alluvion has 6 distinct synonyms spread across 2 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
gradual formation of new land, by recession of the sea or deposit of sediment
the rising of a body of water and its overflowing onto normally dry land
“plains fertilized by annual inundations”
clay or silt or gravel carried by rushing streams and deposited where the stream slows down
Synonyms, antonyms and the broader and narrower relations for alluvion come from Princeton University’s WordNet. Full sources and licences.