Hysteresis and streamflow-sediment relations across the pre-to-post dam construction continuum in a highly regulated transboundary Himalayan River basin

Worldwide more than 16 million reservoirs of dams have trapped 65 Gt of fluvial sediment each year, affecting river ecosystems and human livelihoods. It is important to understand the effect of dam construction and possible shifts in hydrologic regimes. This study aims to create a conceptual framework to explore the interdependent streamflow and suspended sediment load (SSL-Q) shifts across the natural-to-post dam construction continuum in a transboundary Himalayan river basin. We study the sediment supply, erosivity, and hysteresis for sediment provenance and transportation to answer whether the changing patterns of the SSL-Q relationships can explain the cascading effects from natural to operational dam phases during the monsoon and non-monsoon seasons. Results indicate the disruptive effect of dam construction on increasing sediment loads. Such effect contrasts with the sediment trapping in the post-construction stage, which is exacerbated during the non-monsoon season. However, during the monsoon season, dam-released sediment-starved streamflow leads to increased erosion in the basin. Natural and dam-construction hysteresis evidenced an anticlockwise and clockwise figure-eight pattern with variability in SSL-Q scattering, respectively. The regime shift responds to the underlying pro-cesses that the natural sediment regime shifted to sediment-surplus and sediment-deficit conditions during the construction and post-construction phases, respectively. In the post-construction phase, the relationship between streamflow and SSL-Q creates a quasi-linear hysteresis pattern, implying that the dam-controlled discharge altered the carrying capacity in the basin. These patterns can help build resilience in systems affected by the increase in dam construction and the inherent hydropower demand.