Analysis on characteristics and causes of water quality change in Danjiangkou Reservoir bays during full impoundment period

The full impoundment of a reservoir is usually accompanied by an increase in pollution loads and inundation and release of the fluctuating zone, which has a great impact on the water quality of a reservoir, especially the water quality in bays.Based on the monitoring data of eutrophication in 20 typical bays at Danjiangkou Reservoir from 2021 to 2023, the overall spatial and temporal characteristics of water quality in these bays were analyzed.A comprehensive trophic level index method was used to evaluate the nutritional status of the typical bays, and the characteristics and causes of water quality changes in the bays after the full impoundment of the reservoir in the autumn of 2021 and the autumn of 2023 were discussed in depth.The results showed that: ① From 2021 to 2023, the water quality in the reservoir bays fluctuated, and the nutritional status was generally mesotrophic.The water quality in the tail areas of Han Reservoir and Dan Reservoir was relatively poor, and eutrophication occurred in some reservoir bays.② After the first full impoundment, the concentrations of dissolved oxygen, total phosphorus, and chlorophyll a in the bays showed a decreasing trend, and the concentrations of total nitrogen, permanganate index, and turbidity increased significantly.After the second full impoundment, the concentrations of dissolved oxygen and total phosphorus showed a decreasing trend, and the concentrations of total nitrogen, permanganate index, and chlorophyll a showed an increasing trend.Before and after the two full impoundments, the nutritional status of the bays remained stable.③ The heavy rainfall process during the autumn flood season greatly influenced the increase of nitrogen and organic matter in the reservoir bays, and the dilution and purification effect after the water level rise may be more obvious for phosphorus.The research results can provide a scientific basis for accurately preventing water quality safety risks in the bays during the subsequent high water level operation of Danjiangkou Reservoir.