Impacts Of Hydropower Cascade Development In Heihe Basin On Rive Water Environment

Large-scale hydropower construction projects are the general situation of the world's rivers.River and water storage in a noticeable difference between physical,chemical and biological aspects of natural rivers,significantly changed the hydrological characteristics of the river,material conversion and tr ansmission flux,basin to belong to a series of negative environmental impacts,such as river continuum damage,such as water quality decline,which makes the cascade hydropower development is controversial,so how to solve the cascade hydropower developme nt and the contradiction between the river water environment has become the current urgent need to solve the problems in the process of basin comprehensive treatment.At present,there were many researches on the ecological environmental effects of cascade hydropower development in river basins at home and abroad,but most of them focus on some big rivers,while there are relatively few researches on inland rivers,especially on different scales?basins/river sections?.Therefore,this paper takes the construction of cascade hydropower stations in heihe river basin as the research object,focusing on the response of water temperature,water quality,runoff and sediment transport to cascade hydropower development,focusing on the individual level to the system level,and elaborating the water environment impact mechanism of cascade small hydropower development on large and mesoscale?river reach/reservoir area?.The main research results are as follows:?1?After the cascade power station runs,the natural state of the river changed greatly.The runoff series showed a significant increase trend,while the sediment transport series showed a significant decrease trend.The abrupt change years of the two were 2004 and 2001 respectively,indicating that river sediment transport was more sensitive to dam construction than the change of the runoff series.The main reason may be that the change of runoff flow was greatly affected by precipitation.In this year,due to the water storage and flood detention function and interception function of the dam,the flood season of the river has been extended for about 1 month,and the water level of the river has been raised in the dry season.At the same time,the sediment content in the lower reaches of the dam had been greatly reduced,with the maximum reduction reaching 763800 t/month.This indicated that river damming changed the concentration of runoff distribution and restricts the capacity of sediment transport.?2?The water temperature structure of reservoir appeared stratification after impounding.Based on the analyed of the measured water temperature of two high dam reservoirs in baobao and longshou,it was found that the water temperature gradient of longtou reservoir in space was larger than that of the last reservoir.Moreover,the water temperature in both reservoirs was structured and stratified.However,with the increasing of water depth in the reservoir area,the water temperature droped more slowly,and only 0.1⁰.2?decreased for every 10 m drop after 15 m.?3?On the spatial distribution of river water quality in the reservoir?group B?,the optimal reservoir upstream?group A?and to the reservoir downstream?group C?,showed that hindered the pollutants from the upstream to the downstream river power plant construction of the migration process,lead to upstream contaminants by sedimentation on the bottom of the library,reducing the water to the background value of pollutant in water,so as to improve the dam under the condition of river water quality;However,in the natural river section downstream of the reservoir area,the pollutants accumulated along the river.Moreover,the leading factors affecting water quality changing in different river sections were different,among which TP,NH₃-N and COD_Mn were the main indicators affecting water quality change in group A,while EC,TDS,salinity,TN and NO₂-N have A greater impact on water quality in group C.?4?Based on a complete water quality monitoring of hydrological years showed that the Heihe river water quality condition overall was good,the comprehensive water quality categories was given priority to with?classes and class?,but had a certain time and space points to the opposite sex.In terms of time,the order of pollution was dry period>wet period>flow period.The main reason was that there were differences in the replenishment and water exchange frequency of res ervoirs in different seasons,and the accumulation degree of pollutants in rivers was also different.The main pollution sources in group A are organic compounds,follo wed by nutrient salts.Group B was mainly the accumulation pollution of organic matter an d nutrient salt,while the influence of natural factors was relatively weak.The highest contribution rate of group C was biochemical pollution,followed by non-point source nutrient pollution.From the perspective of long time series,due to the multi-stage interception of the dam,river pollutants accumulate at the bottom of the res ervoir year by year,which makes the water quality in the reservoir area have the hidden danger of regeneration pollution,which needs to attract the attention of relevant departments.