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J. Earth Syst. Sci. (2019) 128:211 Ó Indian Academy of Sciences https://doi.org/10.1007/s12040-019-1233-3 (0123456789().,-vol.:V)( 0123456789().,-vol.:V) Erosion–deposition and land use/land cover of the Brahmaputra river in Assam, India 1, 2 3 LALIT SAIKIA * ,CHANDAN MAHANTA ,ABHIJIT MUKHERJEE 4 and SURANJANA BHASWATI BORAH 1 Department of Earth Science, University of Science and Technology Meghalaya, 9th Mile, Meghalaya, India. 2 Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati, India. 3 Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, Kharagpur, India. 4 Department of Environmental Science, Tezpur University, Tezpur, India. *Corresponding author. e-mail: [email protected] [email protected] MS received 16 August 2018; revised 17 May 2019; accepted 24 May 2019 The Brahmaputra is a unique dynamic river in the world with intense braiding and critical bank erosion. Both erosion and deposition are continuous processes in the river in an attempt to reach a new equi- librium in channel geometry and morphology by the ever dynamic nature of flow. Erosion and deposition of the river have link to land use and land cover (LULC) as the land cover is under constant change in a dynamic landscape constantly shaped by continuous erosion and deposition. The objective of the present work is to evaluate the extent of erosion and deposition along the Brahmaputra river and change in the LULC of the Brahmaputra river in Assam, India. Remote sensing and geographic information system (GIS) techniques were utilised to extract information from Landsat images. Total area of erosion and deposition during 1973–2014 was 1557 and 204 km2, respectively. Increase in area (28%) of the Brahmaputra during 1973–2014 is not solely due to bank erosion, but also for the bifurcation of streams without the loss of land. LULC study has revealed that 29% area was occupied by active channels and 71% was occupied by bars in 2014. Maximum reaches experienced reduction of the submerged part in 2014 compared to 1994 in the post-monsoon months with an overall decrease from 37% to 29%. A reduction in natural grassland and forest has been observed with a corresponding increase in agricultural practices in different bars and islands of the Brahmaputra in Assam during 1994–2014. Keywords. Alluvial river; erosion–deposition; LULC; Brahmaputra; Assam. 1. Introduction (Church 2006). Different categories of alluvial river patterns, i.e., straight, meandering and braided Erosion, transportation and deposition are the three rivers (Leopold and Wolman 1957)areduetothe major processes in a fluvial system (Schumm 1977) sediment supply along with the flow regime. Brai- that are influenced by the supply of the sediment at ded rivers are those that flow in two or more the upstream end and sediment that is locally ero- channels around alluvial bars or islands. The pri- ded from the bed and banks (Charlton 2008). Thus, mary causes of braided channels are the overloading the morphological changes of an alluvial river of sediments and steep slopes (Lane 1957). Highly channel are the consequence of sediment erosion, variable discharge, erodible banks and a high width/ sediment transport and sedimentation in the river depth ratio are the other factors responsible for 211 Page 2 of 12 J. Earth Syst. Sci. (2019) 128:211 braiding (Schumm and Kahn 1972; Knighton 1998). (within Assam) were 544 and 920 km2, respec- They have a braided look at the low flow stages with tively, whereas the corresponding amount exposed bars, but all or some of the bars are of deposition were 145 and 68 km2 (Sarkar et al. submerged during the high flow stages. 2012). The recurrent erosion has caused irreparable The Brahmaputra is an extremely dynamic and damage to many important places along the river predominantly braided river (Coleman 1969) in the bank in addition to the permanent loss of cultivable world. The Brahmaputra originates as the Yarlung and homestead lands. Tsangpo from the Angsi glacier near the Mana- Both erosion and deposition are continuous sarovar lake in the Kailash range in southern processes in the river in an attempt to reach a new Tibet, and it is a trans-boundary river flowing from equilibrium in channel geometry and morphology by the Himalayas to the Bay of Bengal through China ever dynamic nature of flow. Erosion and deposition (Tibet), India and Bangladesh. The river is unique of the river has link to land use and land cover due to its peculiar drainage pattern, diverse geo- (LULC) since the land cover is under constant logical setting, high sediment load and critical change in a dynamic landscape constantly shaped bank erosion problem (Mahanta and Saikia 2015). by continuous erosion and deposition. The objective The Brahmaputra supplies an estimated 1000 of the present work is to evaluate the extent of million ton of suspended material to the Bay of erosion and deposition along the