Abstract:

The middle and lower Yangtze River develops a large number of channel islands, which attract research interests after impoundment of Three Gorges Project (TGR). The data of average annual runoff and sediment discharge of Yichang, Hankou and Datong During 2000-2011 are analyzed. The 20 representative channel bars are selected as research objects based on remote sensing images in 1999, 2001, 2003, 2005, 2007, 2009 and 2011. The area change of channel bars are closely linked with the annual runoff and sediment discharge before and after the impoundment of TGR. A conceptual model is developed to analyze the relationship between changes of bars area and the runoff and sediment discharge. It shows that: 1) after the impoundment of Three Gorges Reservoir (2003-2011), average annual runoff of Yichang, Hankou and Datong decreased by 61%, 73% and 95%, respectively; sediment discharge decreased by 903%, 721% and 669%, respectively; average sediment concentration decreased by 896%, 896% and 896%, respectively; 2) before the impoundment, 12 bars (60%) area is increasing year by year, with an average rate of 268% on the basis of 1999; after the impoundment, only 9 bars (45%) area also increased, the average rate reduced to 221%, 11 bars (55%) area was reducing with an average rate of 194%; 3) the conceptual model shows that there is the positive correlation between the change of channel bars area and water level and erosion. Moreover, water level and erosion have the positive correlation with flow discharge and suspended load reduction. The area change of channel bar is mainly controlled by water level change. This relationship produces an illusion of channel bars evolution in the middle and lower reaches of the Yangtze River: the area of the minority of channel bars increases after the impoundment of TGR because the serious riverbed erosion occurred, but actually they are undergoing the erosion process