Lindingyang bay is a typical complex tide-dominated estuary.In dry season,fresh salt-water mixing presents obvious temporal and spatial variations and cannot be regarded as a single partial mixing type.In addition,the dredged channel may affect hydrodynamics and material transportation.In order to reveal the dynamic change and fresh-salt-water mixing characteristics of Linding navigation channel before and after the third-stage engineering,a dynamical geomorphology analysis is conducted,and the mechanism decomposition method as well as the three-dimension mathematical model is adopted.The results shows that ( 1) Lingding channel can be divided into three parts,namely upper,middle and lower sectors,and the dynamic characteristics as well as the fresh salt-water mixing mechanism varies in different sectors; ( 2) after engineering,the shelf water intrusion from the bottom into Lingdingyang bay becomes obvious; ( 3) with the increase of the velocity caused by flood tide and ebb tide,the vertical circulation of the channel becomes strong.The surface velocity increases more significantly than that of the bottom; and ( 4) after engineering,the stratification and circulation coefficients both increase,and the fresh saltwater mixing effect is enhanced.