CHAPTER 4 THE BRAHMAPUTRA RIVER

Transcription

1 CHAPTER 4 THE BRAHMAPUTRA RIVER 4.1. INTRODUCTION The total length of the Brahmaj&utra river is 2897 Km from its origin in Tibet to its outfall in Bay of Bengal (flap No,4). It traverses its first 1625 Km in Tibet, the next 918 Km in India and the rest 354 Km in Bangladesh. The total catchment area of the Brahmaputra is 580,000 Sq, Km and the breakup is as follows : 293,000 Sq. Km in Tibet, 240,000 Sq. Km in India and Bhutan and 47,000 Sq. Km in Bangladesh. In Tibet the river is known as Tsangpo. On entering India after girlding round Namcha Barwa peak of the Himalayas, the river is known as Siang in its upper reaches and the Dihang in its lower reaches near Along and Pasighat. After Pasighat near Sadiya as the river is joined by two other Trans-Himalayan rivers, the Dibang and the Luhit, it is known as the Brahmaputra. Its channel is navigable by steamers and boats at an average altitude of 3,650m above mean sea level for a length of upto 640 Km, thus making it one of the most remarkable navigable waterways of the world. The river plunges down from an altitude of 3,000m at Pe in Tibet to less than 150m at Pasighat where it debouches in the Assam plains. It

2 58 does so in a series of cascades through narrow gorges without a single fall of more than 50m (Kedia, 1978, p. 17) SOURCE Till the late nineteenth century there were different school of thoughts about the source of the Brahmaputra. The source was first conclusively identified by the great SuedisJ^ explorer Sven Hedin ( , p. 183) in the later part of the nineteenth century. He mentioned that Langchen- Kabab (mountain) lies South-east of Kailash. On the east of this mountain stands the Tamchok-Kabab which is the source of Tamchok-Kabab or the Brahmaputra. Previously there was an erroneous impression that it is at Manas Sarouar lake itself. The Brahmaputra originates from an altitude of 5,300m, about 63 Km south east of Manas Sarouar lake in south west Tibet. The source of the river lies in the Kanglung Kang glacier (30 31' N : 82 10' E) in the northernmost chain of the Himalaya in the Kailash range; just south of the lake called Konggyu Tsho (4877m). The Brahmaputra belongs to the family of south flowing rivers of the Himalaya, namely, Huang Ho, Yangtze-Kiang and Mekong. Most of the characteristics of these rivers are comparable to the Brahmaputra. Many tributaries merge in the infant stage of their outlet from near the Mayum-La pass (5,150m) and Marnayak-La pass (5,303m). These passes, separate the Brahmaputra basin from Manas

3 59 Sarowar lake, in which two other great Indian rivers, the Indus and the Sutlej have their source. It is a remarkable feature that two great Indian rivers, the Brahmaputra and the Indus, having their source almost at the same place on the either side of the division, but traverse in opposite directions towards the east and the west of the sub-continent COURSE OF THE RIVER The course of the Brahmaputra can be divided into three reaches, namely, (1) upper reach or the Tsangpo river; (2) middle reach or the Siang-Oihang-Brahmaputra river and (3) lower reach or the 3amuna-Padma-Meghna river (Map No.4). 4,3.1# Upper Reach Near its origin the river is known as Matsang Tsangpo or Tamchok-Kabab. Many glaciers contribute their snow melt all along the river in its upper reach. The river is flowing eastward through the southern Tibet and traversing about 1,100 Km keeping a course roughly parallel to and about 160 Km away from the main Himalayas. During this traverse, besides numerous unnamed stream, other tributaries joining the Tsangpo are Naes, Tsa, Menchu, Charta, Raga Tsangpo, Tong Chu, Shang Chu, Gya Chu, Gaim da Chu, Po Tsangpo and Chindru on its north bank, and Kubi, Kyong, Sakya Trom Chu, Rhe Chu, Nyang Chu, Yarlang Chu and Trulung Chu on its southern side.

4 60 The important tributaries on the left bank are the Raga Tsangpo which joins the Tsangpo west of Shigaste, and the Kyi Cho, 300 Km long, on which stands the capital city of Lhasa, On the right bank, the Nyang Chu, which flows by the big trade centre of Gyantse on the Tsangpo itself. In this reach from Lhatse Dzong and upto 80 Km from the confluence of Gya Chu river, the Tsangpo has a wide navigable channel. It is one of the most remarkable inland navigation systems in the world, for boats going up and down for 650 Km at an height of 3659m and more, above the sea level. After meeting the Gya Chu river on its north bank, the Tsangpo flows in an almost straight easterly course for about 110 Km. From here the river enters a most difficult reach. The banks rise thousands of metres up near the river. Only one major town namely Gyastsa Kzong is situated on the Tsangpo before reaching T-sela Dzong (2,95?m). A road is running paralleling the banks, connecting numerous smaller towns. The southern side of the river is dotted by numerous glaciers which partly feed the Tsangpo and partly Subansiri, Oiabhrali and Manas rivers which joins the Brahmaputra in India. On the northern side also, the river drains many glaciers in a wide reach mainly through the Gaim da chu and Yigrong Chu rivers. The river changes its direction from east to north-east, about, 50 Km before reaching Tsela Dzong. At Tsela Dzong, the Tsangpo is joined from the

