valley vs natural depression

valley and natural depression both are nouns.

Word	Noun	Adjective	Verb	Adverb
valley	Yes	No	No	No
natural depression	Yes	No	No	No

As nouns, natural depression is a hypernym of valley; that is, natural depression is a word with a broader meaning than valley:

valley: a long depression in the surface of the land that usually contains a river
natural depression: a sunken or depressed geological formation

Other hypernyms of valley include depression.

valley (noun)	natural depression (noun)
a long depression in the surface of the land that usually contains a river	a sunken or depressed geological formation