Effect of water-sediment regulation of the Xiaolangdi reservoir on the concentrations, characteristics, and fluxes of suspended sediment and organic carbon in the Yellow River.

Water-sediment regulation (WSR) of the Xiaolangdi Reservoir in the Yellow River is different from other water conservancy projects, with sediment resuspending along the river downstream of the reservoir during water regulation while some suspended sediment depositing during sediment regulation. In this study, samples were collected before, during, and after WSR to investigate the effect of WSR on the suspended sediment and organic carbon downstream of the reservoir. The suspended sediment concentration ([SPS]) increased with the river flow velocity (V) as a power function ([SPS]=1.348V(2.519)) during the three periods. The suspended sediment grain size decreased along the river during water and sediment regulations and after WSR; they were generally below 200μm with the fine particles (<50μm) of 68.0%-93.7% and positively correlated with the flow velocity. The black carbon content in suspended sediment elevated along the river during both water and sediment regulations, and it increased with 2-50μm fraction during water regulation and with <2μm fraction during sediment regulation, suggesting that black carbon mainly exists in fine particles and is influenced by both suspended sediment source and characteristics. There was no significant difference in dissolved organic carbon (DOC) concentration during water regulation, sediment regulation, and after WSR, inferring that the effect of sediment resuspension/deposition on DOC concentration was insignificant. The contribution of DOC flux (27.3%) during WSR period to the annual flux was comparable to that (22.6%) of water, but lower than the sediment (32.5%) and particulate organic carbon (POC) (49.5%). This study suggests that WSR will exert significant influence on the concentrations, characteristics and fluxes of POC (p<0.05) and sediment (p<0.05) but have no significant influence on DOC (p>0.1) of the Yellow River.