What does alluvium mean?
The definition of alluvium
Al*lu”vi*um, n.; pl. E. Alluviums, L. Alluvia.
Etym: [L., neut. of alluvius. See Alluvious.] (Geol.)

Deposits of earth, sand, gravel, and other transported matter, made by rivers, floods, or other causes, upon land not permanently submerged beneath the waters of lakes or seas. Lyell.