allegheny has 1 distinct synonym spread across 1 of its 1 sense. 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.
a river that rises in Pennsylvania and flows north into New York and then back south through Pennsylvania again to join the Monongahela River at Pittsburgh which is the beginning of the Ohio River
Synonyms, antonyms and the broader and narrower relations for allegheny come from Princeton University’s WordNet. Full sources and licences.