Reconstruction Of Holocene Paleofloods Along The Abandoned Channel Of The Lower Yellow River And Its Regional Comparative Study

The flood disaster is one of the most serious natural disasters in China,especially with the trend of global climate change.In recent years,frequent extreme hydrological events have received widespread attention.The ancient flooding history is an important way to deal with the flood risk.However,most gage records span relatively short periods（<100 years）,and flood events could only be traced to thousands of years in documents,which is extremely limited in space-time scale.Conducting research on paleoflood deposits can greatly enrich the spatiotemporal flooding records,leading us to understand its frequency law and driving mechanism scientifically and reasonably.Thus,it has become a research hot spot in Quaternary geology and global change science.The alluvial plain is the birthplace of civilization and densely populated area,where flood disasters significantly impact social economies and people’s livelihoods,but the paleoflood research needs to be further strengthen.In addition,sedimentary records of ancient floods are important for Quaternary stratigraphic correlation,water system evolution and hydrogeological structure analysis.The Yellow River Basin is one of the world’s most complex rivers.It is difficult to manage,with the lower Yellow River being known for high susceptibility to perturbations and inundations.At present,paleoflood research based on geological records mostly focuses on the Yellow River’s middle and upper reaches,while the downstream alluvial plain is a poorly studied area.Several scientific questions require further research.What are the sedimentary characteristics and identification markers of paleofloods in the lower plain area?Whether the reported paleoflood records are widely distributed in the lower reaches?Is it possible to compare the paleoflood records in the whole river basin and further explore the flooding factors?To answer these questions,this study selected the frequent channel avulsion areas in the lower Yellow River to conduct the research of paleoflood reconstruction and regional comparison.In general,the specific contents are as follows:（1）Based on the sedimentary facies analysis on fluvial deposits,the typical lithofacies and facies associations were summarized.The investigation of post-disaster of"July 2016 Xingtai Flood"in the North China Plain,modern over-bank deposits of the lower Yellow River,and paleoflood-profiles along the abandoned channel area were carried.Nine main lithofacies types were uncovered,namely,coarse sand lithofacies with laminated and tough crossbeds（St）,fine sand lithofacies with crossbeds and parallel bedding（Sl）,fine sand lithofacies with horizontal bedding（Sh）,fine sand-silt lithofacies with soft-sediment deformation structures（Ss）,silt lithofacies with fine lamination（Fl）,silt-mud lithofacies with massive structure（Fm）,silty sand lithofacies with laminated to massive structure（Scf）,silt facies with massive structure and freshwater shells（Fcf）,and silt-mud facies with pedogenic features（P）.These lithofacies types can be further simplified into flooding and inter-flooding lithofacies associations.The former includes lake and swamp deposits（FA1）and paleosols（FA2）,while the latter includes seasonal overbank deposits（FA3）and special flood deposits（FA4）.In addition,sedimentological analysis results show that paleoflood deposits were characterized by distinct inverse-graded flood deposits,including fine-grained slackwater deposits（SWD）in the lower part and coarse-grained flood sediments in the upper part.Due to the Yellow River basin’s unique geological and geomorphological features,SWD can still be an important indicator for identifying ancient floods.Based on the understanding of sedimentary facies characteristics and significance,particular attention should be paid to the abrupt contact relationship between flood deposits and their adjacent strata.（2）The Holocene paleofloods of typical sections were identified and reconstructed.Longwangmiao（LWM）section in Daming,Hebei Province and Gaocheng site（GC）section in Puyang,Henan Province,were selected as representative natural outcrop and archaeological site profiles in the abandoned Yellow River channel areas.Environmental proxies such as grain size,chemical elements,and pollen were used to quantify the differences in sedimentary environments.Widely accepted flood-identification methods,including end-member analysis,local polynomial regression analysis,principal component analysis,and change-point detection,were used to compare and verify the paleoflood results.Based on radiocarbon and luminescence ages and Bayesian age-depth models,the flood records of different profiles were reconstructed.The LWM section recorded five frequent flood periods（F1:>7.3 ka,F2:3.0～2.8 ka,F3:2.4～2.1 ka,F4:1.7～1.6 ka,F5:<0.7 ka）and one seasonal flood period（SF1:2.7～2.5 ka）since the Holocene.Five major flood periods（F1:3.0～1.6 ka,F2:1.1～0.7 ka,F3:0.5～0.4 ka,F4:0.2～0.1 ka,F5:<0.1 ka）during the Holocene has been reconstructed in the GC site.（3）The stratigraphic characteristics and the corresponding paleoflood records were compared and analyzed in the study area.Our results with the existing research related to paleoflood show that the Quaternary strata in the study area mainly include three sedimentary units:loess-like deposits in the early Holocene,mainly lacustrine deposits during the middle Holocene,and sedimentary cycles of paleosols and flood deposits during the late Holocene.Besides,the two ancient flood records during the Northern Song and Western Han periods are widely distributed in the area with obvious characteristics,which agrees well with the period when the abandoned Yellow River flowed through the area as recorded in historical materials.（4）The Holocene fluvial chronologies of the entire Yellow River Basin were collated and analyzed to explore flood frequencies and related factors.Using the relative cumulative probability distribution function of 86 radiocarbon(¹⁴C)and 166optically stimulated luminescence（OSL）ages,eight river active periods with intervals greater than 200 years were obtained,namely:10,000～9,800,9,600～7,800,7,000～6,500,4,600～4,100,3,500～2,400,1,800～1,500,1,000～600,and 400～0 a BP.Based on this research,the driving forces of floods in different periods were further analyzed and discussed.This study revealed that the tectonic factors control the distribution and diversion of the river channel and the flooding cycle on a large scale,which should not be overlooked in the North China Plain.Flood periods in the Yellow River old course area were mainly affected by climate factors in the prehistoric period.However,the steady improvement of human activities has intensified and changed flood characteristics since about 4.0 ka.