Historical bank-breachings of the lower Yellow River as influenced by drainage basin factors

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The Yellow River is famous for sediment-related disasters in history. The bank-breaching frequency of the lower Yellow River has been adopted as a proxy index to study the historical sediment-related disasters, and then it has been related to various indices describing the natural and human factors of the Yellow River basin. Consequently, some correlations have been established. Climate has profound influences on the bank-breaching frequency of the lower Yellow River. Before the Sui and Tang Dynasties (681–907 AD), natural vegetation in the middle Yellow basin was not yet significantly affected by man. With the increase in the humid index, growth of vegetation became greater and the bank-breaching frequency of lower Yellow River declined. After the 8th Century, however, the relation between bank-breaching frequency and the humid index became inverse, i.e., with the increase in the humid index, bank-breaching frequency also increased. This is because, to a great degree, the natural vegetation had been destroyed since the 10th Century. The bank-breaching frequency can be positively correlated to the frequencies of both large flood and drought disasters. The positive correlation between bank-breaching frequency and frequency of large droughts is because the Yellow River channel decreased in size during dry periods; when large floods came, bank-breaching was more likely to occur. Human activities are the major factor influencing sediment-related disasters in the lower Yellow River. Historically, the increase in population led to a higher bank-breaching frequency, and the periods of low population corresponded to the periods with low frequency of bank-breaching. The increase in the bank-breaching frequency of the lower Yellow River occurred in association with the northward shift of the agriculture–animal husbandry transitional zone, and when the agriculture–animal husbandry transitional zone shifted southward, the bank-breaching frequency declined.