The Evolution Of River-lake Relationship's In The Middle Region Of Yangtze River During Historical Period

This thesis is divided into three parts,a total of four chapters.The first part namely the first chapter is an overview of the river-lake relationship's evolution in the middle region of Yangtze River.In this chapter,there's a brief introduction of the connotation and related information of the river-lake relation as well as the middle Yangtze river basin in the historical period.After the basic understanding of the evolution of the middle reach of Yangtze river,this paper's second part with two separate chapters will analyze in detail the river-lake relation revolution in the middle reaches of Yangtze River.The study of the evolution is discussed in three aspects:the process,the factors and the effect.The process of river-lake relation's evolution is viewed in three angles:the evolving process of rivers,the evolving process of lakes and the evolving process of the water-sediment exchange between the rivers and lakes;the affecting factors include the natural environment and human activities;and the effect varies among three influenced sides:the rivers,the lakes and human activities.The one chapter is about the evolution of the river-lake relation of Dongting lake and the Jingjiang reach.Jingjiang reach's riverbed has experienced the transaction from the floodplains to single straight riverbed finally to the meandering,Dongting lake has been through the period of expanding and shrinking by turns since formed,rivers and lakes' evolution brings the evolution of the river-lake relation,and in cycle the evolution of the river-lake relation promotes the rivers and lakes' evolution.River sediment deposition provides material conditions for human reclamation,in the course of history,human activities gradually become a dominant factor in rivers and lakes'evolution.The other chapter of the second part is about river-lake relation evolution between the Poyang lake and the Yangtze river in Jiujiang section which has experienced alternate evolution in main channel and ebb flow in the historical period.Especially in the Six Dynasties,the main channel of Yangtze river of Jiujiang move directly towards south witch lead to the formation of the Poyang lake today.The filling up of the silt on the outlet bottom land under the action of the river-lake interaction,directly influenced the lake's water discharge volume,which affect the function of the Yangtze river to up-lift the flows of the Poyang lake,and then brings the change to the river-lake relation.In the third part namely the fourth chapter,there is a comparison in evolution of river-lake relation between single connected river-lake and multiple connected river-lakes to find the similarities and differences.The historical period of the middle reaches Yangtze river experienced great evolution.Since the Tang and Song Dynasty,the evolution of Dongting lake has been more and more affected by human activities.With the growing deposition of sediment from Yangtze river,the four branches moving around the Dongting lakes not clear any more.The Dongting lake flows in and out in different ways thus there are eight flows straggling in this lake,jacking with each other.Without enough strong scour,it is too difficult to eject the sediment.That's why the Dongting lake has been though rapid silting up and shrinking since the late Qing Dynasty.Following this rapid trend,the Dongting lake will be forced to disappear like the Yunmeng Lakes in history which is now a sediment deposited land.Consequently,flood will be looking for new ways,either moving further south as the lake does,or overflowing to the plain around the lake.When comes to the evolution of the Poyang lake,the affection of human activity on the river-lake relation evolution is not as much great as on the Dongting lake,nevertheless since the late Qing Dynasty,the human interference has been strengthened.With the continuous deposition and extension of MeijiaZhou,Poyang lake should have continued to expand to the southern area rather than stopping expanding and even to have more apparent deposition of bottom land in the southern lake area since the late Qing Dynasty,and this is a suggestion that human activities'impact should never be ignored.If we human beings continue to reclaim along the lake bank as we used do,the water will have no choice but to keep running to the south.And the conflicts between too much water and the lack of space in the south may lead to the outbreak of the floods,the deterioration of river-lake relation,etc.