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.

Aim To review and synthesize multiple lines of evidence that describe the spatial patterns of land use associated with prehistoric and early historical Native American societies in eastern North America in order to better characterize the type, spatial extent and temporal persistence of past land use.Location Temperate forests of eastern North America, and the Eastern Woodlands cultural region.Methods Ethnohistorical accounts, archaeological data, historical land surveys and palaeoecological records describing indigenous forms of silviculture and agriculture were evaluated across scales ranging from local (10 0 km) to regional (10 2 km) to produce a synthetic description of land-use characteristics.Results Indigenous land-use practices created patches of distinct ecological conditions within a heterogeneous mosaic of ecosystem types. At all scales, patch location was dynamic, and patches underwent recurrent periods of expansion, contraction and abandonment. Land-use patches varied in their extent and persistence, and are broadly categorized as silvicultural (management of undomesticated woodland taxa) or agricultural (cultivation of domesticated taxa). Silvicultural patches persisted for centuries and extended kilometres to tens of kilometres around settlements and travel corridors. The dynamics of agricultural patches varied among groups, with persistence ranging from decades to centuries and extent ranging from less than a kilometre to tens of kilometres around settlements. Beyond patch boundaries, human impacts on ecosystems become indistinguishable from other drivers of environmental heterogeneity. These characteristics of patches are evident across scales and multiple lines of evidence.Main conclusions Our findings challenge the view that prehistoric human impacts on vegetation were widespread and ubiquitous, and build on previous work showing these impacts to be more localized and heterogeneous by providing quantitative descriptions of land-use patch characteristics. Collaborative efforts that combine multiple data sources are needed to refine these descriptions and generate more precise measures of land-use pattern to further investigate the history of human impacts on the Earth system.

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In eastern North America, large prehistoric settlements were concentrated in and along the fl oodplains of the midcontinent, but few sedimentary records have been examined adjacent to these sites to evaluate the impacts of Native American land use on terrestrial ecosystems. Here we report a high-resolution and multiproxy paleoecological record from Horseshoe Lake, an oxbow lake in the central Mississippi River valley that is adjacent to the Cahokia site (Illinois, USA), the largest prehistoric settlement north of Mexico. Palynological and carbon isotope data document pronounced vegetation changes over the past 1700 yr driven primarily by land use, including 900 yr (450-1350 CE) of sustained prehistoric human impacts. Rapid forest clearance was followed closely by the proliferation of indigenous seed crops of the Eastern Agricultural Complex beginning ca. 450 CE, centuries before the emergence of Cahokia at 1050 CE. Agricultural intensifi cation that included the use of maize (Zea mays subsp. mays) followed this initial clearance, with peak land use intensity between 900 and 1200 CE. A large fl ood event ca. 1200 CE marks the onset of agricultural contraction and Cahokia's decline. Reforestation follows the abandonment of the Cahokia region at ca. 1350 CE. The Horseshoe Lake record thus indicates that regional agricultural activity began abruptly at 450 CE and intensifi ed over the following centuries, well before the formation of Cahokia and other large prehistoric settlements. The evidence that a major fl ood coincided with the onset of Cahokia's decline is noteworthy, but will require corroboration from additional records.

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