"deposited by flowing water," 1794; see alluvium + -al (1).
Entries linking to alluvial
alluvium n.
"matter deposited by flowing water," 1660s, from noun use of Medieval Latin alluvium, neuter of alluvius "washed against," from Latin alluere "wash against," from ad "to, against" (see ad-) + -luere, combining form of lavere "to wash" (from PIE root *leue- "to wash").
-al 1
suffix forming adjectives from nouns or other adjectives, "of, like, related to, pertaining to," Middle English -al, -el, from French or directly from Latin -alis (see -al (2)).