Abstract: Field observation data for the middle and lower Yangtze before and after the operation of Three Gorges reservoir (TGR) have been collected and analyzed in this study to examine the relationship between channel erosion and sediment transport in the reaches downstream of TGR and to evaluate the impact of channel degradation on the river stage in flood season and low flow season. Our detailed analysis includes variations in flow and sediment regimes at the hydrological stations, spatial distributions and time variations in erosion volume in the river channel, and river stage variations with different discharges. The results show that the concentrated scour in low-flow channels is related not just to the decrease in flood frequency and longer duration of medium flows, but to the characteristics of channel scour at a high rate under medium flows and a low rate under flood flows. The main erosion zone has been moving downstream over time, and up to the present it has been staying in the Jingjiang reach. At the Jianli station, the annual mean transport of coarse sediment (d > 0.125 mm) has recovered to the same multi-year mean level as before TGR impoundment, while most of the erosion in the long reach immediately downstream of Chenglingji was related to fine grain sediment. Channel erosion caused a significant drop in the medium and low river stages, and such a tendency showed a growing rate in the sandy reaches upstream from Chenglingji. However, the river stage did not drop considerably when the channel flow was above a certain threshold around the bankfull discharge. We estimate the main characteristics of river stage changes discussed above will be maintained in quite a long period in future.

Key words: Three Gorges reservoir, downstream of the dam, channel degradation, river stage variation