During the impounding,channels in fluctuating backwater area become a part of the reservoir as the base level of erosion rises.And those channels are in natural state during rest time of the year.Thus,in response to different conditions,feature of deposition and erosion of channels in this area varies accordingly.Taking Sanjiaoqi shoal as an example,this article focuses on the transformation of shoals’ characteristics of deposition and erosion and navigation-obstructing.Under the natural condition,as the results reveal,the shoal followed the law of sediment deposition on high-water period and sediment erosion on low-water period throughout a year.Besides,interannually cumulative sediment erosion resulting from coming sediment whose quantity sharply decreased is observed and sediment erosion developed as quantity of coming sediment reduced constantly.After the flood season of the year 2008,the storage level of Three Gorges Reservoir lift gradually and Sanjiaoqi shoal became a part of fluctuating backwater area.Rising of the channel’s base level of erosion renders the channel a tendency of sediment deposition.Under the influence of decreasing coming sediment as well as elevated base level of erosion,previously severe sediment erosion is effectively alleviated and the channel shows no cumulative sediment deposition.Meanwhile,the sediment deposition period shows apparent change.Previously,sediment deposited in this area during the flood season could be effectively scoured when the flood receded.While,under the new condition,wholesale sediment erosion won’t take place until the reservoir’s storage level falls next year.Hence it is predictable that the channel condition of Sanjiaoqi shoal may deteriorate with the navigation standard remains unchanged.