Characteristic of the water temperature lag in Three Gorges Reservoir and its effect on the water temperature structure of tributaries

The water temperature regimes of the Yangtze River has changed a lot since the impoundment of the Three Gorges Reservoir (TGR), which lead to a significant effect on the water environment in the TGR. This paper focuses on the analysis of characteristics of water temperature regimes of the main stream in the TGR and the lagging effect of water temperature on temperature structure differences in six typical tributaries. The results showed that water temperature gradually decreased along the main stream and stratified position of water temperature was about 5–10 m arisen in estuaries of six tributaries in heating period. However, water temperature gradually increased along the main stream, and water temperature stratification of the six estuaries disappeared in cooling period. Annual vertical water temperature showed little change. After the impoundment of the TGR, the impact of water temperature is mainly reflected in lag characteristics. The monthly mean temperature difference significantly doubled, and the whole process of water temperature had been put off one month. Through further analyses, water temperature profile tends to be stable with the success of the TGR impounding to 175 m and contributes to water temperature differences in tributary, which is the main reason for water temperature structure changed from three layer to two layer gradually.