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This is a paper about the speed and intensity with which new and intensive human land use in a semi-arid environment can bring about large-scale environmental change. In particular, this paper pinpoints how and why it was that the Yellow River shifted from a long-term condition of relative stability to a later state of frequent floods and course changes in the eleventh century. It is possible to trace the environmental history of this dramatic and sudden change of state with precision and confidence. Historical sources that record the dates and characteristics of flood events downstream align well with those that note the locations and dates of human activity upstream. More important, each aligns well not only with one another, but also with information from environmental science: sediment cores that preserve soil and pollen evidence for the timing and processes of systematic change. [ABSTRACT FROM AUTHOR]