Estuarine fluctuation zone is the key area for pollutant reduction in tributaries in reservoir. Affected by runoff scouring, river channel downcutting is a common problem in estuarine fluctuation zone, which makes the low purification efficiency of the runoff. Aimed at the weak ability of pollutant reduction of the estuarine fluctuation zone, the technology of improving purification efficiency by transforming hydrological elements with multi-stage submerged dam, which can form a surface flow constructed wetland, was proposed. The estuarine fluctuation zone of the Bajia River in the Biliuhe Reservoir was taken as a case study in this paper. By comparing the water quality and Normalized Difference Vegetation Index (
NDVI), acquired through on-site monitoring and satellite imagery of Sentinel-2A and the situation before and after the construction, the influence of the project was analyzed. The water surface area and the water level as well as the hydraulic retention time in the estuary were all effectively improved. With the change of hydrological elements, the emergent plants grew well without human intervention, while the vegetation on the floodplain was significantly improved due to the increase of underground water level. Due to the influence of flooding, the pollutants on the land were released. The precipitation of the original pollutants in the flooded area has a significant impact on wetland water quality. The maximum removal rates of TN and
\rm NO_3^- 
-N reached 72.61% and 75.95% in July 2020, while those in October 2019 were 22.06% and 27.95%, respectively. As a whole, the removal rates were good in surface flow constructed wetlands in China, while the improvement of TP and NH
3-N removal efficiencies was not obvious, which may be caused by the precipitation of original pollutants. The hydrological elements transformation technology can effectively improve the purification capacity of the estuarine fluctuation zone and promote the reduction of pollutants in the basin.
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