[4][5] The Manas River drains 41,350 square kilometres (15,970 sq mi) of eastern Bhutan and northeast India.

[6][7] A part of the main stem of the river rises in southern Tibet at an altitude of 4,500 metres (14,800 ft) before entering India at Bumla pass at the northwestern corner of Arunachal Pradesh.

It is the only river that rises north of the Great Himalayas and it joins the Manas at Gongri Zomsa between Monggar and Pema Gatshel districts.

The Aie River, which rises in the Black Mountains at an altitude of about 4,915 metres (16,125 ft) near the village of Bangpari, is about 110 kilometres (68 mi) in length.

[10] The lowlands north of the confluence with the Brahmaputra, known as the Terai or Duars (a Sanskrit word meaning "passes" or "gates"), encompass a 15–30 kilometres (9.3–18.6 mi) stretch of the outermost foothills and are very fertile.

[11] The Manas catchment is almost wholly mountainous, rising within the space of 140 kilometres (87 mi) from an elevation of about 100 metres (330 ft) near the Indian border to the great Himalayan peaks at over 7,500 metres (24,600 ft) along the main Himalayan range bordering Bhutan and Tibet.

The huge elevation range and varied climatic conditions are reflected in rich diversity of fauna and flora native to the area.

From May to October, the southwest monsoon brings heavy rainfall—more than 4,000 millimetres (160 in) in the southern part—and there is a pronounced dry season in winter.

[5] Out of the large catchment of the river valley, many protected areas or reserves have been specifically demarcated, both in Bhutan and India, which are declared national parks or sanctuaries.

[citation needed] The park features a large diversity of tropical grasslands, moist temperate forests, alpine meadows, and scrublands.

[12] Among the important faunal species living here are the royal Bengal tiger (Panthera tigris tigris), Asian elephant (Elephas maximus), gaur (Bos gaurus), four rare species of golden langur (Trachypithecus geei), pygmy hog (Sus salvanius), hispid hare (Caprolagus hispidus), greater one-horned rhinoceros (Rhinoceros unicornis), and wild Asiatic water buffalo (Bubalus arnee).

[15]The Manas Wildlife Sanctuary, located in Assam, is considered one of "Asia's finest wild life reserves" and is a UNESCO World Heritage Site.

The park is known for its rare and endangered endemic wildlife such as tigers and elephants, as well as the Assam roofed turtle, hispid hare, golden and capped langurs, pygmy hog, one-horned rhinoceros, Asiatic buffalo, swamp deer, barking deer, leopard, clouded leopard, marbled cat, sloth bear, hoolock gibbon, wild boar, and river dolphin.

These measures are meant to minimise human interference in the fragile core zone, to enable the creation of a database, and to carry out research on animal and plant populations for better conservation of the ecosystem.

Bodos have been demanding autonomy or a separate state of their own on grounds that their lands were incorporated into Assam during the British Raj.

Two dams were proposed on the Bhutanese side of the river to provide hydroelectric power and to control the flow of the Brahmaputra on its northern bank and to make way for irrigation schemes.

As the canal would have passed through the Manas Tiger Reserve, the Ministry of Environment and Forests of the Government of India objected to the proposal on the grounds of adverse impacts on the hydrology and ecology of the area that would occur due to the dam.

The late Prime Minister of India Rajiv Gandhi upheld the objections and decided to discontinue the project.