ridge has 3 distinct synonyms spread across 2 of its 11 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.
Synonyms, antonyms and the broader and narrower relations for ridge come from Princeton University’s WordNet. Full sources and licences.
any long raised strip
a long narrow natural elevation on the floor of the ocean
any long raised border or margin of a bone or tooth or membrane
extend in ridges
“The land ridges towards the South”
plough alternate strips by throwing the furrow onto an unploughed strip
throw soil toward (a crop row) from both sides
“He ridged his corn”
spade into alternate ridges and troughs
“ridge the soil”