Along the entire coastline of Nigeria lies a belt of mangrove swamp forest from 10 to 60 miles in width, which is intersected by branches of the Niger Delta and innumerable other smaller rivers and creeks. Beyond the swamp forest is a zone, from 50 to 100 miles wide, of undulating tropical rain forest. The country then rises to a plateau at a general elevation of 2,000 ft but reaching 6,000 ft to the east, and the vegetation changes from woodland to savannah. In the extreme north, the country approaches the southern part of the Sahara.
                                                                                                     
(Source:  CIA World Fact Book http://www.cia.gov/cia/publications/factbook)

The Niger, the third largest river in Africa, enters Nigeria from the Northwest and runs in a southeasterly direction, meeting its principal tributary, the Benue, at Lokoja, about 340 miles from the sea. It then flows south to the Delta, through which it runs into the Gulf of Guinea via numerous channels. Other main tributaries of the Niger are the Sokoto and Kaduna rivers. The second great drainage system of Nigeria flows north and east from the central plateau and empties into Lake Chad.