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Here we establish the timing of major flood events of the central Mississippi River over the last 1,800 y, using floodwater sediments deposited in two floodplain lakes. Shifts in the frequency of high-magnitude floods are mediated by moisture availability over midcontinental North America and correspond to the emergence and decline of Cahokia—a major late prehistoric settlement in the Mississippi River floodplain. The absence of large floods from A.D. 600 to A.D. 1200 facilitated agricultural intensification, population growth, and settlement expansion across the floodplain that are associated with the emergence of Cahokia as a regional center around A.D. 1050. The return of large floods after A.D. 1200, driven by waning midcontinental aridity, marks the onset of sociopolitical reorganization and depopulation that culminate in the abandonment of Cahokia and the surrounding region by A.D. 1350. Shifts in the frequency and magnitude of flooding may be an underappreciated but critical factor in the formation and dissolution of social complexity in early agricultural societies.

Soil morphology changes dramatically across the former transition from forest to grassland in the Midwestern U.S.A. That vegetation boundary shifted as a result of Holocene climatic change and fire suppression following Euroamerican settlement, but the timescale of soil response to those vegetation changes and the factors that influence it are poorly known. On steep colluvial slopes of southeastern Minnesota, Mollisols with thick, dark A horizons typically associated with grassland are found today under deciduous forest. Soils with much thinner and/or lighter-colored A horizons occur immediately up-and downslope of the forest-covered Mollisols. Most of the soils with thick A horizons are not in topographic settings found to favor organic matter accumulation in other landscapes. Principal components analysis highlights important axes of textural and mineralogical variation among horizons of these soils, often related to parent material properties. Soils with and without thick A horizons are separated along one principal component heavily loaded by high pH, reflecting the presence of dolomite fragments in the hillslope sediment that soils with thick, dark A horizons formed in. Stable C isotope analysis reveals that the Mollisols with thick, dark A horizons under forest had C input from vegetation

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