The Collaborative International Dictionary of English v.0.48:

Alluvium \Al*lu"vi*um\, n.; pl. E. Alluviums, L. Alluvia.
   [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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

alluvium
    n 1: clay or silt or gravel carried by rushing streams and
         deposited where the stream slows down [syn: alluvial
         sediment, alluvial deposit, alluvium, alluvion]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "alluvium":
   acres, alluvion, arable land, ash, cataclysm, cinder, clay,
   clinker, clod, crust, debris, deluge, deposit, deposition,
   deposits, detritus, diluvium, dirt, draff, dregs, drift, dross,
   dry land, dust, earth, ember, engulfment, feces, flood, freehold,
   froth, glebe, grassland, ground, grounds, inundation, land,
   landholdings, lees, lithosphere, loess, marginal land, marl, mold,
   moraine, offscum, overflow, overflowing, overrunning, precipitate,
   precipitation, real estate, real property, region, regolith,
   scoria, scree, scum, sediment, settlings, silt, sinter, slag, smut,
   sod, soil, soot, spill, spillage, subaerial deposit, sublimate,
   submersion, subsoil, terra, terra firma, terrain, territory,
   the Deluge, the Flood, the country, topsoil, washout, whelming,
   woodland