BRAHMAPUTRA RIVER MORPHOLOGY IN UPPER ASSAM AND SEISMOTECTONIC RELATIONS

Abstract

The Brahmaputra river system in Upper Assam valley is one of the most dynamic river systems on the Earth’s surface. Frequent channel migration, bank erosion and monsoonal flood are being the most devastating disasters of the valley every year. The present study on morphodynamics of the Brahmaputra river system and seismotectonic relations in Upper Assam valley were carried out using various remote sensing datasets and field studies. Satellite images from Landsat series of satellites have been extensively used for this study. On the other hand, historical toposheets have been used to map the old river channels and palaeochannels. Very high resolution online World Imagery provided by ESRI (Environmental Systems Research Institute) servers are also being very informative for this study. The study covers a time span of 1913-2016. Rivers in Upper Assam valley has shown some very significant morphodynamic changes within the last century, specially the channel belt of Brahmaputra and its north bank tributaries. The average width of main channel of Brahmaputra in Upper Assam valley has increased from 6 km to 10 km within the period of 1913 to 2016. Interestingly, the north bank of the river has undergone more erosion compared to that of the south. The north bank underwent 436 km2 of erosion and 7 km2 of deposition within the mentioned period. On the other hand, the south bank experienced 83 km2 of erosion and 48 km2 of deposition within the same period. This indicates a northward migrating trend of the main channel of Brahmaputra in the valley. The phenomenon is very prominent near Majuli. Major north-bank tributaries of Brahmaputra, like Subansiri, Jiadhal and Sisi are also showing unidirectional lateral migration towards west. The Subansiri river migrated laterally about 10 km westward within the period of 1915-2015. The channel shifting process of the river was gradual. The Jiadhal river avulsed towards west across its fan in 1976. Another avulsion of same river occurred in 2014 monsoonal flood. The avulsion direction of 2014 was opposite to that of 1976. On the other hand, most of the southbank tributaries have maintained relatively stable channels over the last century. Only meander cut off and bank erosion processes can be noticed along the courses these rivers. However, prominent geomorphic markers within the valley indicate very slow but active channel migration of these tributaries. Abstract ii The Lohit river avulsed in 1988 which triggered severe bank erosion near Rohmoria in Tinsukia district and also converted the Dibru Saikhowa region into an island. Landsat image acquired on 27th October, 1988 shows that a small braid branch of the Lohit river got connected with the Dangori river after the 1988 flood. This channel of the Lohit river showed dramatic changes in the later years. The river started to flow along the captured channel to meet the Dangori river, abandoning its older course. By November 1995 the Lohit river totally changed its course and Dangori river became the trunk channel of the Lohit river. However, no tectonic influences could be linked to this avulsion. Unprecedented high flood level of 1988 and typical topographic setting of the region can be attributed to this avulsion of the Lohit river. Longitudinal profiles of the segments of Lohit river flowing south and north of Dibru Saikhowa region have been compared to study slope along the channels. The channel flowing south of Dibru Saikhowa island shows a slight depression in its longitudinal profile in comparison to the northern channel after their divergence. The profiles clearly reveal that topographic elevation along the new channel is 0.5-1.0 m lower than that of the older channel. This gradient variation between the channels is the major factor in changing the course of the Lohit river. Correlation of topographic elevation and basement depth along selected sections within the valley shows some significant correlations between them. The Nazira-Nangalamora region over Nazira basement low exhibits very low topographic elevation, which might be structurally controlled phenomena. On the other hand, very strong correlation of topography and basement depth can be noticed across the Brahmaputra river along east-west direction and along northeast-southwest over the north Brahmaputra plain. In both the cases, topographic elevations decrease with increasing basement depth towards west bank of Subansiri river. Thus, the prevailed topography of the region is not only controlled by aggradation rate of the rivers flowing from the surrounding mountain/hills ranges, the flexured basement has also played significant role, specially the basement lows and highs, although buried under a thick pile of sediment. A few prominent basement lows can be noticed within the Upper Assam plain near the foothills of Naga-Patkai ranges, Mishmi hills and Himalaya. All these basement structures are now buried under thick pile of Tertiary sediments and recent alluvium. However, active tectonic collision in the region is still influencing this flexure. Brahmaputra alluvial plain which is sandwiched between Himalaya and Naga-Patkai ranges is experiencing considerable tectonic Abstract iii stress. Some of the induced strain can be released by coseismic displacement along the faults in the overlying Tertiary rocks of the basement. Under such conditions coseismic subsidence may occur over the basement lows located near foothill regions. Under such tectonic regime the rivers of the region are actively changing their morphology. Extensive studies revealed that coseismic subsidence of the Subansiri basement low region in 1950 Assam earthquake have triggered westward migration of north-bank tributaries. Northward migration of main channel of Brahmaputra near Majuli can also be attributed to the same event. On the other hand, historical coseismic subsidence of the Nazira region over Nazira basement low might have led to course change of the Dikhow river and channel modification of the Disang river. Assam Embankment and Drainage Act, 1953, directed construction of earth embankments on either bank of all major rivers in the valley as a short term control for floods. Constructions were completed by 1990 for almost all the rivers. Since then, the flood situation of the valley has been improved comparatively. But, frequent breaching of those embankments during monsoon has become a major disaster for the state. Such events have been creating havoc in many regions every year, because breaching happens only during embankment full period of the river. Moreover, very high rate of aggradation of Himalayan rivers are decreasing the channel volume of the embanked north bank tributaries in the valley because of in-channel deposition. Thus a few rivers in the valley exhibit higher river-bed elevations than its own floodplain, specially the Jiadhal river. The river has avulsed numbers of times within the period of 1973-2016. Weak maintenance of the Jiadhal embankments may lead to similar avulsion of the river in future. DEM based surface hydrologic modelling of the valley revealed that many regions on the north bank of the river exhibits lower topographic elevation than the channel of the Brahmaputra. Moreover, those regions are not isolated, rather continuous and remnants of palaeochannels. They can provide a path for a new channel of the Brahmaputra river across the north bank. Topographic elevation along the present channel of Brahmaputra is at least 2 m higher than those palaeochannels, which is enough to trigger an avulsion. The river is being protected only by the earth embankment to prevent water flow over those regions. Breaching at certain locations of northern embankment may result avulsion of the river. Such breaching and subsequent avulsion of the Kosi river occurred on 18th August, 2008. The Kosi river changed Abstract iv its course leaving its elevated river bed. Similar avulsion of Lohit river had been noticed near Dibru Saikhowa region.