Water And Sediment Exchanges Between Rivers And Lakes In The Middle And Lower Yangtze River Basin

Water and sediment exchanges between rivers and lakes are key processes of morphodynamical evolution.Numerous lakes scatter in the middle and lower Yangtze River basin,especially the Dongting Lake and Poyang Lake as the two large river-connected lakes.Under the influence of nature and human activities,the hydro-morphodynamics have undergone major changes.Studying the water and sediment regulation capacity of lakes is crucial for water and shipping resources safety and ecological environment protection,and it is also a prerequisite for better understanding of the impact of engineering such as dams.Based on measured data of runoff and sediment characteristics,firstly,we quantitatively analyzed changes in runoff and sediment load,and the degree of water and sediment exchanges between the river and lakes at different time scales,by using the water(sediment)exchange coefficient and water(sediment)distributing(contribution)ratio,respectively(see section 2.2.3 for details).Lag response of the regime shift,and differences of regulation capacity of the two large lakes were compared.Secondly,influencing factors of exchange process between the rivers and lakes were comprehensively discussed.Differences between the dams on the upper reaches and the regional projects in the middle-lower reaches in terms of impact modes,degree and time scale were compared.Finally,based on the analysis of temporal and spatial changes of sediment particle size,we explored the environmental effects of the regime shift of water and sediment exchanges between the river and lakes.The main conclusions are as follows.? From 1960 to 2019,changes of water and sediment exchange between the Dongting Lake and the Yangtze River were unsynchronized with those between the Poyang Lake and the river.Seasonal patterns of runoff and sediment contributed by the two lakes were similar,but the trends in the past 20 years were opposite.Dongting Lake and Poyang Lake successively showed a significant increase in runoff outflow in the 1980 s and 1990 s.Net outflow of the Dongting Lake increased from 18.9 billion m³/y in 1960-1986 to 20.5 billion m³/y in 1987-2019,while sediment load shifted from net input(70 million ton/y in 1960-2004)to net output(11 million ton/y in 2005-2019).Net outflow of the Poyang Lake increased from 14.0 billion m³/y in 1960-1991 to 16.2 billion m³/y in 1992-2019;net output sediment load increased from 9.26 million ton/y in 1960-1996 to 10.75 million ton/y in 1997-2019.However,sediment regulation capacity of the two lakes is declining.Net sediment load output from the Dongting Lake and Poyang Lake in 2017-2019 decreased by 38% and 62%,respectively,compared with those in 2003-2016.At seasonal scale,contribution of the two lakes to the runoff in the Yangtze River was greater during spring and summer period from January to June.During this period,monthly averaged runoff in Chenglingji and Hukou was 19.9 and 14.5 billion m³/month,respectively,accounting for 31% and 23% of the monthly averaged runoff at Datong in the same period.Contribution of the two lakes to sediment load in the Yangtze River was greater during winter and spring period from January to April and November to December.During this period,monthly averaged sediment load in Chenglingji and Hukou was 1.44 and 1.16 million ton/month,respectively,accounting for 31% and 25% of the monthly averaged sediment load at Datong in the same period.? Changes in hydrology and geomorphology caused by dams on the upper reaches of the Yangtze River are the main factors for maintaining the outflow-dominated pattern of the lakes in the middle-lower reaches.Regional projects in the middlelower reaches such as meander cutting-off and sand mining,boosting the sediment output of lakes,it is also the controlling factor of regulation capacity change of lakes at long-time scales.The influence of dams on the upper reaches of the Yangtze River can be divided into two categories.The first is to directly affect flux input and output of lakes by reducing sediment load and adjusting seasonal pattern of runoff into the middle-lower reaches,but it does not significantly change the seasonal pattern of the exchange between rivers and lakes.The second is to indirectly adjust flux input and output of lakes by scouring riverbed and lowering water level of the middle-lower reaches,increasing slope of the connecting channel between rivers and lakes,weakening backwater effect of the river on the lake outflow,and reducing water storage capacity of the two lakes at the end of flood season.Runoff inflow and sediment load input from the three inlets into the Dongting Lake was reduced by half and two-thirds,respectively,after meander cut-off in the lower Jinjiang River.Sand mining in the Dongting lake and Poyang Lake caused more sediment suspension,turbidity of the water body increased,and more sediment flowed out of the lake with runoff.After construction of the dams on lake tributaries,sediment load from the Dongting Four Rivers decreased by 71% from 1960-1979 to 1998-2019;sediment load from the Poyang Five Rivers decreased by 50% from 1960-1974 to 2012-2019,thus reducing sediment load output from lakes to the Yangtze River accordingly.? The coarsening of suspended sediment in the middle and lower Yangtze River is mainly reflected in the increase of sand content.In dry season,the fine sediment supplementation of the two lakes can moderate the coarsening of suspended sediment in the mainstream.Recently,suspended sediment from the two lakes is gradually coarsening.From 1987 to 2019,suspended sediment on the mainstream of the Yangtze River became finer in the upper reaches and coarser in the middle and lower reaches.Median particle size(D₅₀)of suspended sediment at Yichang,Jiujiang and Xuliujing also increased,mainly driven by the gradual increase in sand content from 0 in 2012 to 10% in 2018.In flood season(May to October),the percentage of suspended sediment volume of 10-100 ?m increases more significantly than that in dry season,and the seasonal difference in particle size increases.However,D₅₀ of suspended sediment at Chenglingji(outlet of the Dongting Lake)gradually increased from 3 ?m in 2003 to 11 ?m in 2019;D₅₀ of suspended sediment in Hukou(outlet of the Poyang Lake)gradually increased from 2 ?m in 2006 to 11 ?m in 2019.The supplementation capacity of the two lakes to the fine sediment in the mainstream of the Yangtze River decreased.