河槽放宽率是保持潮汐河口河槽稳定性的主要参数之一。在利用河槽宽度计算放宽率的基础上，提出了以过水断面面积为主要参数计算的天然河口放宽率，能更真实地反映河槽沿程过水能力是否增强以及增强的幅度。1917-2006年长江口南支河槽自白峁沙至吴淞口的放宽率计算结果表明：整体上，近100年来放宽率呈减小之势，与南支河槽不断冲刷相适应，且白峁沙、扁担沙、中央沙、浏河沙等主要沙洲的生长也起到了缩窄河宽，减小河道过水断面面积，缩小放宽率的作用；放宽率在1979年出现最小值，与七丫口-5 m槽宽为历年最窄及充分发挥其节点作用有关；放宽率在2006年增至1.21%，与三峡工程蓄水后流域供沙大幅度减少，南支河槽主要沙体受到侵蚀，特别是中央沙和浏河沙侵蚀最为严重，使石洞口-吴淞口区域过水断面面积增大有关。

 The channel widening rate is one of the main parameters for the stability of tidal estuaries. Based on previous methods of calculating channel widening rate via river channel width, a new method of calculating the channel widening rate of natural estuary which takes discharge section area as the main parameter is proposed in the paper. The channel widening rate from Baimao sand shoal to Wusongkou in the south branch of Yangtze estuary is calculated from 1917 to 2006. The results indicate that the overall channel widening rate has been decreasing for the past century, which adapts to the continuous scouring in the south branch channel. Meanwhile, the development of Baimao sand shoal, Biandan sand shoal, Zhongyang sand shoal, and Liuhe sand shoal has helped narrow the channel width, reduce the discharge section area and decrease the widening rate. The smallest channel width at 5m contour of Qiyakou and its function as one of the nodes in the estuary contributed to a minimum widening rate in 1979. In 2006, the widening rate increased to 1.21% owing to an increased discharge section area from Shidongkou to Wusongkou induced by deducted sediment supply after the Three Gorges Reservoir impoundment and the subsequent erosion at main sand shoals in the south branch, especially at Zhongyang sand shoal and Liuhe sand shoal. The new calculation method could truly reflect the increase of discharge capacity along the channel and the increasing rate.