Human deforestation outweighed climate as factors affecting Yellow River floods and erosion on the Chinese Loess Plateau since the 10th century

Increasing proportions of the global population are exposed to floods, including many living in the Yellow River floodplain. Since records were first kept in 602 BCE, there have been ∼1500 floods on the Yellow River, resulting in the death of millions of people. Counteracting increased flood risk requires an understanding of the relationship between climate, forest cover, erosion, and river flow. However, to assess whether the Yellow River basin is currently experiencing a period of high flood risk requires a long-term perspective. Here we use a variety of paleoenvironmental proxies (pollen, magnetic susceptibility, cladoceran assemblages) preserved in high-resolution, well-dated sediment records retrieved from an alpine lake in the middle Yellow River basin (Chinese Loess Plateau) within the agro-pastoral ecozone to examine how climatic and land-use changes affected aquatic ecosystems over the past ∼2000 years. Further, to examine changes in erosion, runoff production and flood frequency, we synthesize previously published proxy reconstructions of vegetation density, soil erosion, and dry-wet changes, as well as flood records from historical documents. We demonstrate that, following the period around the 10th century, excessive cultural deforestation outweighed climate effects and became the dominant factor that led to an unprecedented flood-rich period when drought conditions were common, cultivation expanded to meet food shortages, heavy silting raised the riverbed, and runoff and flood risk increased. These watershed changes, including enhanced soil erosion, affected the biological communities of aquatic ecosystems that led to the disappearance of planktonic cladocerans that hitherto dominated the assemblages. Increasing temperatures and weakening monsoon precipitation during the past ∼50 years, together with decreased erosion indicated by unprecedented reductions in Yellow River runoff and sediment load resulting from reforestation and damming, led to the re-establishment of planktonic cladocerans. Despite these recent changes, vegetation cover has not recovered to pre-deforestation levels, suggesting that the Yellow River is currently experiencing a high flood risk period. Our results emphasize the importance of considering vegetation–flooding relationships to help improve risk assessments and management protocols for the Yellow River.