Study On The Evolution Of The Liaohe-Daling River Delta In The Past Four Hundred Years

The Liaohe River Delta,a famous river delta in China,is formed by several rivers,such as Liaohe River,Daling River,Daliao River,and Raoyang River.The Liaohe River and the Daliao river are the meandering rivers with large basins and multi-sources,while the Daling River is a braided river with large slope and a large amount of sediments.The history of Liaohe River from Panjin to the sea has only been more than one hundred years,and the river channel in the downstream of Daling River has dramatically changed in the past few hundred years,so this multi-river delta is relatively rare.In addition,the coastline changes in this area are recognizable.The spatial and temporal distribution of river sediments such as Liaohe River,Daling River,and Daliao River and the evolution process of the modern Liaohe River Delta are absolutely significant research topics.Based on historical data,archeological sites,historical maps,and remote sensing image data,the coastline changes and river swings in the Liaohe River Delta are restored for the last four hundred years.The clay mineral characteristics,detrital mineral characteristics and geochemical characteristics of Liaohe River,Daling River,Daliao River,and Raoyang River are obtained from the tests on the samples which recovered from the downstream of the four rivers.A mixed model of sediments from Daling River and Liaohe River is constructed by using the clay mineral characteristics to qualify the contribution rate of the two rivers in the Liaohe River Delta.The results of lithology description,grain size analysis and electric conductivity of stirred clayey sediments,which are carried out on the 28 boreholes drilled in the Liaohe River Delta,are combined with a chronological framework to establish the isochronous stratigraphic framework of the Delta.The geomorphological analysis results of the spatial relationship between the Daling River sediments and Liaohe River sediments,isochronous stratigraphic framework,coastline changes,and river diversions are mutually confirmed to reveal the formation and evolution process and sedimentary dynamic mechanism of the Liaohe River Delta.Changing the name of the Liaohe River Delta to the Liaohe � Daling multi-river Delta emphasizes the important role of Daling River in the formation and evolution of the modern Liaohe River Delta,which provides research ideas for the evolution of multi-river delta in the future.The main results of the research may be summarized as follows:(1)The coastline of the Liaohe � Daling River Delta in 1600 was roughly located at Sihaitun Village,Wenziguan Village,Nanquanhe Village,Longwang Village,Nantuozi Salt Beach,Tianzhuangtai,and Baimiaozi.By 1800,the coastlines of the Daling River estuary and Daliao River estuary had been pushed more to the sea due to much runoff,while the coastlines of rivers with less runoff and sediment transport changed much slower.From 1800 to 1909,due to the formation of Liaohe River(Shuangtaizi River)and the Daling River diverting into Panjin Bay,the area of Panjin Bay was rapidly reduced.From 1909 to 1956,Panjin Bay was quickly deposited,and the coastline advanced to the sea for about 20 km.After the founding of the People's Republic of China,the runoff was reduced due to the construction of reservoirs and sluice gates.The changes in coastline were mainly caused by artificial breeding,land reclamation,and marine engineering.Since 1600,the river channel in the downstream of the Daling River has undergone six huge diversions.(2)The grain size of the Daling River is coarser than that of the Liaohe River,Daliao River,and Raoyang River.The clay mineral assemblage of the Daling River is smectite-illite-kaolinite-chlorite with the ratio of smectite content to illite content greater than 1,while the assemblages of Liaohe River,Daliao River,and Raoyang River are illite-smectite-chlorite-kaolinite with the ratio less than 1.The heavy mineral assemblage of the Daling River sediment is epidote-magnetite-ilmenite-amphibolemagnetite,and the Daliao River is epidote-ilmenite-hornblende-magnetite,while Liaohe River is ilmenite-magnetite-epidote-garnet-hornblende.The mass fraction of heavy minerals of Daling River is greater than 1%,whereas that of the Liaohe River and Daliao river are less than 0.1%.The geochemical compositions of the sediments of the three rivers are not much different.(3)The Daling River Basin,Liaohe River Basin,and Daliao River Basin are dominated by physical weathering rather than chemical weathering.The physical weathering of Daling River Basin is slightly strongest of the three,while that of Daliao River Basin is lowest.The surface rock and climatic conditions of the basins are the main factors affecting the mineralogical and geochemical characteristics of sediments.Clay minerals can be used as tracing fingerprints due to its stability in the estuary.A provenance discrimination system is stablished with a hybrid model.(4)Based on the sedimentary facies,that divided by the lithology,grain size characteristics and conductivity characteristics of the boreholes,chronological framework,and provenance discrimination system,the isochronism stratigraphic framework of the Liaohe-Daling River Delta is constructed for the past four hundred years.Depending on the changes of sedimentary facies,coastline and river locations,and the distributions of different river sediments,the evolution of the Liaohe-Daling River Delta over the past 400 years are divided into 4 phases: 1600~1800,1800~1909,1909~1956 and 1956 to the present.The amount of sediment transported by rivers and the location and quantity of river estuaries are the main controlling factors for the evolution of the modern Liaohe-Daling River Delta.(5)The Liaohe-Daling River Delta is shaped by rivers of different natures,such as the braided river and the meandering river.The multi-stage delta lobes of the two rivers are superimposed on each other,and finally,a unique multi-river delta complex is formed.The formation of the Liaohe River in the past one hundred years covers the role of Daling River.The interaction of multiple rivers,the impact of human activities,and the strong and weak transition of rivers and oceans are the characteristics of the evolution of Liaohe-Daling River Delta,and the evolution model of multi-river delta is proposed.