Assessing the impacts of Three Gorges Dam on lake inundation areas across the downstream Yangtze floodplain

China's Three Gorges Dam (TGD) has received worldwide attention due to its profound impacts on the downstream hydraulic, morphological, and ecological systems. The TGD operation results in direct alternation of the discharge to the middle and lower Yangtze reach, manifested as regulated flow and reduced sediment load. TGD's flow regulation, typically described as water storage in fall while release in winter and spring, interferes with the natural seasonality of downstream Yangtze River levels which are essentially important to the inundation dynamics of surrounding lakes/wetlands in the Yangtze floodplain. Concurrent decrease of sediment load has caused chronic downstream channel erosion which lowers Yangtze level in relation to flow and further affects the sustainability of riparian lakes and the related ecosystems. By integrating satellite observations, in situ measurement, and hydrologic simulations, this study presents a systematic assessment of the TGD impacts on the inundation areas of six major freshwater lakes across the entire Yangtze basin downstream of the TGD, during the time period from TGD's initial impoundment in June 2003 to early 2012. Despite the small number, the six targeted lakes cover a total area of ~5,000 km2 accounting for ~25% of the freshwater lake area in China, and were identified as the only natural lakes that remain in open connection to the Yangtze River across the downstream floodplain. Using daily MODIS imagery from 2000 to 2012, we revealed a significant year-round decline in the aggregated inundation area of the studied lakes by an average of ~580 km2 or 17.7% from the pre-dam to post-dam period (i.e., before and after June, 2003). To diagnose TGD's contribution to such lake area decline, we followed a two-step procedure by first quantifying the TGD impacts on the seasonal level regime along the complete longitudinal range of the Yangtze River downstream from TGD to the estuary [Wang et al., 2013], and then estimating the consequential impacts on lake inundation areas from the calculated Yangtze level changes at the lake confluences/outlets. Results indicate that TGD's water impoundment explained an average of 20.2% of the lake area decline in fall, while TGD's water release increased the downstream lake area by 10.9% and 10.0% of the area decline in winter and spring, respectively. However, concurrent Yangtze channel erosion due to reduced sediment load substantially counteracted the lake area increase by 40.4% in winter and 11.8% in spring, while further reinforcing the lake area decrease by 9.4% in fall. Since TGD's negative impacts on downstream lake areas are restricted to the water impoundment periods in fall, we suggest that the remnant lake area decline in other seasons was a combined consequence of the drying climate, intensified tributary water regulation, and increasing human water consumption in the downstream Yangtze basin during the post-dam period. Note: Figures/numbers in this abstract may be subject to necessary adjustment in the finalized work. Reference: Wang, J., Y. Sheng, C. J. Gleason, and Y. Wada (2013), Downstream Yangtze River Levels Impacted by Three Gorges Dam, Environmental Research Letters, in review.