J. Geographical Studies, 2(2), 62-78, 2018. S. Dey and S. Mandal 62 Original Research Paper Fluvial Processes and Channel Stability of the Torsa River, West Bengal (India) Sourav Dey 1 *, Sujit Mandal 2 1. Department of Geography, University of Gour Banga, Malda, West Bengal, India. 2. Department of Geography, Diamond Harbour Women’s University, South 24 Parganas, West Bengal, India. Abstract Fluvial processes such as bank erosion plays an important role to change the channel stability of the Torsa River in the downstream region. The present study was focused on stream stability assessment of the Torsa River. The study area is situated between the downstream of the Jaldapara Reserve Forest and confluence of Kaljani River. Data of different parameters about 64 bank segments of the Torsa River were prepared using the field work techniques for assessing the stream bank conditions using lateral, vertical and overall reach stability models. The individual results of BEHI and NBS ratings show that out of 64 bank segments only 35 and 19 bank segments classified in higher categories. Overall lateral stability analysis shows that most of the sample bank segments are in an unstable condition. All bank segments are vertically unstable and degrading. Overall reach stability analysis shows widespread instability. BEHI and NBS results are almost similar for most of the bank segments and therefore, BEHI and NBS can be suitable bank erosion hazard predictive models in the study for channel stability analysis. Article history Received: 10 February 2019 Revised: 23 March 2019 Accepted: 30 March 2019 Keywords Bank Stability; Bank Erosion Hazard Index; Near Bank Stress; Bank Erosion Vulnerability Zone; Remote Sensing; GIS. Editor(s) M. A. Siddiqui 1 INTRODUCTION River scientists define that the term ‘channel stability’ evokes a deluge of different interpretations, i.e., ‘equilibrium’, ‘regime channels’ and ‘quasi-equilibrium’ etc. (Rosgen, 2001b). Stream stability is defined as the ability of stream to keep up its geometry and bathymetry without either erosion or deposition. Stability of stream channel depends on the present climatic condition, sediment load transport and water flows produced in the watershed (Rosgen, 1996 and 2001b). Stream instability wants to be evaluated on the spatial and temporal level (Rosgen, 2001b). Stream instability is too critical to recognize natural erosion and mechanics of transport versus human influences (Rosgen, 2001b). Stream instability did not make due to the extensive sediment load and corresponding record of flood (Rosgen, 2001b). River channels that have been unscientifically managed and have less cohesive bank stratigraphy as well as have less riparian vegetation are subjected to accelerated stream bank erosion vulnerability and equivalent channel adjustments are leading to channel instability (Rosgen, 2001b). Stream bank erosion is considered as the potential threat to the riparian areas, because the resources, properties and lives associated with the land on either side of the river is devoured (Maiti, 2016). Form and course of the river channel adjustment and floodplain development depends on the stream bank erosion, which also threatens man-made structures and destroys valuable agricultural land (Knighton, 1998). Bank erosion of stream is an intricate natural process working in a river valley. Stream bank erosion is one of the principal means of sediment supply to streams (Knighton, 1998). Two predominant processes are involved in stream bank erosion, i.e. (1) Hydraulic action, and (2) Mass failure (Knighton, 1998). Hence, some major processes include in stream bank erosion, i.e. surface erosion, entrainment of flowing water (detachment of particle by flowing water at the bank toe), liquefaction or collapse, positive pore water * Author’s address for correspondence Department of Geography, University of Gour Banga, Malda, West Bengal India. Mob.: +91 9476272499 Emails: sourav.cob.dey@gmail.com (S. Dey -Corresponding author), mandalsujit2009@gmail.com (S. Mandal). http://dx.doi.org/10.21523/gcj5.18020202 © 2018 GATHA COGNITION ® All rights reserved.