Loose sedimentary material including clay, silt, sand, and gravel deposited by flowing water, especially in riverbeds, floodplains, and deltas. This fertile material forms some of the world's most productive agricultural soils.
From Latin 'alluvius' meaning 'washed against,' from 'alluere' (to wash against). The geological term was formalized in the 18th century as scientists began systematically studying sedimentary processes and formations.
The incredibly fertile soils of the Nile Delta, Mesopotamia, and the Mississippi Delta are all built from alluvium - these 'cradles of civilization' exist because rivers have been depositing nutrient-rich sediments for thousands of years! California's Central Valley, one of the world's most productive farming regions, sits on alluvium deposited by Sierra Nevada rivers.
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