The impacts of the Three Gorges Dam upon dynamic adjustment mode alterations in the Jingjiang reach of the Yangtze River, China

Dam operation profoundly modifies downstream flow-sediment regimes, resulting in a variety of geomorphologic responses, without uniform laws. The Jingjiang reach of the middle Yangtze River has undergone significant adjustment mode alterations, with intensive channel erosion and major adjustment area changing from the bankfull channel to the medium flow channel at the time boundary of the Three Gorges Dam (TGD) impoundment. While these changes have received some attention, little work has been undertaken to reveal the reason. In this study, based on 172 cross sections measured in the post-flood period from 2003 to 2015, different river patterns, longitude and reach-averaged channel dimensions were assessed to reflect the channel adjustment characteristics of the Jingjiang reach. An index has been proposed to measure the cumulative water transport capacity of each discharge interval and to assess the relationship with reach-scale medium flow channel dimensions. The results indicate that the channel adjustment changes are directly driven by the alterations of flow duration and sediment transport capacity. With the deficiency of floods, medium flow channels respond quite well to small and medium discharges of under approximately 16,500 m3/s. A discharge of 16,500 m3/s which can be regarded as a critical threshold, corresponds to the average top levels of braided and meandering channel bars, which are of morphological significance in constraining the passing flow in the medium flow channel. Small and medium discharges under the critical threshold associate with relatively larger reductions in sediment loads and hold increased geomorphic impacts (QmJP), eventually resulting in adjustment mode alterations. The results present herein for the Jingjiang reach of the Yangtze River can provide a better understanding of dam operation and the corresponding impacts on downstream channel morphology.