hydrographic feature reef A ridge of rocks, lying near the surface of the sea, which may be visible at low tide, but is usually covered by water. ENVO ENVO:00000130 EcoLexicon:reef FTT:202 FTT:203 FTT:729 Geonames:H.RF Geonames:U.RFSU Geonames:U.RFU REEF SPIRE:Reef TGN:21479 Wikipedia:Reef atoll reef bank reef barrier reef fringing reef ledge reef reef flat reefs shore reef submerged reef