THE IMPACT OF RIVER REGULATION ON STREAMFLOW AND SEDIMENT DYNAMICS IN THE EASTERN HIMALAYAN RIVER BASIN

Anthropogenic activities such as dam regulation have altered the streamflow and sediment relationships in the Himalayan river basins. While the effect of dam operations on streamflow and suspended sediment concentrations (SSC) have been widely studied, the effects of dam construction in this context are poorly understood. The goal is to create a conceptual framework to explore the shifts in the streamflow-SSC interdependence across the continuum of natural-to-post dam construction river flows. The river Tista in the Eastern Himalaya will have the highest density of dams in the world, if all the proposed 26 dams commissioned in the future. Currently, a total of 13 major run-of-the-river (RoR) hydropower dams (>25 MW) and a diversion barrage are operating in the basin. We assumed that these dams have altered the downstream streamflow and SSC relations during and after the construction stages. The objectives are two-folded 1) to characterize the effect of dam constructions based on the changes in the streamflow-SSC associations during the three phases, the pre-dam, dam constructions and post-dam, 2) to quantify the impact of reservoirs and diversion on streamflow variations using the river regulation index (RRi). We observe that pre-dam mean annual streamflow was reduced from 345,000 to 247,000 cumecs during dam constructions and 252,000 cumecs during post-dam period. At the same time, the annual total rainfall has shown marginally increasing trend. Conversely, the average SSC increased to 46 Mt/yr during the dam construction period, compared with 11 Mt/yr in pre-dam and 14 Mt/yr during post-dam. Such abrupt increase has an impact on the streamflow and SSC relations (R²=0.46) relative to pre- and post-dam period (R²>0.8). The R² Value reduced by the increasing RRi value due to variations in regulations during post-dam. However, the RRi value is sensitive to barrage diversion than the reservoirs of the RoR dams that hardly hold the water. We conclude that the abrupt sediment injections were principally determined by the local area geology and the stream order on which the dams were constructed. The study of streamflow and sediment is essential to understand the flooding and riverbank erosion downstream of the regulation structures for future planning and management of dam constructions and operations in the Himalayan river basin.