The middle reaches of the Yangtze River is wide enough that many reaches are braided channels with bars and shoals in the river bed. In those braided channels, the scouring and silting of mid-channel bars and point bars change greatly:some bars decrease, and the others may increase greatly;and simultaneously the thalweg and the main streamline migrate frequently and significantly. After the impoundment of the Three Gorges reservoir, the new features of river bed evolution gradually showed up, and the navigation conditions of those braided channels changed accordingly. This paper takes the Jiayu waterway as an example, which is one of the main navigation-obstructing waterways in the middle reaches of the Yangtze River. On the basis of the existing researches and the recent field observation data, the fluvial processes of slightly curved braided reach are illustrated, and the evolution processes along with the characteristics before and after the impoundment of the Three Gorges reservoir are compared, the factors influencing the river-bed evolution are analyzed, and then the influences of the Three Gorges reservoir are discussed. Finally the river evolution tendency and navigation condition changes are presented. With regard to the Jiayu waterway, the Three Gorges reservoir impoundment changed the cyclic alternating pattern between the single-channel and double-channel of the transition section, and also weakened the relationships between the Wangjiazhou shoal and Fuxing bar:before the impoundment of the reservoir, the shoal shrank and the bar expanded; but after the impoundment of the reservoir, the shoal shrank and the bar also shrank. Under the influences of the Three Gorges reservoir impoundment, the shoals and bars in the middle reaches of the Yangtze River will be further eroded in the long term;as for the Jiayu waterway, the entrance of the left branch will be more flat, the main streamline will meander even more, and the navigation conditions will be more in instable due to the riverbed evolution.%长江中游河道宽阔,分汊河段较多,洲滩冲淤消长变化较大,河道深泓主流摆动也较频繁,三峡水库蓄水运行后,分汊河道演变及航道条件随之发生改变.以长江中游主要碍航浅水道之一的嘉鱼水道为例,在已有研究成果的基础上,结合近期实测资料,分析微弯分汊型河段自身演变,认识三峡水库蓄水前后水道演变过程和特点,分析水道演变的影响因素,探讨三峡工程对本水道演变的影响,在此基础上预测水道演变趋势和航道条件变化.分析结果表明:三峡水库蓄水改变了嘉鱼水道过渡段单双槽交替的变化规律,弱化了汪家洲边滩与复兴洲低滩彼此消长的关系,河道洲滩以冲刷为主,预计在三峡工程的长期影响下低滩仍会有所冲刷,左汊进口更趋宽浅,主流摆动空间增大,航道条件的不稳定性较大.
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