Changes of river regime and waterway downstream of a cascade of reservoirs on the upper Yangtze River

The Xiangjiaba and Xiluodu reservoirs, located on the upper Yangtze River, have been operational and the changes in the downstream river regime and waterway conditions have attracted great attention. A two-dimensional horizontal (2DH) flow–sediment mathematical model of the Shuifu to Lanjiatuo reach (Shuilan reach) downstream of the Xiangjiaba Dam was developed. The spatio-temporal characteristics of sediment scouring and siltation after completion of the cascade of reservoirs were simulated using the model, and then the river regime changes in several typical reaches as well as their possible impacts on navigation channels were analyzed. The results show that, after operation of the cascade of reservoirs, the downstream channel is in the state of scour. The general change of the river regime is that the flow in curved reaches tends to be straight though the river pattern still remains curved. Sub-branches tend to experience deposition, while the main channel will be scoured deeper, which can help to increase the navigation depth in the shoals in the transition sections. Deposition in some sections could result in new shoals. In addition, the bottom elevation difference between floodplains and channels will increase, which could aggravate local adverse flow patterns; therefore, navigation conditions in torrential shoals will tend to deteriorate.