Changes Of Channel Patterns And Floodplains Of Parlung Zangbo River And Its Response To Environmental Change During The Past 50 Years

Channel patterns are the product of the long-term development of the river's water-sediment conditions and the environment in which it is located.It is the dominant feature of the river bed when it reaches the equilibrium of erosion and deposition.Under the background of global warming,the climate of the Qinghai-Tibet Plateau is changing in the direction of warming and humidification.Glaciers are rapidly retreating,lake areas are expanding,river runoff is increasing simultaneously,and disasters such as ice avalanches,landslides,and debris flows have occurred frequently,has caused a serious impact on the regional ecosystem and people's production and life.With the imbalance and instability of the "Asian Water Tower" and the intensification of human activities,great changes have taken place in the river process and river channel evolution in this area.Studying the changes of river channel patterns and its response to environmental changes in this area can provide a full understanding of the “Asian Water Tower”.Long-term dynamic changes and development trends provide basic data and theoretical basis.This paper takes the Parlung Zangbo River as the research area,and on the basis of field investigation,selects 8 periods of Landsat remote sensing images from 1973,1987,1990,1995,2000,2010,2015,and 2020 to analyze the typical river sections of the river in the past 50 years.The evolution process of the chanel patterns was analyzed,and based on the observation data of climate,runoff and sediment transport in the region,as well as the related research results of glaciers,ice lakes,vegetation,etc.,the impact of environmental changes and human activities on river processes and river channels was discussed.The preliminary conclusions are as follows:(1)The river channel patterns of Parlung Zangbo are divided into four river types: straight River,meandering river,braided river and anastonmosing river according to the river morphology.Among them,the straight river and the meandering river are mainly developed in the canyon section and the nodal valley section,and the gradient is relatively large.Braided rivers and anastonmosing rivers are mainly developed in wide valleys,and their gradients are relatively small.The segment ratio is only 0.2%.In terms of flow velocity,the velocity of the straight river is the highest at 3.18-3.53 m/s,the velocity of the meandering river is between 2.40-4.23 m/s;the velocity of the anastonmosing river is the smallest,only 1.08 m/s.The channel width and braid index showed the characteristics of gradually widening downstream along the course,but both reached the highest value in the wide valley section between the Cizula and Kange;the channel curvature was relatively high in the canyon section.(2)From 1973 to 2020,Yuxu,Qingduo,Kadai Village,Kada Village and Guxiang reach river area,channel width,and channel count index all showed a trend of expansion.Among them,the Yuxu reach of Bodui Zangbo and In the multi-dip river reaches,the channel area,width and channel count index expansion are the most obvious.There are obvious differences in the evolution process of channel patterns in each reach.For example,during the period from 1973 to 2000,the channel area,channel width,channel count index,and the number and area of bars of the main reaches of the Parlung Zangbo all increased;Although the area,channel width,channel count index,bars area and number of bars showed an increasing trend,the increase was small;the channel width and channel area of the Yuxu reach showed a slight shrinking trend.In addition,in 1973,the channel area and mid-bank area of the Yuxu reach reached the highest level,and in 1987,they shrank rapidly.After that,both the channel area and the channel width showed a slight increasing trend.During the period from 2000 to 2020,the river sections of Kadai Village,Kada Village and Guxiang all showed the characteristics of channel shrinkage.The channel area and channel width reach expanded significantly.(3)In 2010,the channel pattern of Yuxu Reach was transformed from straight to braided river,but the braided river strength was low,and the highest channel count index was 3,which should be in the transition from straight to braided river.In 1990,the entrance of the Yalong Zangbuhui in the Qingduo reach was transformed from the braided river to the straight River;in 2015,the entrance of the Yalong Zangbuhui in the Qingduo reach was transformed from the straight River to the braided river.(4)From 1970 to 2000,the runoff and sediment volume of Parlung Zangbo showed a significant increase trend;from 2000 to 2020,the runoff and sediment volume decreased.Corresponding to the trend of runoff and sediment volume increasing first and then decreasing,during the period from 1970 to 2000,the Parlung Zangbo channel was mainly expanded as a whole.The area of the shoal showed an increasing trend.During the period from 2000 to 2020,the channel width,channel area,channel count index and the number of bars all decreased,while the exposed area of the central shoal increased.The increase of channel area and channel width is the response to the increase of runoff,while the increase of channel count index,the number and area of the bars is the performance of the increase of sediment volume.At the same time,the frequent occurrence of disasters such as ice avalanches,glacial debris flows,and debris flows have also had an important impact on the evolution of regional river channels.For example,in 1973,a large-scale glacial debris flow broke out in Guxiang gully,damming the river,resulting in an increase in the channel area of the Guxiang reach,and a decrease in the number and area of the bars.(5)With the development of economy and society,human activities in the region are increasing,such as the construction of hydropower stations,the channelization of river channels,and the expansion of cities and towns,all of which have an important impact on the evolution of regional river channels.Among them,the construction of the hydropower station in the Yuxu River section has raised the gradient of the channel upstream of the dam,which has widened the channel of the reservoir section,rapidly deposited sediment on both sides,and developed a large number of bars.