以三峡水库为核心的长江上游水库群陆续投用后,水库拦截大量泥沙,“清水”下泄,导致坝下游河道处于长时间、大范围冲刷下切,对河势稳定、防洪安全及供水安全等带来影响。基于实测资料分析了三峡等上游水库拦沙与长江中下游河道冲刷的关联性,得到以下结论:2003年7月1日—2021年10月31日三峡水库泥沙淤积量断面法统计结果为17.84亿m³,考虑库区区间来沙输沙量法统计为22.38亿m³,两者相对误差为25.4%,2种方法计算结果差异不大;长江中下游宜昌—大通河段累计冲刷量断面法测算为31.46亿m³,显著高于输沙量法的8.99亿m³,结合枯水期水位下降规律得到河槽冲刷量来判别,断面法结果更具合理性;流域减沙总量达84.95亿m³,不考虑中下游人类活动影响,流域拦(减)沙量与下游冲刷量比值为1∶0.48,这与典型已建水库的规律基本相符,其中三峡水库淤积量引起长江中下游河道冲刷量约为10.74亿m³。

[Objective] After the operation of the Three Gorges Reservoir, the ratio between siltation volume in the Three Gorges Reservoir and channel scour volume in the middle and lower reaches of the Yangtze River does not conform to the value (1∶0.5) determined in previous studies, mainly due to the substantial reduction in sediment inflow into the Three Gorges Reservoir. This study aims to analyze the characteristics of sediment deposition in the Three Gorges Reservoir and the scouring variations in the middle and lower reaches of the Yangtze River, and to investigate the correlation between sediment retention by the Three Gorges and other reservoirs and the resulting channel scouring in the middle and lower Yangtze River. [Methods] Measured hydrological data and topographic data from 2003 to 2021 were utilized. The sediment transport method, cross-sectional method, and low-water-level verification were comprehensively adopted for the analysis. [Results] The ratio between basin-scale sediment retention and downstream channel scour was 1∶0.48, which was generally consistent with that of typical reservoirs in China such as Sanmenxia Reservoir and Danjiangkou Reservoir. 1) Regarding siltation volume in the Three Gorges Reservoir, the cumulative siltation from 2003 to 2021 was calculated as 2.381×10⁹ t, which was equivalent to 2.238×10⁹ m³, using the sediment transport method. The cross-sectional method estimated 1.784×10⁹ m³ of siltation in the mainstream of the reservoir area. The difference between the two methods was 25.4%, due to differences in calculation principles, but the results were generally consistent. Siltation was mainly concentrated in the perennial backwater zone, totaling 1.853×10⁹ m³. Siltation within the flood control storage accounted for only 0.67%, indicating a relatively small loss of effective storage capacity. 2) Regarding channel scour in the middle and lower reaches of the Yangtze River, the cumulative scour in the Yichang-Datong reach was calculated as 8.99×10⁸ m³ using the sediment transport method. The cross-sectional method yielded 3.146×10⁹ m³. The minimum scour volume inferred from the decline in water level under low-flow discharge conditions was 2.160×10⁹ m³, demonstrating that the cross-sectional method results were more reasonable. The discrepancy was mainly attributed to the underestimation of fine-grained sediment in the sediment transport method and the exclusion of human activities, including sand mining of 6.64×10⁸ m³ and channel dredging of 3.08×10⁸ m³. The cumulative scour of the entire river reach reached 5.03×10⁹ m³. Continuous scour persisted in the Jingjiang reach with no reduction in intensity, and scour in the downstream reaches significantly intensified after 2013. 3) At the basin scale, the total reduction in sediment volume from 2003 to 2021 was 8.497×10⁹ m³. After excluding the influence of human activities, the ratio between sediment retention and scour was 1∶0.48, which was consistent with the sediment retention-scour pattern of large reservoirs. Siltation in the Three Gorges Reservoir itself was 2.238×10⁹ m³, which directly caused 1.074×10⁹ m³ of downstream scour and accounted for 26.5% of the total scour. The contribution in the first decade was significantly higher than that in the second decade, indicating that its impact gradually weakened with the enhanced sediment retention effect of upstream reservoir groups. [Conclusion] It is necessary to optimize sediment release scheduling of reservoir groups to improve the sediment flushing ratio. Measures such as dredged sediment resource utilization and sediment replenishment should be combined to mitigate channel scour, while strengthening sand mining management and long-term monitoring of scour and deposition.