The Guanzhou reach,located in the middle Yangtze River,is a typical anabranching channel with its mainstream alternating between the two branches within a year.After the impoundment of Three Gorges Reservoir (TGR),the channel of Guanzhou changed significantly.The influence of channel adjustment of Guanzhou on the flow diversion at Songzikou became obvious and has been paid more and more attention.Based on the topographical data in the period 2002—2013 and the hydrological data in 1984—2017,the channel evolution of the Guanzhou reach after the impoundment of TGR was analyzed.Then the impact of upstream channel adjustment on flow diversion at Songzikou was studied from the following three aspects: the channel erosion and deposition,the development of left branch,and the duration of the left branch staying as main channel.The results show that,after the impoundment of TGR,the Guanzhou Reach was eroded continuously,and about 90 % of the total channel scouring occurred in the low-water channel.The scour pattern changed from purely downward erosion to left convex bank erosion along with left branch erosion due to finer river bed materials; now the thalweg elevation in left branch is lower than that in right branch which was opposite before.The status of the right branch being the main channel was weakened in dry season.The critical flow rate for the mainstream alternation was decreased.When the incoming flow was less than 20 000 m3/s,the flow diversion ratio at Songzikou was reduced,while the reducing trend was weakened gradually with the channel developing into an equilibrium state.The development of the left branch will increase the diversion ratio at Songzikou in larger discharge,and the longer the duration of the larger discharge,that is,the duration being the main channel for the left branch,the greater the annual averaged flow diversion ratio at Songzikou.It's worth noting that,the over-development of the left branch may cause the mainstream of Guanzhou Reach to cease alternating between two branches within a year,and to stay in the left branch.The field monitoring should be strengthened in the future and proper engineering measures taken if needed.
