Final answer:
The Niger River, approximately 2600 miles in length, is West Africa's main river, flowing through several countries before emptying into the Atlantic Ocean in Nigeria.
Step-by-step explanation:
The river in question, which is approximately 2600 miles long and is considered West Africa's main river, is the Niger River. Starting from its source in the Guinea Highlands, the Niger traverses a substantial portion of West Africa before reaching its delta and emptying into the Atlantic Ocean in southern Nigeria. Along its journey, the Niger River is pivotal to the regions it flows through, providing essential water resources for the western Sahara and Sahel and playing a crucial role in the development of historical trading centers like Gao and Timbuktu.