Channel Morphological Changes Of Niyang River And Its Response To Environmental Changes In Recent 50 Years

In recent years,under the background of global warming and the increasing impact of human activities,the Qinghai-Tibet Plateau is experiencing the phenomenon of increasing disaster risks such as glaciers retreat,permafrost degradation,ice-lake collapse,icefall and debris flow,which seriously affects the sustainable development of regional economy and society.In order to explore the evolution of regional river morphology and its response to environmental changes under the imbalance change of the “Asian Water Tower”,this paper selects the mainstream of the Niyang River as the research object.Based on field investigations,1: 100,000 topographic map in 1970 and Landsat remote sensing images in 1990,2000,2010 and 2020,this paper analyzes the changes of river morphology in recent 50 years.The results are as follows:(1)The upper reaches of the Niyang River flows through high mountains and gorges,mainly along straight and slightly curved single channels.The middle reaches of the Niyang River is distributed alternately in canyons and wide valleys,with meandering and braided channels developed.The lower reaches of the Niyang River is broad valley landforms,with wide and shallow channels and numerous branches.Among them,meandering channel accounts for 46% of the total river section studied,gradient and sinuosity is between 0.23%-3.13% and 1.28-1.7respectively,the average sinuosity is 1.48.Straight channel is developed in the construction section of the Gengzhang Reservoir in the middle reaches,accounting for 1%,with the slope of0.23%,the sinuosity of 1.18.Braided channel accounting for 20%,with a slope ranging from0.31% to 0.52%,an average slope of 0.37%,the sinuosity ranging from 1.18-1.37,an average sinuosity is 1.24.Anastomosing channel accounting for 33%,with the slope of 0.07%-1.04%,an average slope of 0.47%.And the sinuosity of 1.22-4.36,an average sinuosity of 2.07.(2)Through the extraction of the parameters such as the river morphology,sinuosity,the total numbers of mid-channel bars,channel width and the length of the mainline,it is found that the changes of river morphology of the Niyang River in the past 50 years mainly occurred at the two time nodes of 1990 and 2000.For example,the channel changed from multi-channel to single-channel in 1990,and the length of the mainline and channel width were lower than other years.The total numbers of mid-channel bars was small,and the river sinuosity was large.However,in 2000,the channel changed from single-channel to multi-channel.And the length of the mainline of the whole channel and the average channel width was higher than other years,the total numbers of mid-channel bars was more,the sinuosity of the whole channel was small.In addition,the general morphological changes are the entrance position of the tributaries of the meandering river(near Jinda town,Niangqu reach),the braided river channel with wide and shallow valleys(from Gadan village to Jiegang section of Gongbujiangda reach,from Zheba village to Qiangga section of Bahe reach,from Maiding to Selong section of Gengzhang reach)and the anastomosing channel with wider valleys(near Baiba town,Bahe reach and near Xuquebo of Gengzhang reach),as well as the river section with stronger human activities(near Gengzhang reservoir,Gengzhang reach).(3)Based on the analysis of annual average runoff and annual precipitation at five hydrological stations in Gengzhang,Lazi,Yangcun,Nuxia and Lasa from 1979 to 2010,combining with the change characteristics of regional climate conditions in the Qinghai-Tibet Plateau during 1976-2018,found that the annual runoff and annual precipitation had a significant downward trend around 1990,and began to rise after 1998.The overall change of Niyang River is response to regional climate change.At the same time,the increasing impact of human activities makes the Gengzhang and Linzhi reaches channelized obviously.