alluvium

Sediment deposited by flowing water — clay, silt, sand, and gravel carried by rivers and streams and laid down on floodplains, in deltas, and at the mouths of canyons. Alluvial soils are among the most fertile on earth, renewed by every flood. The agricultural civilizations of the Nile, the Tigris-Euphrates, the Indus, and the Mississippi were built on alluvium — soil delivered by the river and spread across the floodplain like a gift.