International Journal of Geology, Earth & Environmental Sciences ISSN: 2277-2081 (Online) An Open Access, Online International Journal Available at http://www.cibtech.org/jgee.htm 2016 Vol. 6 (1) January-April, pp. 43-53/Ahmed et al. Research Article © Copyright 2014 | Centre for Info Bio Technology (CIBTech) 43 A STUDY ON SUSPENDED SEDIMENT DISCHARGE AND BED LOAD GRAIN SIZE OF THE GUMTI RIVER OF TRIPURA AT SOME SELECTED TRIBUTARY CONFLUENCE POINTS *Istak Ahmed, Nibedita Das (Pan) and Jatan Debnath Department of Geography and Disaster Management, Tripura University *Author for Correspondence ABSTRACT The suspended sediment load refers to the fine sediment that is transported in suspension, with only infrequent contact with the bed, whereas bed load sediment is sediment that is transported along the bed. As a river flows downstream it receives lots of tributaries at regular intervals. The tributaries have their own sediment load and their meeting with the main stream leads to a significant increase in the load of the main stream. Gumti River is the largest river of Tripura which is regulated by the Dumbur Dam in its upstream. After 18.9 km away from the dam, the River Gumti started to receive its major tributaries from both sides, which influence the sediment discharge and grain size of the river. In this paper the researchers intend to identify the impact of five major tributaries on the suspended sediment discharge and grain size distribution of bed sediment of the Gumti River. For this purpose cross sections have been taken immediately before and after the tributary confluence points. Besides, suspended sediment concentration and mean grain size of bed sediment have also been measured. The result indicates that all five tributaries have their influence in increasing the suspended sediment discharge of the Gumti River by 20% to 138%. Besides, the mean grain size of bed load varies from fine sand to medium sand in each upstream and downstream of tributary junctions respectively. Keywords: Gumti River, Sediment Concentration, Suspended Sediment Discharge, Grain Size INTRODUCTION Confluence of a tributary with its trunk stream is very much significant to change the water and sediment discharge of that river. Channel confluence is an important component of river system because it affects the morphology and hydrology of the reaches upstream and downstream from the confluence point (Roy and Sinha, 2007). The experimental works of Mosley, 1976; Best and Roy, 1991; Biron et al., 1993a, 2002; Bristow et al., 1993; Axtmann et al., 1997; De Serres et al., 1999; Rhoads and Sukhodolov, 2001, 2004; Stevaux et al., 2009 etc. are very much effective in the study of river confluence. Moreover, during last ten years there has been large scale increase in the study of confluence dynamics (e.g., Parsons et al., 2007, Szupiany et al., 2009, Leite Ribeiro et al., 2012 and Constantinescu et al., 2012). Confluences are key features of river network which provide diverse flow conditions that influence physical channel processes and biological processes (Benda et al., 2004). When two channels meet together, each of having independent flow and sediment discharge regimes creates unique erosional and depositional environments with consequent changes in channel morphology at confluences (Miller, 1958; Best, 1986 and 1988). Due to the abrupt introduction of sediment and organic materials at confluences, different types of changes in channel morphology in the vicinity of confluences takes place (Chakraborty and Mukhopadhyay, 2014). Confluence is very much important for the influx of extra discharge and sediment load to the main stream. The flow structure of main stream goes through unique change at the junction of tributary. Tributaries have their own sediment load. When it is added to the main stream at tributary junction, it experiences sudden increase in sediment load. This sudden increase in load can change the overall channel profile. Higher the input of sediments from the tributaries, greater will be the changes in physical characteristics of the main stream. Besides, change in the mean size of the river bed sediment also took place at the tributary junction. The magnitude of this change depends on the energy level of the tributary to carry