The Niger River is the principal river of western Africa, extending over 2500 miles (about 4000 km). It runs in a crescent through Guinea, Mali, Niger, Benin and Nigeria, discharging through a massive delta into the Gulf of Guinea. The Niger is the third longest river in Africa, exceeded in length by only the Nile and the Congo River (also known as the Zaire River).
The Niger takes one of the most unusual routes of any major river, a boomerang shape that baffled European geographers for two millennia. Its source is just 150 miles inland from the Atlantic Ocean, but the river runs eastward away from the sea into the Sahara Desert, then takes a sharp right turn and heads southeast to the Gulf of Guinea.
Ancient Romans thought that the river near Tombouctou was part of the Nile River, while early 17th-century European explorers thought that it flowed west and joined the Senegal River. While the true course was probably known to many locals, Westerners only firmly established it in the late 19th century.
This strange geography apparently came about because the Niger River is two ancient rivers joined together. The upper Niger, from the source past the fabled trading city of Tombouctou to the bend in the current river, once emptied into a now-gone lake, while the lower Niger started in hills near that lake and flowed south into the Gulf of Guinea. As the Sahara dried up in 4000-1000 BC, the two rivers altered their courses and hooked up. (This explanation is generally accepted, although some geographers disagree.)