Streamflow-sediment relations across the continuum of natural-to-post dam construction periods in the Himalayan river basin

Anthropogenic activities such as dam regulation have altered the streamflow and sediment relationships in the Himalayan River basins. The effect of dams and barrage operations on streamflow and suspended sediment has been widely studied, but the impact of dam construction in this context is poorly understood. The goal of the study is to create a conceptual framework to explore the shifts in streamflow-sediment interdependence across the continuum of natural-to-post dam construction periods in the Eastern Himalayan Tista River basin. Previous studies have either used sediment rating curve (SRC) or hysteresis, but we have employed both to answer whether these two methods are independently diagnostic of changing streamflow and sediment relations in different stages of dam development in the basin? The Tista basin will have the highest density of dams globally if all the proposed 29 dams are commissioned in the future. Currently, a total of 13 major dams for hydropower (>25 MW) in the mountain basin and a diversion barrage in the alluvial plain for irrigation are functional. Cumulatively, the reservoir of these dams and barrage can store ~89 million m³ of water. The interannual and inter-seasonal streamflow and suspended sediment data from the gauge station located at the alluvial plain downstream of all the 14 regulation structures were analysed for the pre-dam, dam-construction, and post-dam periods. We observed that the annual streamflow is predominantly determined by the heavy monsoon rainfall-runoff in the basin that reduced to 28% during the post-dam condition. The same in the non-monsoon post-dam condition was reduced by 58% mainly due to regulation to satisfy the sectoral demand for water. The mean annual sediment was recorded ~11 Mt, ~46 Mt and ~14 Mt during the pre-dam, dam construction and post-dam period, respectively, while the reservoir trapping reduced 56% of sediment during the non-monsoon post-dam period. The SRC exhibited that erosive behaviour (b-value) of the river increased due to massive streamflow during the monsoon season but fairly increased during annual post-dam condition suggesting the role of dam released sediment starved streamflow to erode. High a-value and clockwise hysteresis demonstrated the sediment-surplus condition due to dam construction activities, which altered the mountain landscape through deforestation and excavation of mountain slope, resulting in further erosion and sedimentation. The post-dam high a-value indicates that reservoirs released sediment downstream by drawdown-flushing with reduced streamflow which develops a complex single-valued hysteresis, implying controlled discharge and carrying capacity. Consequently, due to regulation, the uneven sediment distribution across the river continuum has increased flood vulnerability and riverbank erosion, constraining the ecosystem services. The findings of the study will be beneficial for policies on future water-sharing and management of sediment and flood by the river managers and hydropower companies.