Effects Of Human Activities On Sediment Deposition And Flooding In The Lower Yellow River In The Last 4000 Years

According to historical records,the Yellow River’s downstream sediment deposition characteristics,causing frequent flooding over nearly 4,000 years,have brought profound hardships to the people of the North China Plain.Downstream sediment deposition results from the erosion of Loess Plateau soil,with the intensity of soil erosion depending largely on local soil and water conservation conditions.During historical periods,these conservation conditions were primarily reflected in vegetation cover,closely tied to human activities,particularly the agricultural expansion associated with changes in population.As a result,the downstream sediment deposition serves as a vital link between the environmental changes in the Loess Plateau and downstream flooding.Quantifying the sediment deposition process in the downstream Yellow River is thus crucial for unraveling the relationship between human activities on the Loess Plateau and downstream flooding.This study focuses on the Yellow River basin,using field surveys,sediment profile sampling,and optically stimulated luminescence dating to determine the sediment deposition rate in the low river floodplain section at the top of the downstream alluvial fan in Temple Village,Mengzhou City.The research correlates this data with downstream flooding and factors reflecting human activities,such as Loess Plateau population,historical conflicts,cultivated land area,and forest cover.By integrating theories from multiple disciplines,including soil erosion,ecology,and sedimentology,the study reveals the impact of nearly4,000 years of human activities on sediment deposition rates and flooding in the downstream Yellow River.The main conclusions are as follows:（1）Over time,the downstream Yellow River has exhibited a trend of increasing flood frequency,with periodic reductions coinciding with changes in dynasties.Notably,it has shown long periods of alternating"peaceful"(P_I,P_III,P_V,and P_VII)and"disastrous"(P_II,P_IV,and P_VI)phases.The flood frequencies during these periods,namely P_I（2000-309 BC）,P_II（309 BC-11 AD）,P_III（11-602 AD）,P_IV（602-1138 AD）,P_V（1138-1285 AD）,P_VI（1285-1947AD）,and P_VII（1947 AD onwards）,were 0.4,4.8,2.2,28.6,10.9,60.9,and 0 times per 100years,respectively.In terms of spatial distribution,flood points in the downstream Yellow River are primarily situated in the upper and middle parts of the alluvial fan.Along various ancient river channels,there is a pattern where flood points are more prevalent in the upstream areas and become scarcer downstream.This distribution tends to become more evenly dispersed along the river’s course with changes in the river channel.（2）Over the past 4,000 years,the sediment deposition rate in the downstream Yellow River has shown an increasing trend.In five main stages,P₁（3387-748 BC）,P₂（748 BC-44AD）,P₃（44-475 AD）,P₄（475-976 AD）,and P₅（976-present）,the sediment deposition rates at the Temple Village section of the low river floodplain in Mengzhou City were 6.63,30.3,18.6,35.93,and 76.1 cm/100 years,respectively.These rates closely correspond to the fluctuations in downstream flood frequencies,indicating the reliability of historical flood records.Moreover,the study suggests that the duration of the"peaceful"period from the Eastern Han Dynasty to the Tang Dynasty should be around 600 years（11-602 AD）rather than the previously assumed 800-plus years（69-907 AD）.（3）In history,the downstream Yellow River management frequency displayed a stepwise increase but couldn’t effectively control the worsening trend of Yellow River flooding.This was mainly due to an ancient focus on downstream management,neglecting the entire river and prioritizing flood control over sediment control.On one hand,those advocating a comprehensive approach recognized the importance of"whole-river management"but remained at the theoretical level,allowing the downstream-oriented approach,primarily based on levee and dam construction,to dominate.On the other hand,sediment control concepts,which address the root causes of Yellow River flooding,were not emphasized until the Ming and Qing dynasties,resulting in the long-term dominance of flood control strategies.（4）Over the past 4,000 years,the population of the Loess Plateau exhibited temporal characteristics of exponential increase in response to economic growth and reduced taxation and periodic decrease due to dynastic changes caused by wars.In terms of spatial distribution,population density displayed an exponential increase across the entire region,various provinces,and major sediment-producing areas.Additionally,the population center shifted in a southeast-to-northwest direction over time.（5）The cultivated land area in the Loess Plateau experienced four significant expansions in the Western Han,Sui-Tang,Song,and Ming-Qing periods,corresponding to changes in population（1.13×10⁷,1.20×10⁷,1.36×10⁷,and 1.04×10⁸,respectively）.The maximum expansion of cultivated land area in these periods was 87,000 km2,85,000 km2,107,000 km2,and 187,000 km2,with corresponding cultivation rates of 0.14,0.13,0.17,and0.23,respectively.The cultivation rates followed a similar pattern to the fluctuations in the highly sediment-producing regions but remained lower（0.18,0.13,0.21,and 0.3）.（6）Over the past 4,000 years,the sediment deposition rate and flood frequency in the downstream Yellow River have exhibited nearly synchronous changes.These changes are primarily attributed to population fluctuations in Loess Plateau,leading to alterations in land use practices that affect the intensity of soil erosion.This can be summarized as a model:"Population increase-Expansion of cultivated land area-Intensified soil erosion-Increased sediment deposition rate in the downstream-Increased flood frequency;Population decrease-Reduction in cultivated land area-Weakened soil erosion-Decreased sediment deposition rate in the downstream-Decreased flood frequency."The fact that flood frequency in the downstream is synchronously reduced on a timescale of decades or even years due to population decline resulting from wars further highlights that once human interference in the Loess Plateau is alleviated,downstream flood disasters can be mitigated.