5 61 north by the Gian da Chu river, 3.2 Km wide at the point of junction. Further east at Pe (2,950m) the river is still broad placid stretch of water about 600m wide. Then turning abruptly to the north-east and north it makes its way by a succession of stupendous gorges between the huge mountain masses of Gyala Peri (7,235m) and Namcha Barwa (7,755m)which are in one place only 13 Km apart. Through these narrow gorges the Tsangpo rushes down in a series of cascade and rapids, taking the flanks of the range in a hairpin bend. It takes up from the north another river Po Tsangpo, a swift torrent about 75m wide and then turning south and south-west, emerges from the foot hills in Arunachal Pradesh of India. About 1.6 Km before the border there is a fall of about 25ra on the main river. The length of the river between Tsela Dzong and Indian border is about 260 Km. The river enters in Indian territory at an elevation of 660m (bank level) as Siang river. The important places on the north bank are Sangri, Gyat sa Dzong, Teela Dzong and Druk and on the south bank are Kalung Dzong, Lhatse Dzong, Shimatse, Rinphung Dzong, Orang Pe, Pemakochung and Yortong. The drainage area of the northern side is very wide comparing to the southern side and extends to a maximum of 250 Km near Guat sa Dzong. The drainage area of the southern side is very narrow, only 10 Km at certain places. The maximum

6 62 width is 60 Km. The boundary of the drainage basin on the north, formed by the glaciers from west to east of Kailash range (6,100m), Lunkar (6,000m), Kamchung range and Lap Chung range (6,000m), Nyen Chentang Lha range (7,088m) and Nyinochomo range. Between these high glaciers forming the northern boundary and Tsangpo, there are a number of small glacier masses. The great Himalayas runs east-west along the south side of the Tsangpo. The peculiarity of the Himalayas is that it does not form a water-shed, as several rivers, following oweinsp^iring gorges, force their way through it from north to south. The Indus and the Brahmaputra both of which rise near the Mount Kailash in Tibet, first follow east-west and west-east course, respectively, as though seeking a weak spot in the mountain barrier pursuing their way to the Indian Ocean, Between these two great rivers and the Ganges another important river of the sub-continent, a number of smallerones with sources in North Tibet, find their ways through Himalayas and join the three rivers to empty their waters in Indian Ocean. The upper course of the Brahmaputra was long an unsolved mystery. It was even thought at one time that the Tsangpo might be the upper channel of the Irrawadi and might thus have no connection with the Brahmaputra. The knowledge of its course has been doubtlessly established with full

7 63 certainity only during the present century. The credit of solving the identity of the Tsangpo and the Brahmaputra to a great extent must go to Kinthup (Kedia, 1978, p. 19). He was the first outsider in Tibet to have vieued the "Rainbow Falls" in this river and travelled nearly 160 Km lower than any previous explorer. It was known that Tsangpo after flowing at a level of 3,657m on one side of the range reappeared at 305m on the other side and it was thought that there might be tremendous waterfalls hidden away in the mountains. In a stretch of 160 Km of the river marvellous gorges and rapids were found, but no falls over 9m. A gap of 60 Km still remained. In this part of its course also the river descends not by great waterfalls but by rapids and cascades, pent in towering cliffs, and narrowing down in one place to a width of 27.5m Middle Reach The Tsangpo changes its name to Siang or Dehang in Arunachal Pradesh after crossing the Indo-China border and flowing for 1625 Km. It traverses Arunachal Pradesh in a more or less southern direction for 226 Km before reaching Pasighat ending its journey in the mountains. The total distance between its origin and Pasighat is 1,851 Km. The Dehang basin is surrounded by Subansiri basin on its west side and Debang basin on its east side. The

8 64 Dehang river basin consists of sub-basins of its tributaries, namely, Siyom, Yangsang Chu, Sigong, R ingong, S imong Yams, Siku and Sibia besides its own. The Dehang river flows in an almost straight channel all along the steep mountaineous gorges upto Rogung. Between Rotung and Rengging, the river shows a meandering pattern as revealed by the numerous point bar deposits on either bank. The river has a braided form around Pasighat having cut as many as four channels, though the main flow at present is through two alternating channels only. The channel bar along this strip of river at Pasighat consists of assorted boulders, gravels and sand dunes. At least three paired river terraces are noticeable all along the river stretches between Inkiyong to Pasighat. Some of them attain a height of about 250m above the present flood level of the river. From Pasighat, the Dehang travels another 52 Km before joining the Luhit and the Dibang, two tributaries from east and north east, respectively, near Sadiya. The combined flow of these three rivers forms the Brahmaputra. Fr om this point of junction the mighty river rolls ma j e s t i cally, south-west initially for some distance thereafter west ward down the Assam Valley for a length of 640 Km upto Dhubri (Rap No. 5). A feu kilometres below Dhubri, the river enters Bangladesh. From Pasighat to Dhubri, the river passes through an alluvium region changing its channel courses and

