Relationship Between Landslide-dammed Lakes With The Evolution Of Upper Stream Of The Yellow River From Longyang Gorge To Liujia Gorge On Northeast Margin Of Tibet Plateau

Upper reach of the Yellow River located in a transitional place between the Loess Plateau and the Qinghai-Tibetan Plateau that experienced intense rock uplift,also situated in the convergent region among southeastern Asian monsoon,westeastern monsoon and Westerlies.Therefore,such a particular geographic location,the upper reach of the Yellow River,would sensitively reflect either variations of rock uplift of the Tibetan Plateau or climatic changes.Tons of huge paleolandslides(with mass volume of 108 m3)occurred and most of them dammed the upper reach of Yellow River since Late Pleistocene.As an old saying that is prevalent in the northwestern China goes,?the Yellow River breaches every three years and divulses every one hundred year?,which vividly expresses a phenomenon that the breach events occurred so far as the evolution of this ?Mother River? in China.Most of these dams were resulted from landslides especially huge paleolandslides along the River.With regards to the high magnitude,intense distribution,and long duration of the landslide-dammed lake,the dam and dammed lake would probably not only cause risks to life and property,but also result in immense geomorphic effects.A unique local sedimentary environment that differentiates from other breaches of the Yellow River would have its own special sedimentary rules which haven't been studied well so far.Thus by diagnosing three landslides,Gelongbu,Dehenglong-Suozi and Ashegong landslides in Xunhua,Jianzha and Guide basins,we studied the lanslides features,mechanisms and associated magnitude of the dammed lake,as well as the relationship among the landslide,dammed lake and the evolution of the Yellow River after analyzing the evolution of the Yellow River.The follows are the main results and conclusions retrieved from this study:Firstly,on the basis of previous studies,this study analyzed and summarized multiple evolutions of the upper reach of the Yellow River and associated elevations.It also studied 12 terraces sections for sedimentologies and 16 OSL samples for geochronologies,particularly focusing on those associated with landslide-dammed lakes.We preliminarily summarized that the features of the slopes along the upper reach of the Yellow River: All the lower(six)terraces aggradated during glacial or the transitional time from glacial to interstadial and the period of summer monsoon attenuation.Therefore,the aggradation of all these six terraces were dominated by climatic changes,but also positively response to the regional rock uplift which could be indicated by the conincidence between the average incision rate and GPS rock uplift rate.Secondly,five paleolandslides that dammed the River were chosen to conduct the field investigation and twelve OSL samples were retrived to explore their geochronologies.Integrating with previous literature and my master research results,this study statisticly analyzed and assumed that there were five stages of paleolandsides since Late Pleistocene,which correlated to the Marine isotope stage(MIS)5,ca.80 ka,MIS 3,deglacial and the early Holocene.Except landslides occurring ca.80 ka which were probabliy resulted from earthquake,Paleolandslides occurring during MIS 5,MIS 3,and early Holocene majorly resulted from moisture strengthen of summer monsoon,and those happened in deglacial would be probably associated with snow melting and monsoon strengthening.Thirdly,after integrating ages of paleolandslides and terrace aggradation,this study analyzed the influence of climate change upon terraces,landslide and dammed lake in terms of stratigraphy and geochronology: paleolandslies that mainly occurred during interglacial or interstadial often occurred just after the formation of terraces.The paleolandslide mass or dammed lake lacustrine sediments often overlaid the fluvial sediments that aggradated during glacial or the transition from glacial to interglacial/interstadial.Therefore,the occurrence of landslides and formation of terraces were closely related to the uplift of northeastern Tibetan Plateau,but majorly influnced by glacial-interstadial cycles and summer monsoon strengthening since Late Pleistocene.However,some period of paleolandslides and terraces were dominated by rock uplift,such as those occurring around 80 ka.Forthly,this study also unraveled the geomorphic relationship between paleolandslides/dammed lake and river channel,for example: the variation in gradient of the slopes and geometry after paleolanslide damming the river,the aulvsed channrel;plaleolandslide reactivation due to dam breach;the influence of channel variation upon paleolandslides reflecting features of special zonality and periodicity;the formation of dammed lake not only resulting in the sedimentary ratio changes along the upstream and downstream of the dam,but also bringing about huge potential threats to the life and property down stream.