The Red River is an typical example of the South East Asian rivers which have been impacted by bothnatural conditions and human activities. Recently, a series of dams impounded in the river hasaccelerated a significant decrease of the total suspended solids (TSS) flux. The decrease of the riverineTSS flux may lead to the reduction of some associated elements (C, N, P) and has impacted onecological functioning of the downstream section. In this paper, we aim to investigate the presentconcentrations of particulate organic carbon (POC) and chlorophyll a (Chl-a) concentrations in theRed River downstream and then calculate the ratio of POC/Chl-a for the present situation. Theobservation results in the downstream section from Hanoi to Ba Lat during 4 sampling campaigns in2019 showed that POC concentrations varied from 0.6 to 2.48 mgC/L, averaging 1.59 ± 0.46 mgC/Lwhereas the Chl-a concentration ranged from 0.16 to 2.06 µgChl-a/L, averaging 0.57 ± 0.42 µgChla/L. The ratio POC/Chl-a fluctuated in the wide range from 973 to 14,419 mgC/mg Chl-a, averaging4141 ± 2923 mgC/mgChl-a. The high values of the POC/Chl-a ratio for the River River downstreamindicated that the origin of organic matter in the river water mainly came from the soil leaching anderosion in its watershed. This value was higher than the previously observed value may suggest thechange in phytoplankton development in the Red River downstream hydrosystem, reflecting impact ofchange in nutrient supply on riverine ecology.Keywords: POC, organic carbon, Chlorophyll-a, erosion, dam impact, Red River