River System of Assam River System of Assam

The State of Assam comprised of two valleys namely the Brahmaputra and Barak Valley and it is situated in between 90° to 96° North Latitude and 24° to 28° East Longitude. 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 as assessed by the Rastriya Barh Ayog which is about 39.58 % of the total land area of Assam. This is about 9.40% of total flood prone area of the whole country. The flood prone area of the country as a whole stands at about 10.2 % of the total area of the country, but flood prone area of Assam is 39.58 % of the area of the state. It signifies that the flood prone area of Assam is four times the national mark of the flood prone area of the country. Records show that average annual area affected by flood is 9.31 Lakh Hectares. The flood protected area of the state is 16500.00 Sq. Km till date.

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


 

RIVER SYSTEM OF ASSAM

A) Brahmaputra river system

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 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 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)

Arunachal Pradesh

81,424

Assam 

70,634

West Bengal   

12,585

Meghalaya

11,667

Nagaland

10,803

Sikkim

7,300

Total

194,413

The upper portion of the sub-basin lying in Arunachal Pradesh and Nagaland is mostly mountain ranges and narrow valleys. Most portion of the sub-basin lying in Assam, Meghalaya and West Bengal consists of hills, forests and tea gardens. The Cooch Bihar and West Dinajpur districts of West Bengal has fertile plains.

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 –

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.

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.

Table: Category wise Rivers of Assam

1. Category 'A'

       
           
       
   

Sl.
No.

River

Length in Km.

   

1

Brahmaputra

 

   

 

i

R/B (North bank)

468.402

   

 

ii

L/B (South bank)

555.485

   

2

Subansiri

189.515

   

3

Buridehing

211.738

   

4

Pagladia

95.5

   

5

Borak

251.08

   

6

Kushiyara

36.67

   

7

Surma

3.65

           

Total length of embankment in 'A' Category = 1812.04 Km.

           

2. Category 'B':

       
           
   

Sl.
No.

River

Length  in Km.

   

1

Jidhal

14.215

   

2

Ranganadi

69.61

   

3

Dikrong

34.23

   

4

Jiabharali

33.35

   

5

Puthimari

134.00

   

6

Desang

92.927

   

7

Dikhow

110.34

   

8

Kopilli

149.66

   

9

Manas

16.13

   

10

Beki

34.54

   

11

Gabharu

31.50

   

12

Saralbhanga

16.665

   

13

Noa-Dehing

20.95

   

14

Kollong

136.942

   

15

Killing

11.90

   

16

Katakhal

93.05

   

17

Madhura

20.00

   

18

Sonai

50.81

   

19

Rukni

25.35

   

20

Shingla

45.16

   

21

Longai

78.89

   

22

Saktola

19.275

   

23

Barnadi (Puthimari)

1.50

   

24

Aie

28.125

   

25

Gangadhar

14.21

   

26

Kumatia

43.515

   

27

Barapani

1.67

           

Total length of embankment in 'B' Category = 1325.544 Km.

           

3. Category 'C':

       
           
   

Sl.
No.

River

Length  in Km.

   

1

Dhaleswari

16.240

   

2

Ghagra (Borak)

22.406

   

3

Hareng

12.220

   

4

Rangir Khari

6.300

   

5

Larsing

22.270

   

6

Purkhai

3.180

   

7

Asmjur

46.966

   

8

Bali

4.436

   

9

Jatinga

1.000

   

10

Sutarkhal

0.926

   

11

Bolia Badri

0.345

   

12

Kachua

2.800

   

13

Gumti

2.500

   

14

Dalu

3.600

   

15

Rupacherra

1.530

   

16

Amarkhal

1.470

   

17

Pancholi

1.800

   

18

Deolakhal

4.500

   

19

Nurulcherra

3.800

   

20

Singicherra

1.950

   

21

Jaljali

5.040

   

22

Baralia

39.100

   

23

Nona

51.500

   

24

Barnadi

41.415

   

25

Kalajal

9.000

   

 

i

Dighalighaijan = 8 Km.

   

 

ii

Molongjan      = 1 Km.