9 65 form large sand-bars. While in non-monsoon it breaks into numerous channels of varying magnitude but during monsoon, it becomes a sheet of water upto a width of 15 Km submerging the sand-bars throughout its route from Pasighat to Dhubri. The bank of the river is either abrupt on both sides or abrupt on one side where the current sets in and cuts away the banks and shelving on the other* Some of the channels during dry season become dry and in subsequent years leave such channels and forms new courses. In Assam Valley, its channels oscillate from side to side and form many islands. One of these islands is Majuli, the biggest and perhaps the oldest riverine island in the world covering an area of 1,250 Sq. Kms, Unfortunately the size of the island has been progressively diminished since 1950 owing to the rapid erosion. From Sadiya to Dhubri, there are about 40 tributaries on its north bank and 20 on its south bank. All the north bank tributaries originate from sub-himalayan ranges except Subansiri, 3ia-Bharali and Manas which are trans- Himalayan. The notable northern tributaries are Subansiri, Ranganadi, 3ia-Bharali, Dhansiri(N), Barnadi, Pagladia, Manas, Aie, Champamati and Gangadhar. The southern tributaries originate from the Khasi Hills, the Garo Hills, the Mikir Hills and the Patkai Hills. Some of the notable tributaries in the south bank are Noa-Dehing, Buri Dehing, Oisang,

10 66 Dikhow, Dhajl, Dhansiri(S), Kopili and Dingiram. Owing to the alluvium and delluvium action of the channels of the Brahmaputra, the points of outfalls of almost all the subtributaries oscillate. The important towns along the south bank of the Brahmaputra are Oibrugarh, Gauhati and Goalpara, and along the north bank are Tezpur and Dhubri Lower Reach After crossing the Indo-Bangla border near Dhubri, the Brahmaputra takes turn to south and passes through the Bangladesh. From Dhubri, after sweeping round the spur of Garo Hills, the river traverses the alluvium plain of Bangladesh for nearly 250 Km to join with the Ganges at Goalando. There is practically no notable tributary on the east bank, within this reach. The important tributaries on the west bank are Raidak, Dhongla, Tista, and Karatoya. Above the junction point of the Ganges but below the confluence of Tista, the Brahmaputra is renamed as Damuna. From Goalando it is known as the Ganges or the Padma. After traversing another 120 Km from Goalando, the combined courses of the Ganges and the Brahmaputra, join the Bay of Bengal. Near its confluence at the bay, it is called the Pleghna which branches into many wide channels namely the Tutula in the west, the Shahbazpur in the middle and the Bamni in the east all falling into the Bay of

11 67 Bengal. The only important river joining the east bank of Padma or Ganges is Surma which is known as river Barak in Assam. Till the end of 18th Century, the Brahmaputra flowed through the district of Mymensingh to join the Ganges near the Bhairab Bazar, but in the early part of next century as this channel became choked with silt, the stream deserted it and moving westwards across the friable soil of the delta and cuts its present channel along the western boundary of flymensingh. Several centuries earlier, the Ganges has a separate exit into the Bay of Bengal by the side of Calcutta until it moved eastwards breaking in Central Bengal through the Padma, meeting the Brahmaputra THE BRAHMAPUTRA AS ONE OF THE MAJOR RIVERS IN INDIA The river system of India falls into two broad groups viz., the rivers of the Himalayan and the Peninsular origin. The Himalayan river system comprises three major rivers namely the Brahmaputra, the Ganges and the Indus. The Peninsular river system comprises of Mahanadi, Godavari, Krishna and Cauvery, all flowing towards east, and Narmada and Tapti flowing towards the west. The Brahmaputra is the biggest river in India. The study (Table 4) of the length, the catchment and the maximum discharges of the above mentioned rivers reveals that

12 68 out of the nine rivers, the Brahmaputra is the longest river in India. Its total length is 289? Km. The other two major Himalayan rivers, the Indus and the Ganges have the total lengths of 2880 and 2506 Km respectively. Among the Peninsular rivers, the Godavari is the longest, having a total length of 1553 Km, uhich is almost half the length of the Brahmaputra, But the Brahmaputra shous a smaller catchment than the Indus and the Ganges. Its drainage area is Km2, while the Indus and the Ganges have higher drainage area of and Km2, respectively, Cven though the catchment area of the Brahmaputra is less than that of the Indus and the Ganges, yet it has the highest maximum discharge in the tune of cumec. But among the Indian rivers, the (lahanadi shous a maximum discharge of cumec. Of the major rivers of India, the Mahanadi has the least length of 404 Km while the Tapti has the least catchment area of Km2, and the Cauvery the maximum discharge amounting to 3910 cumec uhich being the minimum amongst all the rivers. Amongst the big rivers of the world, the position of the Brahmaputra with respect to its discharge, drainage area and length uere also studied (Table 5), Amongst the 15 great rivers of the uorld namely, Amazan of Brazil, Congo of Congo, Yangtze and Huang Ho of China, Brahmaputra and Ganges of India, Yenisey, Lena and 0b of U.S.S.R., flississipi of U.S.A., Orinoco of Venezuela, Parana of Argentina, Irrauadi

