WordNet records no synonym and no antonym for delta. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
a low triangular area of alluvial deposits where a river divides before entering a larger body of water
“the Mississippi River delta”
an object shaped like an equilateral triangle
Synonyms, antonyms and the broader and narrower relations for delta come from Princeton University’s WordNet. Full sources and licences.