   

26

Digaru

8.330

   

27

Siligurijan

1.000

   

28

Nonoi

81.650

   

29

Noanadi

19.825

   

30

Ghagra(Mld)

18.845

   

31

Belsiri

7.800

   

32

Solengi

19.758

   

33

Nakhanda

6.017

   

34

Morachawlkhowa

21.850

   

35

Pohumara

56.000

   

36

Krishnai

4.250

   

37

Dudhnoi

1.500

   

38

Kanamakra

3.140

   

39

Kaloo

9.300

   

40

Dirak

15.500

   

41

Gainadi

20.045

   

42

Moridhal

31.240

   

43

Silley

8.100

   

44

Korha

18.730

   

45

Champara

13.500

   

46

Jhanji

47.630

   

47

Bhogdoi

38.970

   

48

Dhansiri (Southern)

23.600

   

49

Boginadi

6.900

   

50

Singra

15.700

   

51

Pabha

34.470

   

52

Durpang

16.870

   

53

Koliani

7.500

           

Total length of embankment in 'C' Category = 864.314  Km.

           

4. Category 'D':

       
           
   

Sl.
No.

River

Length  in Km.

   

1

Sonacherra-Balicherra

21.070

   

2

Bogakhal

6.015

   

3

Bachirkhal

2.411

   

4

Pola

4.000

   

5

Nonoi & Haria

31.325

   

6

Depota

3.500

   

7

Diggoz

9.700

   

8

Bordikarai

4.000

   

9

Ghiladhari

12.000

   

10

Brahmajan

25.900

   

11

Balijan

18.000

   

12

Singra Maganijan

6.590

   

13

Kaldia

8.400

   

14

Kukarjan

3.610

   

15

Bhellengi

10.557

   

16

Mora Pagladia

22.000

   

17

Nadla channel

14.700

   

18

Jaha channel

2.410

   

19

Gaurang

3.910

   

20

Champamati

7.036

   

21

Desam

16.520

   

22

Tingrai

10.110

   

23

Dhola

3.700

   

24

Tipling

7.000

   

25

Kakodonga

24.430

   

26

Ghiladhari

21.600

   

27

Meleng

5.690

   

28

Namti

10.480

   

29

Mitong

34.900

   

30

Namdang

2.600

   

31

Darika

16.370

   

32

Demow

15.298

   

33

Dirgha

9.600

   

34

Kakoi

9.000

   

35

Pichala

6.400

   

36

Hatilung

9.100

   

37

Gariajan

3.200

   

38

Somdri

15.070

   

39

Rupahi

9.500

   

40

Bisannalajan

6.000

   

41

Helacha drainage channel

5.000

   

42

Kulsi

5.200

   

43

Sessa

5.050

           

Total length of embankment in 'D' Category = 468.952  Km.

           
           
   

Category

 

Length 
in Km.

   

A

=

1812.04

   

B

=

1328.514

   

C

=

864.314

   

D

=

468.952

   

GrandTotal

 

4473.82

 

River System of Brahmaputra Basin (Source India WRIS)



Satellite image of Brahmaputra Basin (Source India WRIS)



Elevetion Zone, Brahmaputra Basin (Source India WRIS)



Annual average rainfall, Brahamaputra Basin (Source India WRIS)



Land use Land cover, Brahmaputra Basin (Source India WRIS)



Population Density-2001, Brahmaputra Basin (Source India WRIS)

 

B) Barak river system:

The Barak valley has a geographical area of 6922.00 Sq. Km excluding 2(two) hill districts. 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 of the sut lying in India is 41723 sq.km which is nearly 1.38% of the total geographical area of the country. It is be 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. The State-wise distribution of drainage area is given below

State

Drainage area (sq.km)

Meghalaya

 10650

 Manipur

9567

 Mizoram

 8866

Assam

7224

Tripura

4688

Nagaland

728

Total

41723

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.



Sattellite Image, Barak Basin (Source India WRIS)



Elevetion Zone, Barak Basin (Source India WRIS)



Annual Average Rainfall, Barak Basin (Source India WRIS)

 

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