The Gambia River is the country's dominant feature, flowing across a plateau of Miocene-Pliocene sandstone, compacted sediment composed predominantly of quartz grains formed from 23.7 to 1.6 million years ago. In the east, narrow valleys are separated by broad interfluves or flattish hills. In the west, lower and smaller sand hills alternate with depressions filled in with sand so that a flat plain is formed.
The Gambia River in western Africa, 700 miles (1,120 km) long, rising in the Republic of Guinea and flowing westward through The Gambia into the Atlantic Ocean. Its major tributaries are the Sandougou and the Sofianiama. The Gambia is one of the finest waterways in Africa and the only western African river that is easily accessible to oceangoing shipping. It constitutes a unifying factor for the independent state of The Gambia, which consists of a narrow strip of land along both banks of the river. The political separation between The Gambia and Senegal (formally ended in 1982 by the confederation of the two countries) has, however, long hindered the development of the resources of the river and its basin. A first step had been taken four years earlier, however, with the establishment of the Gambia River Development Organization, initially composed of the same two countries (joined in 1980 by Guinea).
From its source in the highlands of the Fouta Djallon, the Gambia follows a winding course to its mouth, which is a ria, or drowned estuary. The dividing and reuniting of river channels--a phenomenon known as braiding--has created several islands along the river's middle course, of which the two largest are Elephant Island and MacCarthy Island. The river is joined by numerous creeks called bolons, the largest of these being Bintang Bolon, which flows into it from the south. The width of the river's valley varies considerably along its course. The river valley is cut into a plateau of sandstone dating from the Tertiary period (66.4 million to 1.6 million years ago).
Dense mangrove swamps fringe the lower river for 60 miles (97 km) inland, after which freshwater swamps and salt flats on low-lying stretches alternate with dense clumps of small trees and shrubs that line the cliffs. On the higher slopes of the riverbank, swamps and shrubs give place to parkland and tall grass. The wild oil palm grows along the valley bottom.
The vegetation of the river and of its creeks provides a favourable habitat for insects, animals, and birds. The swamps encourage mosquitoes and tsetse flies to breed. The river abounds in fish and river creatures, including the hippopotamus and the crocodile. Among the 400 known bird species that have been recorded are the kingfisher, the cuckoo, the swallow, the heron, the sunbird, the hawk, and the grass warbler.
The swampy region closest to the river, with its dense masses of mangrove trees often growing more than 100 feet (30 m) high, abounds in wildlife but has been of little use for either agriculture or human settlement. The grass-covered river flats (known locally as banto faros) of the lower river are rendered useless for cultivation by the salt water that periodically inundates them, and settlements on them are few. The flats of the middle and upper river are of some agricultural value, however. Much of the grassland is regularly cleared, and the light soils are easily cultivable. The annual flooding of the fertile alluvial loams of the middle flats makes them especially suitable for intensive rice cultivation. On the light sandy and well-drained soils of the higher slopes, peanuts (groundnuts) grow particularly well. Cultivation and settlement have therefore taken place in the middle flats and on the higher slopes, with many villages being located on the borderline between the flats and the plateau, thus avoiding both the flooding of the lower slopes and the increasing aridity of the higher terrain.
The Gambia is one of the most navigable of African rivers; its chief value, therefore, has been its transportation function. As the main artery of the state of The Gambia, it is the principal means of transporting passengers, freight, and mail in the territory. North-south river crossings are provided by ferries such as those plying between Barra and Banjul, Banni and Kerewan, Farafenni and Yelitenda, and Kau-ur and Jessadi.
The climate is of the drier tropical type, characterized by a short and intense rainy season occurring between June and October and by a longer dry season. Near the coast the rainy season lasts longer and the rainfall is heavier, diminishing eastward. At Yundum the average annual rainfall is about 51 inches (1,300 millimeters) and the mean monthly temperature is 77 F (25 C), while at Basse Santa Su, about 270 miles inland, the comparable figures are 43 inches and 82 F (28 C). The relative humidity is high but drops from December to April, when the dry northeastern wind known as the harmattan is dominant.
The vegetation cover of The Gambia is savanna on the uplands, various kinds of inland swamp in the low-lying areas, and mangrove swamp along the brackish lower Gambia River. Few wild animals are native to the region, and those that survive are under pressure from the human and domestic animal populations. In the middle and upper river areas there are warthogs, monkeys, baboons, antelope, pygmy hippopotamuses, and crocodiles.
Patterns of settlement in The Gambia reflect three regions, expressed on both banks of the river, consisting of (1) the swamps adjacent to the river (and not extending above Kaur), (2) the riverine flats, known as banto faros (from a Malinke word meaning "beyond the swamp"), and (3) the sandstone uplands. Most rural settlement is concentrated on the uplands, which have the best-drained soils. A number of settlements are located in the banto faros on the middle course of the river, where there is less danger of flooding than in the swamps. Many villages are built on the boundary between the uplands and the riverine flats.