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