Bhutan, with a total length of 3 200 km. The main Manas, or Gong River drains about 18 300 km2 in eastern Bhutan, rising beyond the Great Himalayan range. It enters Bhutan from the Kameng frontier district of India and runs southwest, unlike most of the Bhutan rivers which usually run from northwest to southwest.
Just before reaching Tashigong, the Manas is joined by the Kulong Chu, a large stream from the snowy range along the northern Bhutan frontier. At Tashigong the Manas is about 50 m wide, and its waters flow rapidly over a bed of boulders. The river bed near Tashigong is about 606 m above sea level; it is only about 121 m above sea level where it joins the Tongsa Chu. The Lhobrak, or Kuru Chu, the main central tributary of the Manas, is the only one that rises north of the Great Himalayas and it joins the Manas in southern Bhutan; further downstream the combined stream joins the Tongsa Chu.
The Tongsa (Mangde) Chu, rises in northern Bhutan near Kula Kangri Peak. At Tongsa Dzong, the bed of the river is about 1 666 m above sea level and its flow is very swift. The Bumthang River, also called Murchangphy Chu, joins the Tongsa in southern Bhutan and the combined stream flows into the Manas.
The difference between maximum and minimum river flow in monsoon and the dry months is said to be as much as 20 times.
The maximum and minimum flood levels of the main three rivers during 1975–1976 as per records of the Wireless Department are the following:
| Raidak River at Chukha | Sankosh River
at Damphu | Tongsa River at Sambezong | Manas River
at Tashigong | |
| Maximum | 1 239 m | 156.57 m | 1 226.51 m | 602 m |
| Month | July | July | July | September |
| Minimum | 1 227.57 m | 155.77 m | 1 225.60 m | 599.84 m |
| Month | Nov./Dec. | Nov./Dec. | September | January |
The rain-fed or non-perennial rivers (about 32 in number with a total length of 1 270 km) originate in the lesser Himalayas and are characterized by wide river beds with low flow depths. During the rainy season most of them flow torrentially and meander widely.
Bhutan has more than 59 natural lakes covering a total surface area of about 4 250 ha. Most of them located above an altitude of 3 500 m.
Many of the lakes are above the snowline with no human settlements around. However, when the adviser paid a visit to five natural high altitude lakes (3 500 m approx.) at Dagala in the Thimbu district, he observed that Drukpa tribesmen move to these places with their yaks for grazing purposes and stay there for 3–4 months. At the time of the visit, eight families were staying in the area in temporary huts made of wooden planks, and each one had 30 to 80 yaks. It was also observed that the yak dung is not used by the people and was found lying scattered all around the area as well as close to the lakes. This yak dung could be considered as fertilizer for the lakes.
The details of the four lakes existing below 2 000 m altitude are given below:
| Lake | District | Surface Area (ha) | Altitude (m) |
| Ho Ko Tso | Punakha | 60 | 1 829 |
| Luchika | Wangdi Phodrang | 2.5 | 1 830 |
| Buli | Shamgong | 2 | 1 372 |
| Gulandi | Diapham | 1 | 366 |
A number of natural springs also exist in the country, particularly in the southern zone. During his stay, the adviser found a good number of them in the Survey, Kanglung, Chirang, and Sarbhang areas.
Artificial ponds/or reservoirs are unknown in the country. However, one small reservoir having a maximum surface area of 15.4 ha and a minimum surface area of 3.4 ha is to be created on the Wong Chu for the Chukha Hydel power project.
Bhutan has great diversity of climate, perhaps greater than any other area of similar size in the world. While the climate is temperate in the central mountain valley (1 535 to 2 155), the rest of the country is either extremely cold, as in the north, or hot, as in the plains in the south. The three climatic zones are:
the tundra region of the Great Himalayan range above 4 585 m, which is snow-covered throughout;
the cooler/or temperate region of the inner Himalayas having a microthermal climate, which is quite difficult to classify. Winters range from moderately cool to severe and summers are warm and rainy.
the hot and humid subtropical area of the foothills (the plains).
The data on atmospheric temperatures during different months taken at four different altitudes are shown in Table 1.
The southwest monsoon breaks around mid-June (sometimes in early April) and continuous up to late September, accounting for 80 to 90 percent of the annual rainfall in Bhutan (Table 2). The northern half of Bhutan gets scanty rainfall but on the southern slopes of the foothills it amounts to as much as 4 500 mm/year. The plains in general have an average annual rainfall of more than 2 000 mm. In the inner Himalayas, the rainfall varies with the exposure to the monsoon winds, the Thimbu and Paro Valleys recording an average annual rainfall of about 700 mm.
