Evolution Of The River-lake Pattern In The Dongting Lake Region In The Past 100 Years Based On Historical Maps And Multi-source Remote Sensing Data

Lakes are sensitive areas and typical areas of global environmental change,and inland lakes are regarded as indicators of environmental change and climate variability in the basin.Historically,Dongting Lake was"magnificant and boundless",ranking first among the"Five Lakes"in China.However,in the past 100 years,with global climate change,lake reclamation,sedimentation,the straightening of the Jingjiang River and the construction of water conservancy projects such as Gezhouba and the Three Gorges,the area of Dongting Lake has gradually decreased,and the relationship between rivers and lakes has undergone many adjustments,changing the storage and discharge capacity of rivers and lakes,the integrity and stability of river and lake ecosystems,aquatic biodiversity,wetland functions,and the development and protection of water resources.Predecessors have carried out a lot of research on the evolution of the rivers and lakes pattern in the Dongting Lake area.Both Chinese and foreign documents have such expressions:"Dongting Lake shrank severely,and its area decreased from 6000km² in the late Qing Dynasty to 2691km² in 1978"or"The area of Dongting Lake was 4350km² in the early days of the founding of the People's Republic of China"and so on.However,it is a pity that in the large number of original documents and related research results,it is impossible to find the source of the documents on which it is based.Even though the area data of the Dongting Lake in the 7 periods released by the"Hydrological History of Hunan Province"in 2006,there are few data sources.Therefore,the so-called"Dongting Lake is seriously shrinking"is a recognized objective fact,but the source of the shrinkage data in various periods seems to be an unsolved case,or the existing research on the temporal and spatial evolution of Dongting Lake is incomplete.Based on this,this article takes the riparian lakes,inner lakes,lacustrine beaches and river channels in the Dongting Lake area as the research objects,massive basic geographic data and satellite remote sensing data from the late Qing Dynasty,the Republic of China,the early days of the founding of the People's Republic of China and the 1960s are studied.This article uses remote sensing interpretation,map analysis,historical comparison,mathematical statistics and other methods to carry out research on on the evolution of the rivers and lakes in the Dongting Lake area,originally inverts the time and space evolution of Dongting Lake in the past100 years,especially since the operation of the Three Gorges Reservoir,reveals the mechanism of different factors on the evolution of the river and lake,predicts the future evolution trend,and adjusts the lake management thinking and the ecological protection of the land space in the lake area for the post-Three Gorges era Provide basis for repair.The main research conclusions are as follows:1.Since the Late Qing Dynasty(1896),the area of lakes along the Yangtze River has undergone four sequential stages of change from 1896 to 1949,steeply shrinking in the1950s,rapid shrinking from 1960 to 1970,and basically stable since the 1980s.5216.37km² decreased to 2702.74 km² in 2019,with a shrinkage rate of 48.19%.In terms of spatial evolution,the main manifestations are the enclosed precipitation of Datong Lake,the southward migration of South Dongting Lake and the replacement of lake embankments,the partial remnants of West Dongting Lake,the three-sided enclosure of East Dongting Lake,and the limited"expansion"after the 1998 flood.The shape of Dongting Lake has evolved in a gradually compact and regular direction,and the lake shoreline has become more and more straight.The evolution of Dongting Lake is mainly affected by three major factors:crustal subsidence,water and sand erosion and siltation,and human activities.Since the middle period of the Republic of China,the Dongting Lake area has been reclaimed at 2150.83 km₂,which is the direct cause of the shrinkage of Dongting Lake.However,the future evolution of Dongting Lake will be mainly controlled by the discharge of water and sand from the Three Gorges Reservoir.It is predicted that the Dongting Lake will not disappear before 2150.2.Since the middle period of the Republic of China(1938),the lakes in the embankment have experienced three time series changes:the rapid increase from 1938 to 1958,the rapid shrinking from 1959 to 1978,and the continuous increase since 1978.The number and area of inner lakes have increased from 640 and 994.33km² in 1938 to 1057 and1578.33km² in 2018.Inner lakes with different characteristics have different changes.In terms of scale,the area of the inner lake?1km² accounts for more than 80%of the total area of the inner lake,and its change determines the overall trend and degree of change of the lake in the lake area.In terms of genetic type,the area of the residual inner lake is obviously larger than that of the reservoir type inner lake and the dumb river type inner lake,and its area accounts for more than 70%of the total area of the inner lake.In terms of regional distribution,the lake area in the original polder area shrank from 994.33km²in 1938 to 635.68km² in 1978,and then expanded to 1041.91km² in 2018.The area of the lake in the new polder area increased rapidly to 551.08km² from 1938 to 1958,and after1958 it increased and decreased simultaneously with the original polder area.3.Since the middle period of the Republic of China(1938),the beaches of Dongting Lake have experienced four time-series changes:the obvious expansion from 1938 to1948,the shrinking of the beach from 1948 to 1958,the significant expansion from 1958to 1998,and the slow shrinking from 1998 to 2018.During the dry season,the area of the beaches expanded from 1622.17 km² in 1938 to 1962.28 km² in 2018.With the addition of 980.96 km² of high-level beaches that were reclaimed,the area of the beaches increased by 1,321.07 km².Since the construction of the Three Gorges Reservoir started in 1994,the average annual beach area of Dongting Lake has expanded from 1,634.28km² before the operation of the Three Gorges Reservoir to 1,845.60km² after the operation.The Three Gorges Reservoir adjusted the water supply to the downstream during the dry season,and did not reduce the beach area by increasing the water volume of Dongting Lake.The reasons for the changes in the beaches are the reclamation of the beaches,the changes in sediment and outflow,and the mining of lake sand.Since the middle period of the Republic of China,1025.31km² of high-level beaches have been used for reclamation in the lake area,occupying 1125.51km² of lake water area.Since 1951,the sediment deposited in Dongting Lake has raised the bottom of Dongting Lake by an average of0.85m.Since 2010,Dongting Lake has been unable to make ends meet,and due to the massive mining of lake sand,the beaches of Dongting Lake have shrunk in recent years.4.The modern river channel of the Jingjiang River section has frequent horizontal swings,and the ancient channel plane swing zone of 20-32 km appears spatially.In terms of morphology,it is divided into two types:lateral gradual swing along the main channel and abrupt migration of river channels.The evolution model includes the protruding bank siltation advancing-the concave bank collapse and receding type,the convex bank skimming and cutting the beach-curved crest reduction type,and the center beach drifting along the bank-the concave bank string trench flow type.The drainage area of Songzi,Hudu,Ouchi and Tiaoxian River in Dongting Lake area is 9659.26km².Since the middle period of the Republic of China,the number and length of natural river systems have decreased significantly,from 825 in 1938 and 3,287.75km to 264 and 2364.79km in 2018.The number and length of artificial river systems have increased significantly from 31 in1938 and 104.89km to 290 and 1370.66km in 2018.With the implementation of lake reclamation and flood storage and reclamation projects in the lake area,the water system connectivity,node connection rate and water system connectivity of the Jingnan Four Rivers all show a decreasing trend.The development degree of natural river system is gradually reduced,and the artificial river connection is gradually reduced.The water system has increased substantially.The improvement of river channels and the construction of river channels in the lake area have improved the regulation of the tributaries of Songzi River,Hudu River,and Ouchi River on the runoff of the river.However,the tributary structure of the Huarong River Basin is worrying.