The Gambia lies entirely within the drainage basin of the Gambia River, a basin that extends well into Senegal and Guinea. The river’s channel only descends10 m as it flows from The Gambia’s eastern border to its mouth at Banjul. The river’s flow is very seasonal, and sea water intrudes some 200 km upriver during the dry season. The land is also flat, especially in the western half. In the eastern part of The Gambia, the river carves a meandering path through laterite-capped plateaus; terraces and shallow valleys characterize the terrain. The highest point is Red Rock, at only 53 m above sea level.
The Gambia’s five ecoregions are all transboundary areas that have their counterparts in Senegal. The Gambia River forms a natural boundary between the northern ecoregions and the one in the south — the South Bank Zone (SB). Over a century ago, most of the country was blanketed by Sudanian woodlands, wooded savannas, and gallery forests. Today, most of the more wooded landscapes are found on the south side of the river, where the South Bank Zone extends seamlessly into Senegal’s Casamance (CAS) ecoregion. The woodlands nearer the coast are denser and have much higher biodiversity than those in the east.
Major systems of mangroves and mudflats are found along the coast near the mouth of the river, extending nearly half way up the length of the country. These constitute the Estuary Zone (EZ), an ecoregion that extends well into the Saloum River complex in Senegal. North of the Gambia River, three ecoregions reflect the varying degrees of human transformation of natural landscapes into agricultural ones. The North Bank Agricultural Zone (NBA) is almost entirely devoted to groundnut, millet, and maize cultivation on sandy soils. The Agricultural Expansion Zone (AEZ) is a mix of broad cultivated valleys among laterite plateaus with shrub and tree savannas. In the east, the Eastern Transition Zone (ETZ) is more sparsely populated, and the predominance of lateritic plateaus has spared the region from the more intensive human pressures of the western regions.