Study On The Evolution Of River Networks And Its Water Environmental Effects In The Typical Plain River Network Region Of Lixiahe

The effect of river networks evolution on water environment in the plain river network area is a hot issue in the background of underlying change and its environmental effects.Due to the complexity of hydrology and the diversity of water environmental factors in plain river network area,it is difficult to separate the effect of river changes from many other reasons.The typical plain river network of Lixiahe region,which is in the lower reach of Huai river basin,has numerous Lake-Marsh.Because of the special pot-shaped terrain characteristics,the planning and allocation of water resources have been studied extensively.While it is located in the developing areas,water environment was not fully studied,especially from the perspective of water system structure and connectivity.Therefore,to explore the influence of river networks evolution on water environment is an important topic on theory and practice.Xinyanggang River is one of the four major drainage channels in Lixiahe region.Xinyanggang basin was selected as the study area,which is the typical district of Lixiahe region.The Remote Sensing(RS),Geoinformation System(GIS),field monitoring,model simulation and scenario analysis methods were used to study the relationship between river networks changes and water environment effect in different spatial scales.The influence of river changes on self-purification capacity of water body was taken as the core,revealing the function rule among land use change,river network change and water environment effect.On this basis,water environmental protection measures were found underlying the change of catchment area and river network,in order to serve the sustainable development of economy society demand.The main research contents include the following three aspects:(1)The change of land use and its water environmental effectTaking Xinyanggang basin as study area,remote sensing and field survey were used to explore the temporal-spatial variation characteristics of land use.The Xinyanggang basin were divided into three parts:upstream section,which is the source of Xinyanggang River;middle reach section,which is the urban of Yancheng city;downstream section,which is the estuary of Xinyanggang River.Pollution sources and water environmental capacities were comparatively analyzed during the three sections.In the end,the influence of land use change on water environment was analyzed in combination with the temporal-spatial variation characteristics of water quality.The results showed that:?From 1991 to 2010,urban was transferred mainly by farmland,which had been expanded by nearly 4 times,and farmland had reduced two thirds.The landuse change were mostly happened in the middle reach section.?The pollution sources were also chiefly concentrated in the middle reach section.The contribution rates of chemical oxygen demand(COD),ammonia nitrogen(NH4N)and total phosphorus(TP)pollution in the middle reach section were 73%,65%and 58%,respectively.The contribution rates of COD,NH4N and TP in both upstream and downstream sections were similar and relatively small,which were 13%?17%and 21%,respectively.?The water quality was serious in the middle reach,while relatively good in upstream and downstream.The main pollution factors of the middle reach were TP,NH4N,COD and biochemical oxygen demand(BOD5)and the pollution had aggravated year by year.?The main land use of middle reach section was urban and the pollutant inputs of COD,NH4N and TP were greater than the environmental capacities,while the main land use of upstream and downstream sections were farmland and paddy field and the pollutant inputs of COD were less than the environmental capacities.(2)The evolution of river networks and its water environmental effectBased on the analysis of the land use of Xinyanggang basin,Yandu district,which is the upstream section of Xinyanggang basin and has less industrial pollution and landuse change,was selected as the research area.Construction of the index system of struction and connectivity of plain river network area and field experimental observation methods were used to analyze the temporal-spatial characteristics and mechanisms of river network of Yandu district and its water environmental effects.The results showed that:?1970s?2010s,there was variability among different river grade.According to urbanization,the lower grade(?)rivers had changed earlier and dramatically than the high grades(???)rivers.The number of lower grade river had decreased continuously and the change intensity was violent,while the higher grades rivers had little variety in the number and gain in the area and length to some extent.?The river density and water area rate of the whole Yandu district had been running down steadily,and the decay rate in the slow urbanization period(1970s?1980s)was greater than the rapid urbanization period(1980s?2010s),but the curvature of the river network had shaped little.?There was a spatial difference among the river network change of Yandu district.The change degree was the most dramatical in the highest urbanization area,whatever the number,length,river density or water area rate.?The main factors of river network change,which influenced phorsphrous retention,were the grade and connectivity of the river.Under the circumstances of raining,the concentration of particulate phosphorous(PP)had been increasing in the high grade rivers,while decreasing in the low grade rivers.When water flow exchange between the tributary and the main stream became obvious,the degration rates of phorsphorous in the good connectivity rivers decreased significantly than the poor connectivity rivers even under the likely landuse conditions.(3)The vary of river-lake connectivity and its water environmental effectThe change of river-lake connectivity has an important role in the self-purification capacity of lake.According to the analysis of the spatial-temporal characteristics of river network in Yandu district,Dazonghu Lake,which is the origin of the Xingangyang River,was chosen to analyze the change of river-lake connectivity,especially the change of inflow-outflow rivers and its water environmental effect,using the literature comparision and scenario analysis methods.The results showed that:?The area and shape of the Dazonghu Lake were greatly reduced,especially in the south and north of the lake.There were big problems in the connectivity of inflow-outflow rivers,especially in the Nanzhou River of west and Xingyanjiehe River of the east,which having sever sediment deposition.The purse nets,which were used to raise crabs,had seriously invaded and occupied the lake channels,resulting in poor drainage.?The eutrophication state of Dazonghu lake was mild and the main pollution factors were total nitrogen(TN)and TP.Except for the upstream pollution,aquatic breeding and agricultural non-point source pollution,the degration of water quality were also connecting with the lake shape and river-lake structure.?Comparing the optimization programs of water system structure,the results showed that:river velocities increased after the implementation of the program,especially on the new west-east orientation channel,which could improve the lake water environment.It has certain characteristics that this study took the typical plain river network region of Lixiahe as the research area based on the perspective of the river network structure and connectivity changes,using the chain of "human activities-landuse change-river network change-water environmental effect" as the main line.The research approaches and results also have certain innovations.It could provide a scientific basis for exploring the water environmental protection measures under the changes of river network plain areas.