Alluvium

alluvium \ ə-ˈlü-vē-əm\ noun: clay, silt, sand, gravel, or similar detrital material deposited by running water (plural alluviums or alluvia)

Etymology: Latin alluvius, from Late Latin alluere wash, flow past, from ad- to, in addition + luere atone, fulfill, pay, wash, cleanse

There should be a nursery rhyme incorporating fluvial and alluvium.