13 69 of Burma, Mekong of Vietnam and Nile of Egypt were taken into consideration for a comparative study. In respect of an average discharge of cumec at the mouth, the Brahmaputra*8 position is 4th with Huang Ho. The Amazon is having the highest average discharge at the mouth in the tune of cumec. The Nile being the longest river having 6693 Km length has the louest average discharge of cumec at the mouth. The average annual suspended load of the Brahmaputra is of very high order. It is second only to Huang Ho (Table 6). The average annual suspended load of the Huang *y Ho is 2080 million ton corresponding to ton/km, and that of the Brahmaputra is 800 million ton corresponding to 9583 ton/km, The Nile shous the louest position having 122 million ton of average annual suspended load corresponding to 259 ton/km2. The average annual yield and the specific yield in cumec per sq. km of the catchment area from some of the major rivers of the world depict that the specific yield of the Brahmaputra is the highest (Table 7). This is one of the main reasons for acute flood congestion in the valley THE BRAHMAPUTRA VALLEY Geographical Features The Brahmaputra valley in India is about 800 Km in

14 70 length lying almost east to west in its lower portion but tending somewhat to the north-east in its upper half. It is bounded on the north by the Eastern Himalayas, the lower range of which rises abruptly from the plains, in the east by ths Patkai range of hills running along the Indo-Burma border and on the south by the Assam range of hills. The broadest part of the valley, is where, the river crosses the Dibrugarh district and enter Sibsagar district. The width of the Brahmaputra valley between the foot-hills is only 80 to 90 Km of which the river itself has a width of 6 to 10 Km in most places. Towards the west of the broadest part of the valley, the isolated block of flikir Hills to the south, and the group of the Dafala Hills to the north, suddenly contract it to 64 Km. Further west, it widens out but towards the western end of the Nowgong district, it is again encroached by the Khasi Hills. The hills are close to the river upto Gauhati. Thereafter, the hills recede again and they donot approach the Brahmaputra upto Goalpara, flanking the spur of the Garo Hills. Beyond this point, the valley again widens at Dhubri and finally touches the great delta of Bengal. A few kilometres downstream of Dhubri, the river enters Bangladesh. The Brahmaputra valley has several interesting physiographic features. Although the valley is monotonously plain, it does show some distinct mounds and hills emerging

15 71 out of the alluvium as seen in Goalpara district. The only prominent hill feature in Upper Assam is the Clana Bum range at south of Noa-Oihing. On the foot-hills of either side of the valley, some mounds are seen. There are also several mounds and raised grounds of older Alluvium seen specially on the southern side of the valley and also on the north bank in the Goalpara district. They enclave many of the hills of granitic and gneissic rocks which are seen as inselberg. Such hills are also seen in Sonitpur district, in Kamrup district and then further west of Goalpara district. These rocks are the extension of the famous "Shillong Plateau". The fringes of the older alluvium mounds were formed during the Pleistocene period and was raised to the present heights much above the recent sand and silt deposited by the present day Brahmaputra. It is believed that these areas were raised in block by the vertical movement of the basement complex. The valley along with the surrounding hills on its three sides vary in their details of formation as well as geohydrologic and meteorologic environment. They vary from basin to basin and even from area to area in minute details. The sub-himalayan range on the north of the valley, particularly on the west of Subansiri consists of soft sandstone and weathered and otherwise disturbed overburden. These soft formation and debriload, are the perennial source of silt, sand and debris which are washed down during heavy rains in

16 72 the valley by the rivers and streams emerging out of this range. This is the reason for the large part of sandy deposits on the northern bank in the eastern part. Further to the east of Subansiri, in Siang, Kameng and Luhit districts of Arunachal Pradesh, even though the rocks are hard yet susceptible to weathering. The rivers in these regions carry boulders, pebbles, sand and silt. The land formation of this part of the valley is composed of those deposits. The Patkai and the Naga hills on the east of the valley have soft sedimentary rocks. Ouring monsoon, huge quantities of sand and silt are washed from these two hills. Rivers and tributaries, flowing through such catchments to the south of the Brahmaputra valley, brings down heavy load of fine sand and clayey sediments. Thus this deep undulating lowlying portion of the valley was gradually built up of these particular types of fine argillaceous soil, silt, sand and detritious load carried down and deposited in layers by the respective tributaries. The granulometric charancteristics of such layers, their density of deposition and extent of coverage was guided by the energy potentialities of the flood volume based on the run-off, sediment and debrilosed, grads of flowing surface, direction of flow, tutbulance and on the rate and nature of changes of such energy potentialities during different stages of floodings and obstructions faced on the path of flow and spilage. At areas, nearer to and along the foot-hills, there

17 73 are de po si ti on of porous detritus layers extending for considerable distance towards the lower portion of the valley below a thick layer of upper soil having comparatively a shallower grade composed of less permeable layers. In such permeable layers, high volume of water infiltrates and gets reserved as ground water. At places where such permeable layers are interfered by layers of finer deposits, less permeable in nature or exposed below a shallow overburden, the flow being obstructed as a result, it emerges out on the surface as spring. If it is favoured by comparatively more permeable layer below, deeps further down and emerges in a nearby river. Such springs and streams are commonly seen in the valley at places on the northern bank of the Brahmaputra within a feu kilometres south of the foot hills such as in the basin between Pagladia and Kaldia rivers, and further in the west. Most of the streams in the north bank become dry or carry very less discharge during winter, owing to the effect of these springs. The characteristic feature of the hills is the deep V-shaped valley, specially well marked in the Himalayan ranges. These valleys have steep gradients and hence the river water shows high velocity that gives tremendous ca pacity to carry the load of detritus during its journey. As the rivers emerge into the plains, the pebbles and the boulders carried by these become tools for cutting the sides and the

