Niger River vs Niger

Niger River and Niger both are nouns.

Word	Noun	Adjective	Verb	Adverb
Niger River	Yes	No	No	No
Niger	Yes	No	No	No

As nouns, Niger River and Niger are synonyms defined as:

Niger River (noun)	Niger (noun)
an African river; flows into the South Atlantic	a landlocked republic in West Africa; gained independence from France in 1960; most of the country is dominated by the Sahara Desert
	an African river; flows into the South Atlantic