Temporal dynamics of suspended sediment transport in two regulated rivers in Chile

Abstract

Dams alter the hydrosedimentary regime of rivers, not only modifying the longitudinal profile of the river and thus its energy gradient, but also trapping a significant portion of sediment load. Consequently, the transport of suspended and bedload sediments downstream is generally reduced. Chile exhibits diverse climatic conditions, ranging from the arid north to the humid south, with twenty-six large reservoirs. This study focuses on characterising suspended sediment transport at different temporal scales in two regulated rivers located in different climatic regions in Chile: the Loa River in the arid zone and the Biobío River in the humid zone. Additionally, sediment transport was studied in two unregulated rivers as benchmarks for natural regimes: the Salado River, a tributary of the Loa River, and the Diguillín River basin, adjacent to the Biobío River. Daily flow records and daily suspended sediment concentrations were obtained from monitoring stations located upstream and downstream of the reservoirs. Hydrological and sediment transport alterations were assessed using indicators at various temporal scales and in relation to flood magnitude and frequency. The results showed a highly scattered relationship between discharge and suspended sediment concentrations in both rivers. Contrary to expectations, the mean suspended sediment concentrations and loads were higher downstream in the Loa River, and similar upstream and downstream in the Biobío River, altogether being the consequence of the remarkable role of various large tributaries in supplying fine sediments between the reservoirs and the downstream monitoring stations. Cumulative suspended sediment frequency curves revealed that sediment transport was more irregular upstream, i.e., linked to the catchment dynamics of erosion fine sediment production, than downstream of the dams, evidencing the role of reservoirs at changing the temporal patterns of suspended sediment fluxes. Overall, reservoir regulation in the Loa and Biobío rivers did not appear to drastically alter their hydrology or sediment transport, though it did generate more constant trends of suspended sediment load downstream.