18 74 bottom of the river valley. There is a drop in the velocity, and all the detritus being dropped build up the foot-hills with such coarse materials. The Brahmaputra plains are monotonously flat with a very low gradient. Sadiya, the town in the eastern border of Assam, is about 137m above the mean sea level, while Dhubri, the town in the western border is 43m above the mean sea level. This shows a fall of 94m in a total length of about 650 Km i.e. a fall of 0.15m per kilometre. This lou gradient determines the character of the river course in the plain. As the river has hardly any strength to cut its channel deep, it cuts sideways. As this process progresses, the river shows meandering, the loops become longer until eventually the river cross cuts the narrow neck forming ox-bow lake. Such abandoned courses are common in the Brahmaputra valley Geological Features The Brahmaputra valley evolved during the Q u a t e r nary period by alluviation of foreland depression in between the Hiraalayan orogenic belt and the crystalline massif of the Shillong Plateau. Some portions of the valley are experiencing upliftment while others are sinking. All the hills and mountains of this region are the result of such upliftment. The sediments which were deposited in an ancient sea had been raised to 1500m or more in the foot-hills of the

19 75 Himalayas. The rocks further north also contain quartzites and limestones which were deposited in the older seas. Even the crystalline rocks further north had been uplifted to their present heights during the process of the formation of the Himalayan mountain system by movements which had pushed up successively the older rocks on to the younger rocks. Similarly, the Patkai, Naga and Lushai Hills, and the Shillong Plateau had been uplifted, both due to the vertical and the lateral movements. Relative to these upliftments, the Brahmaputra valley had sunk, the sinking was greater in Upper Assam where the streams from the Himalayas and the Plikir Hills deposited sediments into the ambayment connected with open sea in what is now the Naga Hills and further south. The geological successions in the Shillong Plateau and in the Naga-Patkai ranges are at variance with those seen in the Himalayan catchment. The oldest formations are in the E-U Shillong Plateau and Mikir Hills, the projection of which are seen in inselbergs within the basin as far east as longitude 94 at Numaligarh. West of this longitude and north of the main plateau, the inselbergs are seen jutting out at several places close to the Brahmaputra river. Towards east of Numaligarh, the total absence of the basement exposures, and the gradual thickening of the Eocene-Recent sediments as met in the oil-fields of Upper Assam, clearly indicate the regional slope of the basement towards the eastern side. The

20 76 basement constituents are composed of high grade metamorphites, gneisses, granite, etc., of the Shillong Plateau. These are overlain by the low grade metasediments of the Shillong Group. The inselberg zone gradually gives way to alluvial plain on the northern side of the Brahmaputra valley, abutting against the Siualik ridges of the Himalayan front running in the ENE-USU direction. The Siwaliks are followed by a highly tectonised zone of Palaeozoic sediments on which the gneisses and schists are overthrust. In the Lohit area, on the easternmost side of the Brahmaputra Basin, the Siwaliks and the Palaeozoics are not exposed, and the alluvial deposits, by and large, directly abutting against the metasediments. These are successfully followed by gneisses and high grade schists. However, it may be assumed that the basement of the entire Brahmaputra Basin is made up of the Shillong Plateau-Mikir Hills type of metamorphites. The fluvial or continental Gondwana sediments provide the palaeozoic history of the Brahmaputra Basin. The continental sequence of the Gondwana is best developed in the Kameng district of Arunachal Pradesh. The outcrops of this sequence peter out towards ENE, and only some carbonaceous shales with thin lenses of coal are seen upto Siang Valley. East of it, the Gondwanas are not discernible due to overthrusting of the older rocks.

21 77 The undisturbed Gonduana or Permo-Carboniferous rocks are not exposed in flrunachal Himalaya. But the technically overlying sediments are phyllitas and quartzites; and some variegated purple green and red shaley to calcareous beds and pink quartzites. The ash beds and the volcanics are successively forming the sequences tectonically overlying or upthrust against the Permian Gonduana and the Permo- Carboniferous marine beds. The dolomite and the quartzitephyllite sequences of S iang and Subansiri districts may be older than the Miri quartzites and hence may represent middle to early Palaeozoic sequence. The metasediments and gneisses of Kameng and Subansiri and parts of Siang districts are considered to be early Palaeozoic or Precambrian in age. Thus the Palaeozoic environment of the Brahmaputra Basin and its catchment were represented by an arcuate NE-SU to NU-SE sedimentary volcanic environment. The Permo-Carboniferous marine-cura-continental environment developed on its outer side. A part of this environment is preserved in the frontal range of Arunachal Himalaya uhile the rest may be underneath the Tertiary-Wesozoic cover in the eastern part of the present Brahmaputra plains. The Tertiary sedimentation is the most important part of the geological history of the Brahmaputra Basin. Essentially, the Upper Assam behaved as a platform with pericratonic blocks or foredeeps on its southern margin, giving

