Abstract: According to the field data of continuous measurements of current velocity, current direction,concentration of suspended sediments and salinity during two tidal cycles at eighteen stations in the North Channel, the North Passage and the South Passage of the Changjiang Estuary in the spring tide in August, 1978. by making an analysis of water and suspended sediment flux of a width unit across the main channel of each tributary, the action of each term of suspended sediment transport in the turbidity maximum of the Changjiang Estuary is discussed and the relative importance of each term in three different tributaries is compared in this paper by introducing techniques of mechanism decomposition. The result indicates that: (1) The dominant mechanism of net transport of water and suspended sediment is that of non-tidal steady advection transport, mass Stokes drift. tidal trapping and net vertical circulation. (2) The net transport of water and suspended sediment induced by mass Stokes drift is of considerable importance in each tributary of the Changjiang Estuary and has a feature of landward movement. which restrains the seaward transport of suspended sediments and induces the increase of suspended sediment concentration within the estuary. (3) The magnitude of suspended sediment transport induced by tidal trapping is greater than that induced by vertical shearing diffusion. Tidal trapping with the resuspension of bedload play an important role in the formation of turbidity maximum in the area where the action of tidal trapping is distinct. The resuspension of bedload and the asymmetric sediment transport are the main factors that induce the distinct action of tidal trapping. (4) In different tributaries, the contribution of each suspended sediment transport term to net sediment transport varies with the relative importance of runoff and tidal current. Mass Stokes drift and tidal trapping play important roles in the formation of turbidity maximum in the South Passage where tidal current is stronger. In the North Channel where runoff is relatively strong. the non-tidal steady advection transport and net vertical circulation occupy dominant positions and the later is the main factor fn the formation of turbidity maximum. In the North passage,it is between the South Passage and the North Channel.

Key words: Changjiang Estuary, turbidity maximum, tidal trapping, vertical circulation