ANALYSIS OF THE URBANIZATION EFFECT ON THE TAIHU PLAIN RIVER NETWORK——A CASE STUDY OF WUCHENGXIYU REGION OF TAIHU BASIN

For its rapid rate and high degree of urbanization,Wuchengxiyu Region is selected as the study area for studying the effect on the plain river system in Taihu hinterland by quick urbanization in recent years.Based on the river system of three periods:1960s,1980s and 2003,the river network is classified into three levels according to the width of channels.Then from the view of river geomorphology,the river structure characteristics,such as the river network density,the water surface rate,the complexity and structure stability of river network,the fractal of river network,are calculated quantitatively,as well as the long-term water level changes.The results show that:1) nearly all the river system features are shorter;2) river length and area of rivers and lakes of different levels are mostly in a decreasing trend except the area of First Level River.The river network density,the water surface rate and fractal dimension of river network all become smaller.Also the complexity and structure stability of river network weaken.The river evolution tends to trunk and single;connectivity of rivers and lakes reduces.There are obvious differences between the conditions of pre-urbanization and post-urbanization.Furthermore,the decrease trend is more obvious since 1980s.In the other hand,the reducing of river system has great impacts on the storage and flood control capacity of rivers,under the condition of urbanization.It makes the annual mean water level and the mean water level in flood season show the increase trend,leading to a high probability of the occurrence and risk of flood.As a result,during the period of urbanization,the rules of river network evolution should be respected to protect the river system and get the aim of making human and river system coexist peacefully.