Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

G. B. Gilfellon ¹, J. N. Sarma ², K. Gohain ³

Affiliations
1 Research and Development Department, Oil India Limited, Duliajan - 786 602, Assam, IN
2 Department of Applied Geology, Dibrugarh University, Dibrugarh - 786 004, Assam, IN
3 Geological Department, Oil India Limited, Duliajan - 786 602, Assam, IN

Abstract

The Brahmaputra, one of the largest braided rivers of the world, flows through three countries, viz. China (Tibet), India (Arunachal Pradesh and Assam) and Bangladesh. A study on a 183 km long reach of the river in the eastern part of the Assam Valley reveals that the average bed levels were consistent in some sections and variable in others at different times. The Brahmaputra consists of primary and secondary channels. Primary channels are large, deep and active, while secondary channels are much smaller and shallower than the primary channels. The width and depth of the primary channels as well as the bed profiles vary significantly from place to place with time. Four topographic levels have been recognized on the Brahmaputra river bed. Levels 1 and 2 are subjected to modification by low flows, while level 3 is sculptured by monsoonal flows. Level 4 is affected by peak discharge during the monsoon months. The river bed of the Brahmaputra is infested with bars and islands. There are basically two types of bars, namely, compound bars consisting of mid-channel and side bars which have complex depositional histories attached to them, and unit bars with simpler depositional histories. The Brahmaputra channel has widened significantly with time, and its cross-sections have also changed with time. Migration of the banklines of the river has been inconsistent with distance and time.

Keywords

Geomorphology, Brahmaputra River, Assam.

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D. Bezbaruah ¹, P. Kotoky ¹, J. Baruah ¹, J. N. Sarma ²

Affiliations
1 Regional Research Laboratory, Jorhat - 785 006, IN
2 Dibrugarh University, Dibrugarh - 786 004, IN

Abstract

Wetlands are conspicuous features of the Rood plains along the Brahmaputra River, Assam. The sequential change in numbers and areas of wetlands (Bils) for a stretch of 260km have been studied using available topographic maps of the Survey of India (1914,1975) and Indian Remote Sensing satellite imagery (1998). The total number of swamps as evident from the study was 529 in 19 14,783 in 1975 and 781 in 1998. Moreover, total areas covered by the wetlands as observed was 93.73 km² in 1914, 161.23 km km² in 1975 and 182.73 km km² in 1998. The low lying nature of the basin, shifting of river channels in a relatively short span of time, associated tectonics of the region and allogenic processes in relation to flood control measures adopted have played a major role in extensive changes to the wetlands over this period. The data presented here aid in formulating an effective flood control strategy to be adopted for this perennial flood prone area, stabilisation of landforms and different wetland management approaches.

Keywords

Gepmorphology, Swamp, Floodplain, Brahmaputra River, Assam.

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J. N. Sarma ¹, M. K. Phukan ¹

Affiliations
1 Department of Applied Geology, Dibrugarh University, Assam - 786 004, IN

Abstract

A study was carried out to assess the amount of bank erosion and bankline migration of the Brahmaputra River in Assam, during the twentieth century in two different periods. The study reveals that the net effect during the first period from 1912-28 to 1963-75 was a loss of 980 km² area due to bank erosion whereas in the second period from 1963-75 to 1996 there was a gain of about 112 km² new area due to fill-Up of earlier channel. The overall effect of erosion from 1912 to 1996 was a loss of about 868 km² area along the entire length of the river in Assam. The maximum average annual rate of bank erosion and fill were respectively 0 178 km² and 0 343 km² per unit length of the channel. The shifting of the banklines occurred both in north and south directions. The net maximum shift of north bank towards north was observed as 7740 m, at an average annual rate of 97 m, whereas that of north bank towards south was 8754 m at an average annual rate of 129 m. The net maximum shift of south bank towards south was 7090 m, at an average annual rate of 86 m, whereas that of south bank towards north was 6375 m, at an average annual rate of 59 m. The study reveals presence of alternate southward and northward shifting in the Brahmaputra similar to a meandering river. The bank erosion is dependent upon composition of bank materials.

Keywords

Brahmaputra River, Bank Erosion, Bankline Migration, Channel-Fill, Assam.

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