Fine Grained Braided River Sedimentation in Brahmaputra River, Majuli, Assam, India

The Brahmaputra River in Assam exhibits a typical braided pattern and is characterized by a sediment load comprising of sand and mud. The river contains channel-bars of varied types and dimensions and embraces the world's largest river island - Majuli, which at present is under the effect of severe bank-erosion along its southern margin. The objective of this study is to examine the characteristics of fluvial sedimentation prevailing along the southern margin of the Majuli landmass. Both channel as well as bank sediments were analyzed to understand the mechanism of transportation, depositional process and hydrodynamic behavior. Both channel and bank sediments consist of very fine to medium sand with subordinate amount of silt and mud. Majority of the sediments exhibit well to very well sorting and positive skewness. The Probability curves of grain size distribution indicate transportation of sediments dominantly by saltation mode with minor amount by suspension and surface-creep mode. CM plot suggest transportation by graded suspension (saltation) with subordinate rolling and uniform suspension process by means of tractive current. The internal stratifications are dominated by planar cross stratifications followed by horizontal stratifications, trough cross stratifications, ripple laminated layers and massive sand. The bank sediments in few locations exhibit a general fining upward sequence which ends up with the deposition of a mud layer. The lithofacies sequences of the study area show prevalence of both lower as well as upper flow regime energy conditions indicating fluctuations of energy conditions during their respective sedimentation.