Incised Meandering And Modern Valley Development Of The Upper Taohe River

Incised meandering and fluvial terrace are well developed at the upper reaches of Taohe River,which are good carrier to study watershed system evolution and river valley development.Meanwhile,incised meandering used to be main basis for inferred that development forms of river valley was antecedence hypothesis or superimposition hypothesis,and fluvial terrace was ancient riverbed in the past,which formation and development were closely related to regional tectonic activity,climate change and lithology.Therefore,detailed study of incised meandering and fluvial terraces in this area will help to clarify evolution of drainage system and development of fluvial valley in the upper reaches of the Taohe River.This paper selects the upper reaches of the Taohe River as the research area,and analyzes geomorphic feature and influencing factors of incised meandering in the upper reaches of the Taohe River by combining geomorphological parameters of watershed,modern meander form index,tectonic and climate indicators.Meanwhile,ESR dating technology is used to obtain formation age of main fluvial terrace sequences in the area,and modern fluvial valley development process in the upper reaches of the Taohe River is reconstructed.Existing data is used to explore response of valley development to tectonic activities,climate change and river processes.The main conclusions are as follows:?1?According to the extraction of meandering form index,it is found that channel gradient in the upper reaches of the Taohe River is between 0.07 and 0.7%,curvature of these rivers are mostly above 2,river width is more thasn 100m.By comparison between watersheds,channel gradient of the Heihe River,the Baihe River and the Jialing River are 0.02⁰.7%,but the curvature are more than 2,while the river crankshaft length of Jialing River is more than 3 km,river width is between 200 and1000 m.The curvature of the Bailong River and the Minjiang River is between 1 and2,channel gradient is reduced to 0.15-3%,and river channel width is below 100 m.In addition,according to the extraction and comparison of average regional curvature,it is found that average regional curvature?₁₀₀ of the upper reaches of the Taohe River is1.17-1.22,PDF value is relatively high,and slope of CDF curve is greater.However,drainage area of the Jialing River is larger,and more tributaries,drainage networks,larger runoff,and river width are much higher than the other drainages,average regional curvature?₁₀₀ extracted by Jialing River is much larger at 1.24-1.39,the PDF value is relatively lower,and slope of CDF curve is smaller.Especially,there are many alluvial meandering and oxbow lakes in the Heihe River and the Baihe River,channel curvature is larger,but average regional curvature is relatively smaller due to frequently river cutoff.?2?Combined with the meandering forms above the planation surfaces at upstream of Taohe River,the alluvial meandering and incised meandering characteristics widely developed in adjacent area and the tectonic evolution since the late Cenozoic,it indicates that the strong tectonic uplift in the northeast of Tibet Plateau is the main background of incised meandering development in this area.In addition,the upstream of Taohe River is entirely underlined by Triassic strata and the regional lithology is dominated by metamorphic rocks such as sandstone and shale.The difference of curvature has a low correlation with Schmidt hardness,and lithology has little influence on the curvature.Regional mean curvature(?₂₀)is mainly controlled by change of regional climate index.Higher precipitation and runoff increase river hydrodynamics,river is dominated by lateral erosion,leading to higher curvature,but more higher precipitation makes river transporting capacity exceed accumulating capacity,and forming straight channel with lower curvature easily.?3?Detailed field investigation found that at least 10 river terraces were developed in the upper reaches of the Taohe River.It is best developed in the Luqu basin,where T1-T2 terraces are fill terraces,others are strath terraces,and development and preservation of T5 and above terraces are incomplete.The other river reaches are mainly developed with 4-5 terraces,which are distributed mainly in incised meandering convex bank.The top of the T10-T1 terrace gravel layer above modern river are 235 m,205 m,185 m,134 m,106-120 m,60-71 m,36-55 m,22-26m,11-16 m,2-4 m in the upper reaches of the Taohe River.ESR dating method is used to study age of gravel accumulation in the main reach fluvial terrace.It is found that ages of T10-T2 terraces were 1115±216 ka,1059±206 ka,891±181 ka,714±114ka,630±73 ka-673±129 ka,471±91 ka-523±88 ka,254±51 ka-374±73 ka,165±32ka-207±43 ka,64±13 ka.?4?According to spatial and temporal distribution characteristics of river terraces,it can be inferred that modern valley in the upper reaches of the Taohe River formed at 1.12 Ma.River valley cutted rapidly and formed fluvial terraces and incised meandering under the background of tectonic uplift.Fluvial terraces responded to climate periodic changes of orbital scale under the background of tectonic uplift.ESR chronology results showed that gravel layer of fluvial terrace was accumulated in the glacial period,such as the discovery of T10-T1 gravel layer correspond to the L₁₃?T10?,L₁₁?T9?,L₉?T8?,L₇?T7?,L₆?T6?,L₅?T5?,L₄?T4?,L₂?T3?,L₁?T2?.At the same time,combined with the downstream of Taohe River,Daxia River,Lanzhou section of Yellow River,Weihe River and other watersheds,it indicates that cutting erosion of river and formation of fluvial terraces mostly occurred in the transition stage between glacial and interglacial periods.