Abstract: Interaction of the Yangtze River and the Poyang Lake strongly affects the Poyang Lake water resources and flood and drought potentials in the lake basin. This interaction and its variation are examined in this study for the period 1957-2008. In addition, changes in the relationship between the Yangtze River and Poyang Lake after the operation of the Three-Gorge Dam (TGD) are evaluated using the TGD operation data available for 2004-2006. Major findings are shown as follows. 1) At interannual to decadal timescales the variation in the Poyang Lake water level is primarily determined by fluctuations of the climate and the inflow from the five major tributaries in the lake basin. By influencing the lake level these fluctuations also affect the lake-river interaction. 2) At the seasonal timescale, the Poyang Lake has the largest outflow to the Yangtze River and exerts a strong pressure on the mainstream during April-June, when the lake receives annual peak inflow from its tributaries. The Yangtze River's blocking and/or reversed flow to the Poyang Lake are the strongest during July-September. 3) The operation of the TGD has changed little in terms of seasonal variations of these lake and river forcings. In most of the seasons, the influence of the TGD has resulted in less than 10% of the variation in the Yangtze River flow. However, by damming or releasing water, the TGD can result in seasonal changes in the Yangtze River flow and river-lake interactions. These subtle effects from the TGD operation are quantified and compared to the climate effect on the river-lake interactions. These comparisons show enhancement in some seasons while weakening in the other seasons of the climate effect on river-lake interactions and flood or drought potential in the lake basin by the TGD operation.

Key words: Yangtze River, Poyang Lake, river-lake interactions, climate, the Three-Gorge Dam