In the upper reaches of Taihu Lake Basin, the river courses assume branches in shape and fan shaped drainage system distribute in the lower reaches. Throughout the drainge area river, lakes and link up one another. Affected by sea tide and the river flow. the hydrographic net of lower river has become changeable and the capacity of preventing flood and fighting calamities weak. In recent years such functional variations have happened as "moderate rain with heavy calamities", the inadequate water resources and the serious water pollution, etc. The main cause is that with the effect of natural and artifical factors, many new problems and controdictions in the human being and nature, present and future. economy and environment are brought forward and need to be solved further.