22 78 way to the Upper Cretaceous-Palaeocene-Oligocene geosyncline, now teansformed into the Naga Patkai ranges. The Palaeocene- Eocene sedimentation commenced with large volumes of miogeosynclinal sediments viz. Disangs towards the southern side, while towards the northern platform the Eocene Daintia Group was deposited in the Upper Assam plain. South of the Shillong Plateau, these Palaeocene-Eocene sediments extend westwards over the Cretaceous fringe on the shelf and platform. Thickness relation and sub-surface data indicate that the regional palaeo-slope of the platform during Palaeocene-Oligocene period was towards south-east, the provenance for the sediments being essentially on the northern and the western sides. During Oligocene period the marine conditions were gradually replaced by deltaic and continental environments and the provenance areas must had been the upland on the north(himalayan area), west (flikir Hills) and east (Luhit upland) and geanticlinal crest (axials of Naga Hills). The sub-surface data also indicate significant thinning of the typical Barails in the northern direction, that is, towards the Himalayan front. A large part of the region became a landmass towards the end of the Oligocene period, probably due to the upliftment of the Barails in the geosynclinal area, and one of the uplift phases of the Himalayan terrain. In the post-oligocene period tectonism and upliftment resulted in sinking of the

23 79 southern margin of the Himalayan front to give rise to the Himalayan foredeep the site of Siualik sedimentation. The Siualik sediments, south of Gonduana, may be divided into two fault blocks. The sediments of the southern block are in general correlative with the Tipams, Namsangs and Dihings. The southern limit of the foredeep is not fully understood. The northern limit of the inselbergs may roughly delineate the platform and the Siualik foredeep. East of the Siang river, the NE-SU Siwaliks are illexposed but a tendency of swerve in the strike to NU-SE is discernible. Again east of the Siang river, the boulder beds are conspicuously thick which through the Sadiya plain join the Dihings. It is logical that as the Pliocene sea regressed southwards in Burma, the piedment deltaic or fluvial deposits might advanced in that direction until the master slope was changed in Plio- Pleistocene period. During the PIio-Pleistocene period, the Disang thrust and the Naga thrust along with the accelerated tectonic activity in the Luhit Himalaya, the combined Luhit- Siang system perforce took the westerly course through the top of the Dihings and its platform facies. It was perhaps because of the intense south-westerly directed force from Luhit Himalaya during Pleistocene period, the present westerly flow direction of the Brahmaputra occurred. The faulting and fracturing in the inselbergs further accentuated this change in the drainage pattern.

24 80 The Brahmaputra valley is in the process of sinking. The subsidence of the basin might have taken place along some basement faults, rejuvenated from time to time, by tectonic activities. The effects of the earthquakes in the valley particularly during 1897 and 1950 showed sharp changes in the level along fault scraps, changes in the river courses and extensive landslides suddenly contributing un us ua l amount of scree materials to the rivers THE TRIBUTARIES OF THE BRAHMAPUTRA The Brahmaputra valley has two distinct sets of tributaries on the north bank and the south bank of the river. The north bank tributaries with larger catchment and heavier precipitation are bigger than those of the south bank. The characteristic of these two sets of tributaries are also having basic differences. The northern tributaries, originate from the Himalayas show considerable discharge with very steep slope and shallow braided channels for considerable distance from the foot-hills and in certain cases upto the very outfall. They generally carry heavy silt charge, and have unstable sandy bed and bank, susceptible to flash floods. All the north bank tributaries have northsouth courses except at the confluence with the Brahmaputra where the mouths are shifted westward by frictional drag (Barthakur, 1978, p. 8). The tributaries on the south bank

25 81 originate from Patkai, Khasi-Daintia and Garo hill ranges. Host of them have a comparatively flatten grade and deep meandering channels. The soil of these rivers ranges from fine sand to alluvial clayey loam. The silt and the sediment charges are comparatively low The North Bank Tributaries The Brahmaputra has 40 tributaries on the north bank. The principal tributaries are Dehangf Dibang, Subansiri, Ranganadi, Dia-Bharali, Dhansiri, Barnadi, Pagladia, Manas, Aie, Champamati, and Gangadhar Dehang Dehang is one of the three tributaries forming the Brahmaputra. But it is actually the main river which after traversing through Tibet enters India and joining with the other two tributaries forms the Brahmaputra. Dehang has a catchment area of about Sq. Km. The average annual discharge of the river is 5722 cumec Dibang Dibang is also one of the three tributaries forming the Brahmaputra. It originates from the Himalayan range and traverses towards south. Dibang has a catchment area of Sq. Km. The average discharge of the river is 4715 cumec.

