The Niger and Benue rivers including their tributaries drain most countries.
The Niger rises in Guinea highlands, flows through Mali, Niger Republic and then into Nigeria where it then enters the Atlantic Ocean by a Delta. The Niger is the main and of course the largest river in Nigeria and in it is the Kainji Dam. We also have river Benue.
These two rivers are the main rivers in Nigeria that have their sources outside the country.
It is however worthy to note that the Niger River has a lot of tributaries in Nigeria and the major ones are:
Anambra River, Moshi River, Kaduna River and the Awum River.
The Benue River on its part takes its source from the Eastern highlands of Mandara Mountain and its main tributaries are Donga River, Katsina Ala River, Gongola River and West River.