The river Brahmaputra flows down from the Eastern Himalayas. In certain stretches this river is 16 km wide. But if you look at the map, it is not only the Brahmaputra that flows through these plain tracts. There are 23 tributaries that flow down the Eastern Himalayas into the Brahmaputra. Some of these tributaries are on a par with the Ganga and Jamuna and present a landscape most unlike the foothill plain tract of the northern Himalayas. In the upper reaches of the Brahmaputra valley the average annual rainfall is 80 inches, making the land rich in wetland values. Over centuries the valley has been ecologically dynamic witnessing the succession of a host of bio-regions, the most prominent being the establishment of the wet alluvial grassland , a distinct eco system which is still well represented today in undisturbed places like the Kaziranga National Park. The south of the Brahmaputra valley is hemmed in by what is known as the Assam range of hills occupied from the east to west by the tribal communities of Nagas, Jaintias, Khasis and Garos. This range comprises the Karbi and Shillong plateau. The southern edge of the plateau near Cherrapunjee has some stupendous gorges overlooking the plains of Bangladesh.

It is in the Shillong plateau that British scholar-administrators (in 1870) found Old Stone Age antiquities and these specimens are preserved in Pitt-Rivers Museum, Oxford. Early man existed here some ten thousand years ago. Recent researchers have also discovered evidence of new stone age settlements of hunter gatherers in several sites in the Garo Hills of Meghalaya. A large number of Neolithic tools have been excavated here. In the Daphabum area of the eastern Himalayas, of what is now a part of the present Namdapha National Park (Arunachal Pradesh), old stone age antiquities have been discovered.

The isolated hills on the banks of the Brahmaputra near Guwahati, Goalpara and Tezpur are the northerly extensions of the Shillong plateau. And it is here that the ancient and early medieval settlements were established. The earliest epigraphic record is the 5th century rock inscription written in Sanskrit verse and can still be viewed in the Nilachal hills of Guwahati, where the Kamakhya temple is located.

The temple of Goddess Kamakhya on the Nilachal hill is today the most visited spot in the North East . Few of these visitors know what this temple stood for before it was rebuilt in 1565. The stone slab images strewn all over this hill and icons in the nooks of the main temple has helped recent research claim that this was an ancient hill fort and for a period a Tantric Buddhist shrine. In the rebuilt temple the Sakta cult has assimilated Saiva and Buddhist Tantric elements. Jain images are very prominent on the boundary walls.

In the beginning of the Christian era except for the plain tract areas around the isolated hills of Tezpur/Silghat, Guwahati and Goalpara, the rest of the valley was still marshy. The great river overflowed the country for several miles during rains. Late in the 7th century AD the Chinese traveler Hiuen Tsang visited Kamarupa (present Guwahati area) and found the country 'low and damp'. Even the Mahabharata referred to the people of this province as inhabitants of marshy land. The Eastern Himalayas (Bhutan and the present Arunachal Pradesh) which lie to the north of the Brahmaputra valley had several passes and routes to China. For centuries the ancient states in Guwahati and Tezpur had traditional links with the neighboring countries of Bhutan, Nepal and Tibet. These routes were used by waves of Tibeto-Burmans to enter this area. Most parts of the valley's terrain were marsh country. Over centuries an ecological process of alluvial flood deposit and vegetative succession, as also human interference, made such areas firm and dry. The entire Brahmaputra flood plain and valley highlands are ecologically dynamic and fragile even today.

The Himalayas continue east, then arches southeast and joins the Patkai hills. Today the Patkai Hills forms the natural boundary between Assam and Myanmar. In geological history the Himalayan upheaval is of recent origin. The Patkai-Barail range (part of present Nagaland) was formed in this upheaval. Technically it forms the northeast end of the Himalayas. These hills in the present Nagaland was affected by glaciating and today glaciated features can be noticed in the highland and valleys surrounding south western parts of Nagaland as also the Jafu Peak (3015 metres) near Kohima. It is in these hills that megalithic customs had a pervading influence and survives as a living form even today. Haimendorf (1946) recorded hundreds of megalithic monuments scattered over Angami country. The Khasis and Jaintias are renowned megalith builders. In the Shillong plateau concentration of dolmen and menhirs are located in six different sites. Today while driving up from Shillong to Cherrapunjee megaliths can be easily viewed from the road. Megalithic influence is prominent in the architectural forms of the 'Chessmen columns' at Dimapur, the medieval capital of the Dimasa-Kachari kingdom. Here the megaliths are ornamental and not bare stone slabs. In the whole world North-East India is one pocket where megalithic customs survive till today.

Of the Assam-Myanmar routes in the east, one that runs through Cachar-Manipur helped the migrations of the racial elements from South-East Asia. The Himalayas have the bhabar tracts on the foothills where vegetation cover is not luxuriant due to low water level. Today the Manas National Park and the Nameri National Park are the only representative areas of such bioregions. Only two decades ago this habitat type stretched uninterrupted right from the eastern most corner of the Eastern Himalayas (Pasighat) to North Bihar. These areas were also plain routes for migration in prehistoric and ancient times. But the earlier and more frequent route lay on the west of the valley. In medieval times the great Tai race came from the Shan state of Myanmar by the Patkai route and gave an impetus to the process of colonization of the marshy valley. Less than fifty years ago the Dalai Lama fled Tibet via the Chuthangmu pass on the Macmohan line in the Arunachal border and came down to the Assam valley.

Geologically the Shillong plateau is of much earlier origin and this plateau, which forms the central highland of the present state of Meghalaya, is an extension of Peninsular India. In bio-geographic terms the Brahmaputra valley and its hill ranges are an amalgamation of the Indo-Malayan, Indo-Tibetan and peninsular India. This bio-geographical amalgamation has produced a very high level of faunal and habitat diversity. Because of which the North East is an ideal destination for natural area visitors. The Namdapha National Park is representative of altitude diversity of the eastern Himalayas. The subdivision by deep valleys and the existence of rain forest is pronounced in this protected area. The high levels of habitat diversity especially of the bhabar tracts with its mixed forests and riparian grassland are still undisturbed in Nameri and Manas National Park. The terai biome in northwest Assam has been recently constricted. The same patterns of diversity are even more apparent in the fauna of this region, wherein over half of all of genera of Indian mammals are represented here and which is also particularly rich in birds and reptiles. But due to a variety of reasons sighting of animals today is very difficult except in Kaziranga National Park.