Impact of hydropower dam on total suspended sediment and total organic nitrogen fluxes of the Red River (Vietnam)

Abstract. The Red River is a typical example of the Southeast Asian rivers, which has been strongly affected by human activities. This paper analyses the change of total suspended sediment (TSS) load of the Red River from 1960 to 2015 in which numerous new dams in both China and Vietnam have been constructed. A strong decrease of TSS load of the whole Red River (from 79 ± 26 × 10 6  t yr −1 in 1960s to 6 ± 1 × 10 6  t yr −1 in 2010s) allocated to the dam impoundments in spite of population and deforestation increase. Base on the experimental equation describing the relationship between TSS and total organic nitrogen (TON) concentrations, and on the available data of TSS concentration and river discharge, the longterm TON concentrations and fluxes were calculated for the three tributaries and the whole Red River. The annual average of TON concentrations spatially varied from 0.41 to 3.19 mg L −1 , averaging 0.98 mg L −1 for the whole period; the lowest was found for the Da River where the new dams have been impounded. The highest TON concentrations and fluxes occurred in the wet season in relationship with the highest sediment loads and river discharges. The riverine TON fluxes transferred to estuary significantly decreased from 141 × 10 3 ± 38 × 10 3  t yr −1 (equivalent to 902±247  kg km −2  yr −1 ) in 1960s to 32 × 10 3 ± 5 × 10 3  t yr −1 (equivalent to 207±35  kg km −2  yr −1 ) in 2010s. The TSS flux decrease has driven a clear reduction of associated elements like nitrogen, which let to hypothesis a change in biogeochemical processes in the coastal zone.