African Unification Front
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THE GAMBIA BASIN
Location: Atlantic West Africa
Size: 70,000 km2 [Spread over 3 countries].
The Gambia basin is one of the smaller basins but it is one of the 52 basins that are transboundary. The Gambia River Basin occupies about 5.4% of the West Coast. It straddles the republics of Senegal, Guinea, and Gambia. This Basin borders the much larger Senegal Basin, and the Volta Basins, as well as numerous other small basins that stretch southward and west along the western African coast. The West Coast is the region grouping all the basins draining to the sea from Senegal to Nigeria. Altogether the smaller watersheds, including the Corubal, Scarcies, Mana-Morro Basins cover 4.7% of the continent and spread over 13 countries.
The Gambia River has its sources in the high rainfall mountainous Fouta Djallon in the north of the Central Guinea region. The total quantity of water leaving Guinea for Senegal is estimated at 3 km³/year. The river then flows northwards to enter The Gambia in the extreme east of the country. Contradictory information exists about the discharges entering The Gambia. According to different sources, they range from 4 km /year [average of 1951-1990] to nearly 10 km³/year. Its flow is highly seasonal: the peak discharge is about 2000 m³/s, but for six months the inflow at the Gambian border is less than 10 m³/s. In May it falls below 0.5 m³/s.
Because of the flat topography of The Gambia and the low river discharges during the dry season, salt water moves up to about 70 km upstream in the wet season and 250 km upstream in the dry season. The tidal variation at the mouth is about 1.6 m.
The higher, upstream part of the basin in Guinea is badly eroded. Irrigation would be possible in the downstream part, where the potential has been estimated at 20000 ha. There are 60000 ha of suitable soils in the Gambia basin in Senegal. There is a plan to construct a dam at Kekreti for hydropower and this could irrigate an estimated 15000 ha in Senegal and 55000 ha in The Gambia.
Soils suitable for irrigation in The Gambia are estimated at 80000 ha. There are about 104200 ha of swamps, of which 33500 ha are cultivated. Mangroves account for an additional 67000 ha. In the dry season, the salt tongue moves upstream at a rate of 15-20 km/month. It is thought that an additional withdrawal of 1 m³/s would increase the penetration of the salt tongue by 1 km/month.
The safe limit for irrigation from the Gambia River without major dam construction is, therefore, estimated to be no more than 2400 ha in the dry season. However, if the planned Kekreti dam on the Gambia River in Senegal is constructed, it is expected that 15,000 ha can be irrigated in Senegal and 55000 ha in the Gambia. Moreover, this dam could contain salt intrusion during the dry season. The development of these 55,000 ha would require 0.275 km³/year of water. A further 25,000 ha of mangrove cultivation would require 0.125 km³/year of water.
The salinity problem affects rice fields, especially those along tributaries. Another possible solution to this is the introduction of new varieties of seed which grow well in both upland and swampy areas. Although the annual irrigation water requirement is only 10% of the discharge, any water abstraction within the basin in the dry season should be studied very carefully until the Kekreti dam is constructed, in view of the low discharges in the dry season and the danger of increasing salt intrusion from the sea.
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