26 Subansiri The Subansiri has its origin in Tibet and receives several hill torrents and sub-tributaries. It flows down through a canyon into the plains of Lakhimpur. It has a total length of 442 Km covering 153 Km in the plains of Assam, and drains a catchment of Sq, Km with maximum discharge around cumec. It brings down boulder, gravel, sand and debris during the flood Ranganadi The Ranganadi originates from Dafla hills. It is known as Pamir Nadi in the hilly region and Ranganadi in the plains of Assam. It has a total length of 154 Km of which 122 Km in the hilly region and 32 Km in the plains. The river has a catchment area of 5180 Sq. Km and maximum discharge of 1259 cumec. The bed of the river consists of boulder of different sizes ia-Bharali The Bhoralli or Dia-Bharali or Kameng originates from the Himalayas, at an altitude of 6565m which receives several sub-tributaries. It emerges in the plains at Bhalukpong through a narrow gorge and flows between two low hill ranges, and then turns to the south and receives the subtributaries sueh as Upper Dikrai, Bor-Qikrai on the left

27 83 bank, and uppar and lower Sonai on the right bank. The total length of the river is 264 Km of which 66 Km falls in Assam. It has a catchment area of Sq. Km with a maximum discharge around 5886 cumec. The river brings down sand and heavy debris Dhansiri It originates from the Bhutan Himalayas and flows in the southern direction through Darrang for 90 Km. The river has a maximum discharge of about 1088 cumec. The bed of the river consists of boulder of different sizes Barnadi It originates from the Bhutan Himalayas. After flowing 6.5 Km in Bhutan, it enters Darrang district of Assam where it flows north-east to south-west direction for a length about 120 Km and falls into the Brahmaputra near North Gauhati. It has a maximum discharge of 169 cumec Pagladia The river flows doun from the sub-himalayan range for 95 Km before joining the Brahmaputra, out of which 75 Km falls in Kamrup district of Assam. It carries fine micaceous sand and silt with high velocity and turbulance at the time of fresh floods. This quickens sand and silt deposits in the

28 84 bed and the bank which get suspended, flow as a liquid mud. Sometimes the river is obstructed by the existing high shoal dash against it or dash against a bank in a meander splashes near the bank. The areas between the Kaldia and the Pagladia were built up by the tributaries swinging their courses from east to west. The river Pagladia has a maximum discharge of 3743 cumec Nanas The Nanas rises from the great Himalayan range in Bhutan and emerges in the western part of Kamrup district and flows through Goalpara district. It receives on its bank the Aie. The total length is 375 Km of which 194 Km falls in Assam. It drains a catchment of 37,500 Sq. Km with a maximum discharge of 7641 cumec Aie It originates from the hilly range of Bhutan. After travelling 50 Km in Bhutan region, it enters Assam and flows in a south westerly direction through the plains of Goalpara district for 43 Km. It meets Nanas near Lengtisinga, finally outfalling in the Brahmaputra near Oogighopa. It has a maximum discharge of 2035 cumec.

29 Champamati It originates from the Bhutan Himalayas, having a total length of 102 Km of which 90 Km falls in Kokrajhar district of Assam. The maximum discharge is 2289 cumec Gangadhar It originates from the southern slope of the Himalayan ranges in Bhutan. The river is known as Sankosh in upper reaches and takes the name of Gangadhar in the plains of Dhubri district till its outfall in the Brahmaputra. The total length is 169 Km of which 90 Km falls in Assam The South Bank Tributaries The Brahmaputra has 20 tributaries on its south bank, the principal among them are Luhit, Noa-Dihing, Buri Dihing, Disang, Dikhou, Ohaji, Dhansiri, Kopili and Dinjiram Luhit It originates from Patkai ranges. It is one of the three tributaries forming the Brahmaputra. It has a ca tc h ment area of Sq. Km. The average discharge is 5736 cumec

30 B Noa-Dehing It originates in the Singpho hills on the border of Burma and the northern spur of Patkai range. It traverses for 154 Km. The river takes uesternly direction upto fliao where it bifurcates into northern branch known as Noa-Dehing and southern branch known as Kaikhe which joins Buri Dehing. The Noa-Dehing falls into Luhit about 20 Km upstream of Saikhouaghat. The principal sub-tributary of Noa-Dehing is Dirak on the left bank Buri Dehing This is one of the most important tributaries of the Brahmaputra on the south bank. It is formed out of several sub-tributaries of Yapauap Bum range and Patkai Bum range and flows in the westerly direction through gorges and receives one sub-tributary viz., Tirap near Ledo. It then emerges from the hill near Plargherita and flows onwards. Below Plargherita, the river starts meandering acutely. There it receives several sub-tributaries such as Digboi, Tipling, Tingrai, Sisa on the right bank, and Dirak, Namsang, Disam on the left bank meeting the Brahmaputra at about 32 Km downstream of Dibrugarh. The total length is 362 Km of which 162 Km in the hills and 200 Km in the plains. It drains a catchment area of 8473 Sq. Km with maximum discharge of 2476 cumec. The sediment load of the river varies from coarse to fine silt.

31 Disang It originates from Patkai hills where it is known as Tisa. Its total length is 230 Km. It travels northwardly for a distance of 61 Km. It is joined by tributaries viz. Tawai jo on the right bank and Tirat jo on the left bank. The river then flows in the plain and takes its course towards north west. The length of the river from the foothills to the outfall Is 150 Km. It drains a catchment area of 3950 Sq. Km, and having a maximum discharge of 1087 cumee Dikhow It originates in the Naga Hills at an altitude of m. The river traverses a length of 200 Km before it joins the river Brahmaputra out of which 96 Km falls in plains. The river at the upper most region is known as Longa river. After meeting a sub-tributary viz., Chimel on the right bank, at a distance of 45 Km from its origin, the river travels by the name of Dikhow and passes through deep gorges, meets another sub-tributary Yangamum on its right bank, then moves further north and emerges from some hills near Naginimara, It is very deep and heavily meandering In the plains and joined by the two subtributaries viz,, Namdang and Darika. The river drains a catchment area of 4372 Sq. Km with a maximum discharge of 1379 cumee.

