Abstract: Due to the effect of monsoon climate and river-lake interactions, a seasonal water-level fluctuation pattern, i.e. rising seasons, flooding seasons, retreating seasons and dry seasons appearing consecutively, has been formed in the two river-connected lakes, the Dongting Lake and the Poyang Lake. In the recent three decades, dramatic changes occurred in these two lakes due to climate variation and built dams on the main stream of the Yangtze River. The Dongting Lake and the Poyang Lake are both located in the south of Yangtze River with similar climates, and probably with rather similar drainage structures. While, the Dongting Lake and the Poyang Lake are also linked by the upstream and downstream relations. Hence, the climate variations and river-lake interactions affect water regimes of those two lakes both similarly and differently. Contrasting of water regime variation of those two lakes is conducive not only to understanding river-lake interaction in middle reaches of the Yangtze River, but also to improving lake governance. In this research, taking the year 2003 as the breakpoint (There are two periods:1980-2002 and 2003-2014), comparative analyses were performed about the hydrological data of both two lakes and the multi-site in a single lake. How river-lake interactions affect the water regime variation in those two lakes was also discussed. The results showed that (1) compared to the former period, in both dry and rising seasons, the water level declined in the upstream region of each lake during the later period, while the lake water level fell even more in the middle region of each lake. Eventually, the lake water level rebounded decreases in the downstream region of each lake. 2) In both the flood and the retreating season, magnitudes of water level fluctuations in both lakes have changed considerably since 2003. For the Dongting Lake, the biggest water level decline in flooding season occurred at the upstream region and the middle region of the lake, while the biggest decline in the retreating season occurred at the middle region and the downstream region of the lake. Unlike the Dongting Lake, the declining amplitude of water level in the Poyang Lake expanded from the upstream region to the downstream region in both the flood season and the retreating season. (3) The water level drops in the Dongting Lake were significantly less than that in the Poyang Lake. Different mechanisms and intensity of river-lake interactions are mainly responsible for these disparity in variations magnitude of those two river-connected lakes in the middle reaches of the Yangtze River.

Key words: river-connected lakes, Dongting Lake, Poyang Lake, seasonal water-level fluctuations