The Brahmaputra Valley has an average width of about 80 Km. The main river of the valley, Brahmaputra is one of the largest rivers in the world and rank fifth with respect to its average discharge. The river originates from the Kailash ranges of Himalayas at an elevation of 5300 M. After flowing through Tibet it enters India through Arunachal Pradesh and flows through Assam and Bangladesh before it joins Bay of Bengal.
The catchments area of Brahmaputra in Tibet is 2, 93,000 Sq. Km; in India and Bhutan is 2,40,000 Sq. Km and in Bangladesh is 47,000 Sq. Km. The Brahmaputra basin extends over an area of 5,80,000 Sq. Km up to its confluence within Bangladesh.
The average width of Brahmaputra is 5.46 Km. The maximum discharge of Brahmaputra at Pandu near Guwahati was recorded as 72,779 cumec on 23.08.62 and minimum discharge was recorded as 1757 cumec on 22.02.63. The average annual discharge is about 20,000 cumec and average dry season discharge is 4,420 cumec.
The river slope is very steep till it enters India. A drop of about 4800 M is achieved in a length at about 1700 Km in China (Tibet). This average slope of about 2.82 m/Km gets reduced to about 0.1m/Km in Assam valley. Due to this sudden flattening of river slope, the river becomes braided in nature in the Assam valley. During its course in Assam valley from Kobo to Dhubri the river is joined by about 20 (twenty) important tributaries on its North bank and 13 (thirteen) on its South bank. Joining of these tributaries bringing high sediment load activates braiding.
Brahmaputra sub-basin extends over an area of 580,000 sq.km lying in Tibet (China), Bhutan, India and Bangladesh. The drainage area lying in India is 194413 sq.km which is nearly 5.9% of the total geographical area of the country. It is bounded on the north by the Himalayas, on the east by the Patkari range of hills running along the Assam-Burma border, on the south by the Assam range of hills and on the west by the Himalayas and the ridge .separating it from Ganga sub-basin. The sub-basin lies in the States of Arunachal Pradesh, Assam, Nagaland, Meghalaya, West Bengal and Sikkim. The State-wise distribution of drainage area is given below:
|State||Drainage area (sq.km)|
The most predominant soil type found in the sub-basin is the red loamy soil and alluvial soil. Other important soil types are sandy, loamy, clayey soils, their combinations and laterite soils. The culturable area of the sub-basin is about 12.15 M. ha which is 6.2% of the culturable area of the country.
All the tributaries of the valley area are rain fed and foam up with rain. The precipitation here is mainly due to South West monsoon. Heavy precipitation occurs here from May to September. All its tributaries experience number of flood waves as per rainfall in respective catchments. If the flood of the tributaries coincides with the flood of Brahmaputra, it causes severe problem and devastation. The tributaries namely Subansiri, Ronganadi, Dikrong, Buroi, Borgong, Jiabharali, Dhansiri (North) Puthimari, Manas, Beki, Aie, Sonkosh are the main tributaries on the North while the Noadehing, Buridehing, Desang, Dikhow, Bhogdoi, Dhansiri (South), Kopilli, Kulsi, Krishnai, Dhdhnoi, Jinjiran are the main tributaries on the south bank of the river Brahmaputra. The characteristics of the north bank tributaries are different than that of the south bank tributaries, which may be summarized as below :
Besides these, there are several other small streams also which drain directly to the river.
The meteorological conditions in catchments of Brahmaputra in Tibet and in India are different and lie in different climatic zones. The mean annual rainfall over the entire catchments including Tibet and Bhutan is about 2500 mm. The rainfall in Brahmaputra basin is mainly due to South-West monsoon and out of total annual rainfall, 85% occurs during the monsoon months from May to September. Besides the valley gets a good amount of rainfall in the month of April and May due to thunder storm activities which account for flood during heavy rain in June, when the soil is already saturated and river bank in full stage.