32 Dhaji It originates in Naga Hills at an altitude of m. The river travels 108 Km of which 55 Km falls falls in the plains of Assam. From the source the river moves towards north-west. In its upper coarse the river is known as Melek. After a traverse of about 15 Km in the north-west direction, the river turns towards north-east and moving about 20 Km meets the first tributary viz. fluning Nadi. It continues its north-east journey for 16 Km further and turns towards north and gushes out of the hills. Moving further north, it meets its tributary viz. Tori Nadi, thereafter, the river meanders towards north-west direction through the alluvial plain, meets the sub-tributary viz. Teok on its left and falls into the Brahmaputra. It drains a catchment area of 1349 Sq. Km, and has a maximum discharge of 644 cumec Dhansiri The Dhansiri rises in the south-west of Naga Hills below the Laishiang peak. For the first 37 Km, the river flows in north-westerly direction and then turning north-east it flows for about 75 Km upto Dimapur. It then takes an abrupt turn and flows west to join the Brahmaputra at Dhansirimukh. The Dhansiri shows meandering all throughout its course, and the meandering is very marked down-stream of Golaghat. Beyond Golaghat and upto its outfall in the Brahmaputra there are

33 89 about 18 big loops. The total length of the river is 254 Km, of which 100 Km is in the plains and drains a catchment of Sq. Km with a maximum discharge of 2296 cumec. The sediment load varies from medium to fine sand, and s ilt Kopili The Kopili is one of the major tributaries of the Brahmaputra. It originates from an altitude of 1630m on the Barail Parbat in the North Cachar Hills of Assam and flows down through Khasi-Daintia Hills d istrict of Pteghalaya. It receives several sub-tributaries on the way down to the plain near Panimur Ardra, and thereafter receives a major subtributary namely Diyung, and flows in a north-west direction. The major sub-tributaries on its right bank are Lankajan and Damuna, and on its left bank are Barapani, Umium and Llmtru. At about 6.5 Km downstream of Oharamtul, it connects with the Kalang river and then flow together through low lying areas and join the Brahmaputra near Raja Hayang. Its total length is 297 Km. From its origin, it travels 127 Km through the h ills in a zigzag way by passing through several gorges and tiny narrow valleys meeting some h ill streams. The river drains a total catchment area of Sq. Km with a maximum discharge of 1942 cumec.

34 Dinjiram The Dinjiram river originates from a Beel known as "Urpad Beel" near Agia. The river flowing in an east-west direction for about 40 Km from its source to Dhumerghat. It then takes an abrupt turn to south west and flows upto Phulbarighat and again turns to south to meet the Brahmaputra in Bangladesh. The total length of the river is 160 Km of which 120 Km is in India. It drains a substantial area of Garo Hills of Heghalaya through 9 sub-tributaries, all on the left bank. Important sub-tributaries are Ajagar, Ghagna, Ringgi and Rongai. The total catchment area of the river is 3467 Sq. Km with a maximum discharge of 168 cumec.

35 TABLE 4 : Length, Catchment and Maximum Discharge of the Major Rivers of India. Name of the river Length in Km Catchment area in Km2 I Maximum Recorded Discharge in Cumec Brahmaputra Ganges Indus Mahanadi Godavar i Krishna Cauvery Narmada Tapti

36 TABLE 5 : Discharge, Drainage Area and Length of the Major Rivers of the World. Name of R iver Country Average Discharge at month in Cumae Drainage Area in Km2 Total Length in Km. Amazan Brazil Congo Congo Yangtze China Huang Ho China Brahmaputra China India Bangladesh Yenisey U.S.S.R Miss iss ipi U.S.A Orinoco Venezuela Lena U.S.S.R Parana Argentina Irrauadi Burma Ob U.S,S,R Ganges India Mekong Vietnam Nile Egypt

37 TABLE 6 : Average Annual Suspended Load of the Major Rivers of the World. Name of River Country Drainage Basin in 1000 Km2 Average Annual Suspended Load Million Tons Tons per Km2 Average Discharge at mouth in 1000 eumec Huang Ho Ch ina Brahmaputra China India Bangladesh Yangtze China Indus Pakistan India Ganges India Amazan Brazil Mississ ipi U.S.A Irrauadi Burma Missour i U.S.A Mekong Vietnam Colorado U.S.A Red Vietnam Nile Egypt

38 TABLE 7 * Average Yield and Specific Yield of the Major Rivers of the World. Name of the R iver Average Yield in Cumec Specific yield in Cumec par Km2 Brahmaputra Amazan Mekong Yangtze Congo Ganges r i no co Danube Indus Parana St. Lawrence Mackenzie Yenisei Volga Riss issipi Nile Lena