Brahamaputra Basin

Brahamaputra Basin
1.2 Topography
The Brahmaputra, a unique river drains such diverse environment as the cold dry plateau of Tibet,
the rain-drenched Himalayan slopes, and the landlocked alluvial plains of Assam and the vast deltaic
low-lands of Bangladesh.
The upper portion of the basin (Indian part) lying in Arunachal Pradesh, Nagaland and the some
lower portion of the basin falling in Sikkim is mostly mountain ranges and narrows valleys. Most
portion of the basin lying in Assam and Meghalaya consists of hills, forests and the wide
Brahmaputra valley which is about 80 km wide on an average. In West Bengal, also the basin covers
of hills and forests besides the plains in the districts of Koch Behar and Jalpaiguri.
The basin part falling in India consists of 5 physiographic parts (NATMO, 1981). Topography of the
each region are described below-
i) Assam Valley: The plain Assam/ Brahmaputra valley is long and narrow. It is 640 km long and the
width varies from 64 km to 90 km.The valley is bounded in the north by high Himalayan
mountain ranges, in the east by the Patkai hill ranges, in the south by the lower (Assam) hill
ranges of Garo, Khasi and Naga hills and in the west, it is continuous with the plains of
Bangladesh. Extending from the eastern most tip of Upper Assam near the syntaxial bend of
Eastern Himalaya to the west of Dhubri on the border of Bangladesh, the valley (about 720 km x
80 km) covers an area of about 56,274 km 2 .
The Assam/Brahmaputra valley built mostly by the aggradational work of the Brahmaputra and
its tributaries is almost a flat level plain with very little slope from its north-east corner at Sadiya
to Dhubri in the west.The general level of the valley ranges from 130m in the east to 30 m in the
west with a fall of about 12 cm per km.Physically the valley is demarcated from its surrounding
ranges by the 150 m contour.The valley in its northern margin is characterised by the steep
slope, having an almost immediate fall from the foothills of the Himalaya. But the southern
margin has a gradual fall from the soutrhern Hill ranges. Interesting geomorphological feature of
the valley is the presence of a good number of isolated hillocks on both the banks of the river,
right from Tezpur and Mikir hills to as far west as Dhubri, detached from the Meghalaya plateau
by the degradational work of the river. Assam valley covers the maximum geographical area of
the Brahmaputra Basin within the Indian teritory.
ii) North Bengal Plains: Though the northern and western fringes lying in Jalpaiguri and Darjeeling
encompass areas slightly above 150 m, Siliguri, Koch Behar and Jalpaiguri area of North Bengal
plains are primarily flat terrain.
iii) Eastern Himalaya (East): The Eastern Himalayan mountainous part covers the Arunachal
Pradesh, and Sikkim and Darjeeling district (excluding Siliguri sub-division) of West Bengal.This
region shows the highest elevation range of more than 6000m with snow capped ranges.
iv) Meghalaya-Mikir Region: This is geologically a part of the Deccan Plateau covering Meghalaya
and Karbi Anglong of Assam. The Garo Hills has an average elevation of 600 m or less. The Khasi
and Jaintia hills show elevation ranging upto 2000 m gradually slope down towards the
Brahmaputra valley and forms the sub-mountane region of the central Meghalaya. The Mikir
Hills is an isolated hill surrounded by plains on three sides.The Mikir Hills area has roughly a
northerly slope with the outer ranges having an average elevation of about 450 m. In the central
portion the average elevation is about 1000 m. The northern ranges which extend from Dabaka
(Nagaon Dist.; Assam) in the south-west to Bokakhat (Sibsagar Dist.) in the northeast have an
average elevation of 600 m. The southern ranges known as the Rengma Hills are higher with an
average elevation of about 900 m and extend from
v) Purvanchal: Brahmaputra basin part of this unit comprising of parts of Naga Hills (parts of
Nagaland), Tirap, Changlang, Anjaw and part of Lohit (Arunach Pradesh). The elevation increases
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