river vs watercourse

river and watercourse both are nouns.

Word	Noun	Adjective	Verb	Adverb
river	Yes	No	No	No
watercourse	Yes	No	No	No

As nouns, watercourse is a hypernym of river; that is, watercourse is a word with a broader meaning than river:

Other hypernyms of river include stream.

river (noun)	watercourse (noun)
a large natural stream of water (larger than a creek)	a conduit through which water flows
	a natural body of running water flowing on or under the earth
	natural or artificial channel through which water flows