Abstract: After the construction of the Three Gorges Reservoir (TGR),sediment-containing flow in the downstream channel of the reservoir had been in a severe sub-saturation state for a long time. The sediment transport changed from a relative equilibrium state to an unbalanced state,and the sediment transport intensity varied significantly compared with that in natural condition. Based on the cross sectional data,bed sediment and hydrological data of major hydrological stations in the midstream of the Yangtze River before and after the operation of the TGR (1992-2017),we looked into the sediment transport intensity of compound cross sections from the perspectives of sediment transport rate,bed sediment particle size,and bankfull discharge. We found that after the impoundment operation of TGR,the sediment that enters the downstream channels reduced remarkably,and the sediment transport rate of the same flow level at hydrologic stations along the midstream of Yangtze River decreased obviously. The bed sediment was coarsened in different degrees along the river. Such coarsening was most obvious in the Jingjiang reach and less obvious in the Luoshan-Wuhan reach. At hydrologic stations of Luoshan and Hankou,the sediment transport intensity in the adjacent of bankfull discharge reached the maximum under natural conditions,and the corresponding sediment carrying capacity reached peak.

Key words: downstream of reservoir, compound channels, sediment transport rate, bankfull discharge, sediment transport capacity