riverbed vs river bottom

riverbed and river bottom both are nouns.

Word	Noun	Adjective	Verb	Adverb
riverbed	Yes	No	No	No
river bottom	Yes	No	No	No

As nouns, riverbed and river bottom are synonyms defined as:

riverbed and river bottom: a channel occupied (or formerly occupied) by a river

riverbed (noun)	river bottom (noun)
a channel occupied (or formerly occupied) by a river	a channel occupied (or formerly occupied) by a river