Abstract: Lake wetlands play an irreplaceable role in maintaining biodiversity, ecological balance, and land surface mass balance. Poyang Lake is important for flood control and water source of the middle and lower Yangtze River and provides a unique hydrological condition for its rich habitats and biodiversity. Since the Three Gorges reservoir was put into operation, Poyang Lake has experienced an earlier and longer dry period, especially its abnormally low water stage in October that greatly imperils its function as an habitat for wintering birds. This paper presents a quantitative analysis of the water stage changes of Poyang Lake in October using a fuzzy division clustering model; and examines its influencing factors such as the river runoff, the Three Gorges reservoir’s water storage after flood period, and the corresponding change in the river and lake geography. Results show that a significant decrease in the Poyang inflow and the Yangtze mainstream runoff is the main cause for the lake’s stage falling, and that the reservoir impounding after flood season has further accelerated this falling. These results can provide a basis for lake management and improving the lake ecosystem and environment during the dry period of Poyang Lake.

Key words: Poyang Lake, water stage, Three Gorges reservoir, fuzzy division clustering model, quantitative analysis