RESEARCH ARTICLES CURRENT SCIENCE, VOL. 112, NO. 7, 10 APRIL 2017 1533 *For correspondence. (e-mail: tapas1976@gmail.com) Study of channel instability in the braided Brahmaputra river using satellite imagery Tapas Karmaker 1, *, Hemanta Medhi 2 and Subashisa Dutta 3 1 Department of Civil Engineering, Thapar University, Patiala 147 004, India 2 Department of Civil Engineering, Indian Institute of Technology Kanpur, Kanpur 208 016, India 3 Department of Civil Engineering, Indian Institute of Technology Guwahati, Guwahati 781 039, India In the present study, instability of the river reach of Brahmaputra was analysed for braided belt width changes, braiding index and bar area. The river reach of the Brahmaputra from its confluence of Lohit, Dibang and Dihang to its confluence with the Tista river was studied from 1973 to 2009. The study was carried out using remotely sensed data from Landsat satellites at different dates. Discharge data synchro- nized with satellite data was collected by maintaining near-similar water level or discharge. Wavelet of the braided belt change was analysed to get the wavelet power and spatial extent of the changes. Finally, stream power was analysed from the average dis- charge data during the monsoon period to determine its effect on the instability of parameters considered. Results indicate that stream power does not directly relate to local changes in the braided belt or braiding index. However, with decrease in stream power, an in- creasing trend of bar area was found. Maximum wavelet power within a period showed a threshold be- haviour at stream power of 5 W/km, beyond which the wavelet power raised sharply to a high value with in- crease in stream power. River response to the stream power was found at a global level rather than local level. Finally, a gradual decrease in stream power over time indicates the stable river reach. However, changes due to local bank erosion cannot be predicted using this analysis. Keywords: Braiding index, braided belt, plan form, stream power. BRAHMAPUTRA river is one of the world’s largest, braided and dynamic river systems. The dynamics of the river are attributed to high rates of riverbank erosion, in- crease in braided belt, change in channel courses and braiding pattern 1 . Also, due to high rates of bank erosion and frequent changes in the channel flow, it becomes dif- ficult to take efficient river training measures 2 . It is there- fore necessary to understand the dynamics of the river and identify the stable and unstable reaches through tem- poral analysis. Continuous monitoring of river’s morpho- logical changes can be considered as a cost-effective measure for efficient management system of the bank erosion problem 3 . For an efficient morphological study of Brahmaputra river, spanning over hundreds of kilometre length, it is necessary to carry out multi-date satellite re- mote sensing data analysis. Brahmaputra flows a total distance of approximately 2900 km from its origin at Manasarovar (Tibet) to Bay of Bengal. The river mainly flows from east to west (in India) up to the Indo-Bangladesh border and then from north to south (in Bangladesh). The braided belt varies considerably from ~1.3 km at Pandu (near Guwahati) to 18.5 km near the Indo-Bangladesh border as studied with the help of Landsat-7 satellite imagery in 2009. Notably, the narrowest reach (Pandu) of the river is bedrock- controlled and hence no bank erosion was evident. The river flows through the Himalayan valley which is com- posed of Paleozoic sediments 4 . The eastern side and the southern side (Sillong Plateau) of the river are composed of high-grade metamorphites, gneiss, schists and granites overlain by metasediments 4,5 . The south-eastern side of the river basin is composed of sandstones, shales and mottled clays which are easily erodible. The longitudinal bed slope varies from 1 in 3000 near Passighat to 1 in 12700 near Dhubri 6 . The high annual rainfall (100– 600 cm) in this river basin is linked with high magnitude floods (bank full stage and higher) during monsoon every year. The discharge of the river varies widely from 1757 m 3 s –1 (20 February 1968) to a maximum of 72,984 m 3 s –1 (23 August 1962) as recorded at Pandu (Guwahati) gauging station. Bankline migration and fluvial erosion of Brahmaputra have been studied earlier 7–11 with Survey of India to- posheets and/or satellite imagery. Recent studies on river morphology have extensively used satellite remote sens- ing imagery 3,12–17 . Takagi et al. 18 studied the river reach of Brahmaputra in Bangladesh and concluded that the river is in dynamic equilibrium stage with its stabilization waves propagating to the downstream. Baki and Gan 14 reported that the larger sandbars in Brahmaputra are more stable than the smaller ones. Akhtar et al. 3 examined the stream power relation with braiding pattern and con- cluded that with low stream power, braiding intensity increases, which in turn intensify bank erosion. Lahiri and Sinha 16 carried out a geomorphologic study of Majuli