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Rivers and Drainage System of Assam : Assam Geography Study Materials & Notes

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The State of Assam comprised of two valleys namely the Brahmaputra and Barak. The geographical area of Assam is 78,438.00 Sq. Km out of which 56,194.00 Sq. Km and 22,244.00 Sq. Km fall under the Brahmaputra and Barak Valley including 2 (Two) hill districts respectively. The flood prone area of the state is 31,500.00 Sq Km, which is about 39.58 % of the total area of the state and 9.40% of total flood prone area of the whole India.

River System of Assam

A) Brahmaputra river system

The main river of the valley, Brahmaputra is one of the largest rivers in the world and is a trans-boundary river which flows through China, India and Bangladesh. With 3,848 km in length, it is the 15^th longest and tenth largest river in the world by discharge.

The river originates from the Kailalsh 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 as the Jamuna. It merges with the Padma, the popular name of the river Ganges in Bangladesh, and finally the Meghna and from here it is known as Meghna before emptying into the Bay of Bengal.

The river drains the Himalaya east of the Indo-Nepal border, south-central portion of the Tibetan plateau above the Ganga basin, south-eastern portion of Tibet, the Patkai-Bum hills, the northern slopes of the Meghalaya hills, the Assam plains, and the northern portion of Bangladesh. The basin, especially south of Tibet, is characterized by high levels of rainfall. Kangchenjunga (8,586 m) is the only peak above 8,000 m, hence is the highest point within the Brahmaputra basin.

The river is often called Tsangpo-Brahmaputra river. The lower reaches are sacred to Hindus. While most rivers on the Indian subcontinent have female names, this river has a rare male name, as it means “son of Brahma”.

The Brahmaputra is an important river for irrigation and transportation. It is a classic example of a braided river and is highly susceptible to channel migration and avulsion. It is also one of the few rivers in the world that exhibit a tidal bore. It is navigable for most of its length. 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 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. This average slope of about 2.82 m/Km in China (Tibet) 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.

The drainage area lying in India is 1,94,413 sq.km which is nearly 5.9% of the total geographical area of the country. The sub-basin lies in the States of Arunachal Pradesh, Assam, Nagaland, Meghalaya, West Bengal and Sikkim.

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.

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 severity of flood problem of the state has been further aggravated by the acuteness of erosion on both banks of river Brahmaputra and its tributaries. Study reveals that an area of 4.27 Lakh Hectare of the state has been eroded by the rivers since 1950, which is 7.40 % of area of the state. The average annual rate of erosion is 8000.00 Ha. The world’s largest river island Majuli is also under the grip of erosion by river Brahmaputra and about 60 % of its original area has already been engulfed by the river.

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 –

The North Bank Tributaries:

• Have very steep slopes and shallow braided channels for a considerable distance from the foot hills and in some cases right up to the outfall.
• Have boulder, pebble and coarse sandy beds and carry a heavy silt charge.
• Generally have flashy floods.

The South bank Tributaries:

• Have comparatively flatter grades and deep meandering channels almost from the foot hills.
• Have comparatively low silt charge.

Right Bank Tributaries of the Brahmaputra River

Left Bank Tributaries of the Brahmaputra River

Barak River system

Barak is the second largest river system in Assam as well as in North East. The river with a total length of 900 km from source to mouth drains an area of 52,000 sq. km. The Barak is also a perennial river of the state.

It originates from Japvo mountain of Manipur hills at an altitude of 3,015 m, near the border of Manipur and Nagaland and forms a part of the northern boundary of the Manipur State with Nagaland where it is known as Kirong. Then it flows south through mountainous terrain up to Tipaimukh near the tri-junction of the three states: Assam, Manipur and Mizoram. Here, the river takes a hairpin bend and debouches into the plains of Cacher district of Assam and forms the border of Assam and Manipur states up to Jirimat. The river then flows through the Barak valley of Assam and then it enters Bangladesh where it forks into the Surma and Kushiyara rivers. From the source to the Indo-Bangladesh border, the Barak River flows for 564 km.

The local rainfall run off of the valley along with that of adjacent hilly areas flows through river Barak and its various tributaries and is drained out to Bangladesh. The Katakhal, Jiri, Chiri, Modhura, Longai, Sonai, Rukni and Singla are the main tributaries of the valley. The tributaries are mainly rain fed and cause flood problems when precipitation occurs.

The Barak sub-basin drains areas in India, Bangladesh and Burma. The drainage area lying in India is 41723 sq.km which is nearly 1.38% of the total geographical area of the country. It is on the north by the Barail range separating it from the Brahmaputra sub-basin, on the east by the Na Lushai hills and on the south and west by Bangladesh. The sub-basin lies in the States of Meghalaya. Manipur, Mizoram, Assam, Tripura and Nagaland.

There are two major physiographic regions in the sub-basin, namely, the hilly region and the plain plains are thickly populated and extensively cultivated. The predominant soil types found in the sub-basin are laterite and red and yellow soils. The culturable area in the sub-basin 0.893 M-ha which is only about 0.5% of the culturable area of the country.

Statewise Drainage Area of Barak River

• Meghalaya – 10,650  Km²
• Manipur 9,550  Km²
• Mizoram 8,280  Km²
• Assam 7,224  Km²
• Tripura 4,725  Km²
• Nagaland 728  Km²

Total Drainage Area of Barak Basin- 41,157  Km²

Hydropower Potential – The Hydro power Potential at 60% load factor for Barak River is 3908 MW.

Major Projects – Tipaimukh Dam Project, Tista Champamati and Dhansiri barrages.

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