river bottom vs riverbed

river bottom and riverbed both are nouns.

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

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

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

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