The Collaborative International Dictionary of English v.0.48:

Alluvion \Al*lu"vi*on\, n. [F. alluvion, L. alluvio, fr. alluere
   to wash against; ad + luere, equiv. to lavare, to wash. See
   Lave.]
   1. Wash or flow of water against the shore or bank.
      [1913 Webster]

   2. An overflowing; an inundation; a flood. --Lyell.
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   3. Matter deposited by an inundation or the action of flowing
      water; alluvium.
      [1913 Webster]

            The golden alluvions are there [in California and
            Australia] spread over a far wider space: they are
            found not only on the banks of rivers, and in their
            beds, but are scattered over the surface of vast
            plains.                               --R. Cobden.
      [1913 Webster]

   4. (Law) An accession of land gradually washed to the shore
      or bank by the flowing of water. See Accretion.
      [1913 Webster]

WordNet (r) 3.0 (2006):

alluvion
    n 1: gradual formation of new land, by recession of the sea or
         deposit of sediment
    2: the rising of a body of water and its overflowing onto
       normally dry land; "plains fertilized by annual inundations"
       [syn: flood, inundation, deluge, alluvion]
    3: clay or silt or gravel carried by rushing streams and
       deposited where the stream slows down [syn: alluvial
       sediment, alluvial deposit, alluvium, alluvion]