Study On River Health Assessment Under The Background Of Urbanization

In the 21^st contrary,the rapid urbanization has become an irreversible development trend.Although urbanization generally could improve the economic development,it also threatens river ecosystem health,resulting in water pollution,hydrological disorder and biodiversity decline.With this in mind,how to maintain river health in the context of urbanization is a major problem that is urgently needed to be solved.River health assessment could help to understand and analyze the river’s physical,chemical and biological integrity,and provide decision support for river management.Thus,for better management and restoration options,it was important to establish a river health assessment system to balance the contradiction between urban development and environmental protection.Aiming to rivers in urbanized catchment,this thesis established an evaluation system,and applied it to assess the rivers’condition in Liangjiang New Area,China.Moreover,this thesis also analyzed the mechanisms by which urbanization affected river health components.The main research contents and conclusions can be summarized as the following:（1）The health definition for rivers in urbanized catchment was identified.A thematic content analysis has been used to extract a set of common themes from 37health definitions in literatures published in the last 30 years.Four criteria which appeared most frequently were identified,including:maintain system structure integrity,meet the demands of society,sustainability and stability.Combining these four criteria and the characteristics of urbanized river in geomorphology,water quality,biological community and social values,a health definition for urbanized river is presented:under the background of urbanization,river can sustainably maintain stable ecosystem structure and continuing meet social needs and expectations.Specifically,under the background of urbanization,（1）a healthy river should have stable channel structure;（2）a healthy river should keep native species dominant and contain integrated ecological components;（3）a healthy river should have good water quality and sufficient water quantity to support river function;（4）a healthy river should sustainably meet the social meets and expectations.（2）The evaluation system for rivers in urbanized catchment was established.As the influences of catchment urbanization on river health components include multiple spatial scale processes,the indicators of river health assessment were determined at catchment scale（i.e.catchment land use condition）,riparian scale（i.e.riparian vegetation,channel structure）and habitat scale（i.e.water quality,biological community,substrate and water flow condition）.Besides,the social functions,such as flood control ability and landscape quality were also included in the indicator system.On the other hand,the fuzzy comprehensive assessment method was applied to establish the assessment model.Moreover,the assessment software was developed based on MATLAB language.（3）The influences of urbanization on river health components were investigated.Take Liangjiang New Area as the study area,the influences of urbanization on river water quality,macroinvertebrate community and physical habitat was analyzed.Firstly,in Liangjiang New Area,nutrient and microbiological contaminants,which were the principle water quality problems,were significantly different（p<0.05）between catchments with different urbanization levels.Pearson correlation analysis and stepwise regression analysis indicated that the urban land cover was the principle contributor to increased nutrient and fecal coliform concentrations.Secondly,14,297 individuals belonging to 14 orders and 46 families were identified.The macroinvertebrate community compositions were found to differ significantly（p<0.05）between catchments with different urbanization levels.The sensitive species（e.g.EPT taxa）were mainly centralized in the catchments with low urbanization level,while the tolerant species（e.g.Tubificidae,Chironomidae,Physidae）were dominant in catchments with high urbanization level.The biotic indices,including taxa richness,Shannon dversity index,BMWP and FBI,were also significantly different（p<0.05）among catchments with different urbanization levels.The environmental variables at habitat scale explained largest 30.7%of macroinvertebrate community variation,while catchment and riparian variables explained 10.6%and 22.6%of the variation.The relationship between urban land use and habitat variables indicated that catchment urbanization resulted in less diverse but more tolerant species mainly by increased nutrient pollution and sedimentation.Thirdly,there were no significant differences（p>0.05）in phytoplankton and zooplankton taxa richness and diveristy between catchments wth different urbanization levels.Fouthly,the comprehensive habitat quality index was significantly different（p<0.05）between catchments with different urbanization levels.In the low urbanization level catchments,the substrate quality and channel structure condition were significantly better in high urbanization level catchments.（4）The rivers health conditions in Liangjiang New Area were assessed.The principle health status for rivers in high,moderate and low urbanization catchments was“Good”,“Fair”,and“Poor”,respectively.The feature of rivers in low urbanization catchments were better than that of rivers in high urbanization catchments in land use condition,channel structure,water quality and macroinvertebrate community.The threshold of impervious surface cover for degrading river health status is about25%.Pearson correlation analysis indicated that the catchment land use condition,riparian vegetation quality and channel structure were significantly correlated（p<0.05）with river water quality,substrate condition and macroinvertebrate community.In this ease,the catchment land use planning,riparian vegetation habitation and natural channel protection were important for river ecosystem health.In summary,this thesis revealed the influences of urbanization on river health,and developed the evaluation system to assess the river conditions in urbanizated catchment.The results of this thesis could significantly help to guide the river management and rehabilitation in urbanized catchment.