The Niger River (/ nadr /; French: ( le) fleuve Niger, pronounced (( l) flv ni ) ) is the principal river of West Africa, extending about 4,180 km (2,600 mi). Its drainage basin is 2,117,700 km (817,600 sq mi) in area. Its source is in the Guinea Highlands in southeastern Guinea. It runs in a crescent through Mali, Niger, on the border with Benin and then through Nigeria, discharging through a massive delta, known as the Niger Delta or the Oil Rivers, into the Gulf of Guinea in the Atlantic Ocean. The Niger is the third-longest river in Africa, exceeded only by the Nile and the Congo River (also known as the Zaire River). Its main tributary is the Benue River.

What is the source of the river niger? OK. Let's think. My stream of consciousness:
The relevant information to answer the above question is: Its source is in the Guinea Highlands in southeastern Guinea. The answer is in the Guinea Highlands in southeastern Guinea.