Alluvia

a deposit of sand, mud, etc., formed by flowing water.
the sedimentary matter deposited thus within recent times, especially in the valleys of large rivers.
Historical Examples

The source of the elements which compose the Pampean alluvia is very uncertain.
The Argentine Republic Pierre Denis

It was only in the extra-glacial tracts that alluvia of interglacial age were at all likely to be preserved in any abundance.
Fragments of Earth Lore James Geikie

Hence, I believe that some of our so-called “post-glacial” alluvia will eventually be assigned to an interglacial horizon.
Fragments of Earth Lore James Geikie

Then comes the Negro down in the alluvia, with dark skin, woolly hair, and prognathous development.
History of the Negro Race in America From 1619 to 1880. Vol 1 George W. Williams

We see clearly the alluvia of two streams converging from right and left, and uniting to pass to the sea through Brading Harbour.
The Geological Story of the Isle of Wight J. Cecil Hughes

noun (pl) -viums, -via (-vɪə)
a fine-grained fertile soil consisting of mud, silt, and sand deposited by flowing water on flood plains, in river beds, and in estuaries
n.

“matter deposited by flowing water,” 1660s, from Medieval Latin alluvium, neuter of alluvius “washed against,” from Latin alluere “wash against,” from ad- “to, against” (see ad-) + -luere, comb. form of lavere “to wash” (see lave).
alluvium
(ə-l’vē-əm)
Plural alluviums or alluvia
Sand, silt, clay, gravel, or other matter deposited by flowing water, as in a riverbed, floodplain, delta, or alluvial fan. Alluvium is generally considered a young deposit in terms of geologic time.

alluvial adjective