Study Of River System Structure, River-lake Connectivity And Function In The Core Regions Of Taihu Lake Basin

The rivers in the Core Regions of Taihu Lake Basin belong to a plain river networks. There is a huge number of crisscrossing rivers and river system is well developed. The four cities named Suzhou, Wuxi, Changzhou and Zhenjiang of Jiangsu Province and a part of Shanghai City are located in this area. The economy of those cities is prosperous and the development is rapid. However, the activities of human are strongly affecting the river system in recent years. The length and area of rivers and lakes are changed, and then the pattern of river system changed as well. At the same time, the connectivity of rivers and lakes weakened because of the channels are silted, blocked and landfilled. The river and lake function is decided by the structure of rivers and the river-lake connectivity, which relates to the water supply, the ecosystem and the flood control of an area. The study of river structure, river-lake connectivity and function of the Core Regions of Taihu Lake Basin can meet the needs of making the river system better and keeping the health of rivers. It’s the requirement of relieving the flood control pressure as well, which also has a very important meaning for the scientific development of an area.The Core Regions of Taihu Lake Basin were selected as the study area, which were the Wu-Cheng-Xi-Yu Region (Wu Region for short) and Yang-Cheng-Dian-Mao Region (Yang Region for short). These regions were analyzed from three fields:the pattern of river system, the connectivity of rivers and lakes, and the function of rivers and lakes by using Geomorphology, Historical Geography, GIS, RS image interpretation, Graph Theory, Complex Networks and other methods. The characteristics and rules of the changes were found and the methods were compared in this study.(1) The changes of river system structureAccording the condition of study area, the rivers were well classified. Then based on the long-term and rich experiences of our team, and by using a scientific method, a combined index system of river networks structure was established and the study area’s river systems in1960s,1980s and2000s were analyzed from three ways: normal features, structure features and connectivity features. Not only the pictures of three periods showed the river system was weakened, but also the analysis results of index system indicated the length and area of rivers and lakes became smaller, and the indices of river density and ratio of water were weakened in Wu Region and Yang Region in recent50years. But the connectivity of main streams increased because human paid more attention to the main streams, and these rivers were connected and dredged up by human. Moreover, the rivers’changes of different grade have commonness and differences. In general, the river system tended to be trunk and single. And human’s activities drastically affected the rivers of lower grade. The historical rivers’evolvement of Suzhou City’s ancient town was studied and the situations of river system and floods and droughts in different periods of history were described qualitatively. The result showed that the Ming dynasty was the peak period of ancient town’s river system for the number and the length of river were the maximum.(2) The changes of river-lake connectivityAt first, on the basis of forefathers, the Graph Theory and GIS were combined and improved. Xinqu (new town) of Wuxi City was selected as the typical area and its topology of river system was constructed by Graph Theory. Then the evaluating indicators of Landscape Ecology were adopted to evaluate the connectivities in1960s,1980s and2000s. The changes of connectivity reflected the urbanization made the river network become thinner, but the connectivity of channels left was stronger. And different zone location had differnet effects on rivers.And then, the Complex Networks Theory was used to analysis the typical area: Taicang City. L Spatial Network of Taicang City’s river system was established. And the indices such as shortest path length, clustering coefficient, betweeness and others were used to comment the network, the vertices and the importance of a vertex in a network:Point13was the key vertex of the net. At the same time, by means of Network Resilience, the hypothetical change of some edge was analyzed.Furthermore,4sets of gauging stations in the study area were selected to analysis the long-term hydrological connectivity between them. The results showed that hydrological connectivity of Changshu-kunshan set tended to increase slightly and the connectivities of the other3sets were decreasing. And the connectivity of Yang Region was better than Wu Region, and the fluctuation was narrower. Then, the water level was classified into three types:Normal Water Level, High Water Level and Low Water Level. And the relationship between the connectivity of Changzhou-Wuxi set and the three water levels was focused on. The results suggested the hydrological connectivity was influenced greatly by the water project operation which should meet the requirements of flood control when the water level is high.(3) The analysis of river-lake functionThe function of rivers and lakes was divided into3categories, which are Natural Function, Social Function and Ecological Function and13sub-categories. And the functions of the important rivers and lakes in the study area were explored. The influencing mechanism of urbanization and water conservancy project on river-lake function was researched and then "Yangtze-Taihu Water Diversion" was studied in detail which is a major project in Taihu Lake Basin. According to these studies, this project had both positive and negative effects on river and lake function, and it was just a emergency measure for improving the Taihu Lake.By the studies from three fields, some improving thoughts and measures were raised replying on the practical situation of the Core Regions of Taihu Lake Basin to make the pattern of river system and river-lake connectivity better, strengthen the related river-lake function, and then promote a harmonious development between the economy, the society and the environment.