Brahmaputra river Bengal (Milliman and Meade 1983; Milliman and and change in the LULC of the river. In the post- Syvitski 1992; Hay 1998). There is an additional monsoon season, the most important use of the strong indication that erosion is a major factor of LULC study is to evaluate the newly emerged river instability in the Brahmaputra due to the morphology after the flood and erosion events. This very large amount of sediment intrusion from bank will help in planning mitigation measures for both erosion itself. This sediment causes further insta- flood and erosion in a correct perspective, instead of bility downstream, triggers more bank erosion, and drawing from the morphology of the previous year apart from the loss of land and flood protection, which undergoes much change during the monsoon hampers navigation (ADB 2007). The valley of the flood. In this paper, the erosion and deposition Brahmaputra river has been facing a heavy insta- behaviour of the Brahmaputra river was directly bility of landmass due to river bank erosion, studied for 1973 and 2014. LULC due to erosion and believed to be accelerated after the 1950 earth- deposition was studied for a later period covering quake. The stretch falling within Assam, India, has 20 yrs, i.e., 1994–2014. Time periods of 1973–2014 already lost about 7.4% of its total land due to for the erosion–deposition study and 1994–2014 for bank erosion and channel migration (Kakati and the LULC study were chosen as per the availability Changkakati 2013). Erosion during 1990–2008 in of satellite data and to study the change during the the north bank and south bank of the Brahmaputra 20-yr period. Figure 1. Map showing different reaches of the Brahmaputra river in Assam. J. Earth Syst. Sci. (2019) 128:211 Page 3 of 12 211 2. Materials and methods pre-processed for image enhancement techniques like haze reduction, brightness and contrast to make the The erosion–deposition of the Brahmaputra river process of information extraction easier. Images were for the period 1973–2014 was studied from Landsat then stitched to create a single seamless mosaic images using remote sensing (ERDAS Imagine) image for the entire stretch of the river which was and GIS techniques (ArcGIS 10.1). utilised to visually interpret and extract the bank Images were procured for the same season of the line. The Brahmaputra river in Assam in three dif- year to minimise the inconsistencies in the data. ferent years, i.e., 1973, 1994 and 2014 was divided Post-monsoon data were used due to low cloud cover into 16 reaches at a length of 40 km and the reaches and proper channel and sandbar definition available were numbered from 1 upstream to 16 downstream during this season. Raw data consisting of individ- as shown in figure 1. Erosion–deposition and LULC ual bands of each satellite image were combined were studied for different reaches to know the in ERDAS Imagine image processing software to longitudinal variation. create a composite image. The images were then LULC analysis for 1994–2014 was carried out by categorising the Landsat images (table 1) into dif- Table 1. Satellite dataset used. ferent classes, i.e., river/water, sandbar, vegetation (including natural grass land) and agriculture (in- Spatial cluding human settlement), using unsupervised Sl. Path/ Date of resolution classification in image processing software, i.e., no. Sensor row acquisition (m) ERDAS Imagine. Error in the unsupervised clas- 1 MSS 145/41 15-11-1973 60 sification of the LULC maps of 1994 and 2014 were 2 MSS 146/41 16-11-1973 15% and 12.5%, respectively. The overall classifi- 3 MSS 147/41 05-12-1973 cation accuracy was 85% and 87.5% in 1994 and 4 MSS 147/42 22-11-1973 2014, respectively (table 2). 5 MSS 148/42 21-02-1973 6 TM 135/41 20-11-1994 30 7 TM 136/41 26-10-1994 3. Results and discussions 8 TM 136/42 26-10-1994 9 TM 137/42 01-10-1994 3.1 Erosion and deposition in the Brahmaputra 10 TM 138/42 11-12-1994 in Assam 11 OLI 135/41 27-11-2014 12 OLI 136/41 18-11-2014 Both erosion and deposition of the channel mate- 13 OLI 136/42 18-11-2014 rials continue to be happening constantly. Erosion 14 OLI 137/42 09-11-2014 and deposition in both the banks of the Brahma- 15 OLI 138/42 02-12-2014 putra river in Assam during 1973–2014 are shown Table 2.. Error matrices. Reference data Producer User accuracy Classified data River Sandbar Agriculture Vegetation accuracy (%) (%) Kappa Error matrix for 2014 classification River 10 0 0 0 90.91 100 1 Sandbar 0 10 0 0 71.43 100 1 Agriculture 1 2 7 0 100.00 70 0.6364 Vegetation 0 2 0 8 100.00 80 0.75 Overall classification accuracy 87.50% Overall kappa statistics 0.83 Error matrix for 1994 classification Water 8 2 0 0 80.00 80 0.7333 Sand 0 9 0 1 81.82 90 0.8621 Agriculture 1 0 8 1 100.00 80 0.75 Vegetation 1 0 0 9 81.82 90 0.8621 Overall classification accuracy 85.00% Overall kappa statistics 0.8 211 Page 4 of 12 J.

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