How does the three gorges dam affect the spatial and temporal variation of water levels in the Poyang Lake?

•STL decomposition is efficient for analysis of hydrological data and can avoid stochastic disturbance.•Factors related to the Yangtze River have greater impact in the northern area than in the southern area, while factors related to the Poyang Lake itself have greater impact in the central area.•The operation of TGD results in an obvious recession of water level in the Poyang Lake, but it also limits the maximum and minimum water levels in a narrow range. The Poyang Lake is the largest freshwater lake in China located at the midstream of the Yangtze River, and the water level has witnessed a decline both interannually and annually since the operation of the Three Gorges Dam (TGD) located at the upstream of the Yangtze River in 2003. However, the impact of TGD on the spatial and temporal variation of water levels in the Poyang Lake remains to be elucidated. In this study, the long-term observed water levels at different stations in the Poyang Lake and Yangtze River were analyzed by seasonal-trend decomposition based on loess to determine the impact of TGD on the spatial and temporal variation of water levels in the Poyang Lake. The results reveal that TGD has no obvious impact on the interannual variation of water levels, but accounts for a major part of the annual variation of water levels in the impounding season of TGD (autumn). The effect of the Yangtze River is larger in the northern area that is close to the Yangtze River than that in the southern area of the lake; but the effect of the Poyang Lake itself is most obvious in the central area. TGD has both positive and negative consequences on the water level of the Poyang Lake. It can result in an obvious decrease in the water level and prolong the duration of drought, but it can also reduce the variation amplitude of water levels anywhere in the Poyang Lake and control the maximum and minimum water levels. This study may provide insights into the impact of TGD on the Poyang Lake and thus contribute to water resources conservation and ecological restoration of the lake in the future.