say it: RIV-ur-bed
riverbed is a noun meaning "a channel occupied (or formerly occupied) by a river". It carries one recorded sense.
a channel occupied (or formerly occupied) by a river
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for riverbed come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.