受强人类活动和极端洪旱气候等因素综合影响,长江中下游江湖水沙情势、冲淤分布及江湖关系等均发生改变。三峡水库蓄水后,长江中下游干流年均输沙量大幅减少,减幅达70%~93%,年内流量过程总体有所坦化,但汛后因水库群蓄水而退水速率加快;洞庭湖四水及荆江三口年均入湖径流量无明显变化,入湖沙量呈减少趋势,入湖年均水、沙量分别减少了9%和38%;鄱阳湖五河年均入湖水、沙量分别减少了2%和57%,出湖年均水、沙量分别增多了1%和5%。水沙过程的改变引起长江中下游冲淤格局变化,2003—2021年长江中下游河段累计冲刷50.3亿m³,年均冲刷量2.65亿m³;洞庭湖区由淤积转为微冲,荆江三口洪道以冲刷为主;鄱阳湖区总体上由淤转冲,入江水道冲刷下切明显。未来30 a长江中下游干流河道仍将保持冲刷态势,至2050年末宜昌至大通河段、荆江三口洪道累计冲刷量分别为35.8亿m³和1.17亿m³,洞庭湖区呈微淤状态,鄱阳湖区呈微冲状态。基于长江中下游江湖冲淤新格局,从防洪、供水、航运、生态及涉水建筑物安全等5个方面,较为系统地阐述了江湖演变的影响效应,并针对性地提出了若干对策和建议。

Under intense human interference and extreme climate events, the flow-sediment regimes, deposition and erosion patterns, and river-lake interactions in the middle and lower reaches of the Yangtze River have undergone significant transformations. After the impoundment of the Three Gorges Project, the annual upstream sediment supply to the middle and lower Yangtze River has decreased by 70% to 93%, and the flow process has become more concentrated. However, the post-flood recession has accelerated due to the operation of cascade hydropower stations. The annual water supply from the four rivers and three outlets flowing into the Dongting Lake has shown no significant adjustments, with a decline of 9%, while the annual sediment supply has decreased significantly by 38%. The annual water and sediment supply of the five rivers into the Poyang Lake have decreased by 2% and 57%, respectively, while the annual water and sediment outflow from the Poyang Lake have increased by 1% and 5%, respectively. These adjustments have altered the deposition and erosion patterns in the middle and lower Yangtze River. To be specific, from 2003 to 2021, the cumulative erosion of the mainstream reached 5.03 billion m³, with an average annual erosion of 265 million m³per year. The deposition-erosion state of the Dongting Lake has shifted from being deposition-dominated to slight erosion-dominated, and the riverbed of the three outlets generally exhibits an erosion trend. Similarly, the deposition-erosion state of the Poyang Lake has changed from deposition to erosion, and the channel connecting the Poyang Lake to the mainstream Yangtze River shows severe erosion and down-cutting. A predictive model indicates that over the next three decades, the mainstream of the middle and lower Yangtze River will continue to experience significant erosion. By the end of 2050, the cumulative total erosion of the mainstream from Yichang to Datong and the three outlets will be 3.58 billion m³ and 117 million m³, respectively. The Dongting Lake is expected to be slightly silted, while the Poyang Lake area will be slightly eroded. Based on these findings, the impacts of the river-lake evolution on flood control, water supply, navigation, ecology, and safety of water-related structures are expounded systematically. Countermeasures and suggestions are also put forward.