Impact of human intervention structures on the rivers: An investigation of the spatiotemporal variation of grain size in the Tista River, eastern Himalayas

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Research Article

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Earth Surface Processes and Landforms


In order to investigate the effect of intervention structures like dams and barrage on the sediment grain size distribution, we examined a ~75 km stretch of the Tista River in the foothill alluvial plain of the eastern Himalayas. However, studies on the impact of dams on streamflow and sediment in the Himalayan rivers are constrained due to inadequate historical gauge data. Therefore, we employed a simple method to track the initial formation of some of the existing river landforms that formed before the dam/barrage operations from yearly satellite images (1987–2017) for in situ sediment sample collection. Hence, the collected sediment grain sizes analysis is used here as a proxy to comprehend the pre-, syn- and post-dam river sediment-transport pattern. We have grouped 23 landforms into three temporal clusters: (i) those formed before 1997, earlier than the dams/barrage interventions (TOL); (ii) formed between 1998 and 2010 during the upstream dam construction (TML) and (iii) formed after 2010 (TNL) when upstream dams and the barrage were fully operational. A total of 116 sediment samples from these landforms indicate the overall grain size reduction for TML and TNL sediment due to reservoir sequestration of coarser grains. In comparison, the coarsest TOL sediment with downstream fining trend represents the pristine river condition. The coarsening trend with a poor correlation coefficient of TML is primarily assigned to partial trapping of the coarse bedload and massive sediment generation by the upstream under-construction dams, resulting in the downstream erosion of the riverbed and transport of the coarser bedload further down. Similarly, post-dam sediment trapping and controlled flow were responsible for the weaker trend with a distinctly finer size of TNL. The historical satellite images and data on streamflow and suspended sediment reveal that contemporary river behaviour is consistent with the earlier interpretation.

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