Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/52

What is the Niger Delta? this feature is most dramatically visible in Port Harcourt, where the old town drops suddenly to the Nembe Waterside.

The sea level continued to rise and fall during the lower (or older) Quarternary period, further shaping the changing Delta. Examples can be seen at Forcados and Bonny. At one time a sudden drop in level caused the rivers to rapidly erode new paths through deposits to the receding coastline. When the sea rose again it flooded these valleys to form the deep, wide estuaries of today.

4.3 LOCAL CLIMATE

The tropical hot monsoon climate of the Niger Delta characterises the area around the Bight of Biafra, including the lower Cross River Basin and the Southwesterly slopes of Mount Cameroon. This climate is a function of location.

The Niger Delta straddles latitude 5 degrees North of the equator. It extends into the Gulf of Guinea, dividing the Bight of Benin (to the West) from the Bight of Biafra (to the East). Except in the Northeast, where it rises to 10-15m., most of the Delta is less than 6m above sea level; it shares the monsoon climate of the surrounding Bights.

Mean annual rainfall is always high, although it varies within the Delta. Between 1948 and 1980, average rainfalls ranged from just below 2000mm. at Port Harcourt to over 4000mm. on the Southwest coast (because of the Northeast tradewinds) and around Bonny (because of the concave nature of the coast here). The Delta has such high rainfall levels because it is so near to the Atlantic, the source of the moisture; the high equatorial temperatures cause it to evaporate from the ocean surface and rise, to be carried Northwest by the Southeast trade winds. Funnelled up the Bight of Biafra, when these winds hit the Adamawa massif and rise upwards, the cooled water vapour condenses and falls as rain on the Niger Delta and on the Cross and Sanga river basins, making the area one of the wettest on earth.

Heavy rains commence in February and continue through to November, with peaks around July and September (a little earlier in Warri and a little later in Port Harcourt); relative humidity is over 80% throughout the year. However, temperature is moderated by the high cloud cover and proximity to the sea; the mean temperature as measured at Brass ranges from 24 to 27 degrees Celsius.

4.4 HYDROLOGY

The hydrology of the Niger Delta has to be understood in terms of two water types; freshwater, either 'white' or 'black'; and brackish water, which is salty (although less salty than seawater).

4.4.1 FRESHWATER – 'WHITE-WATER' 

Water coming into the Delta from the Niger/Benue river system is termed 'white-water'. It is so called because the sediment it brings down from the interior makes it very cloudy. As the rivers branch and lose velocity, this sediment is dropped and over time has formed the Delta itself.

Most white-water discharges into the Forcardos and the Nun-Ecole river systems (see Map 3B), but some also reaches the Orashi/Brass system, especially during the annual floods. Then the Niger itself spreads across to the Orashi, below Onitsha. These rivers all peak in October when the whole Niger/Benue is carrying down its maximum load, and flooding is severe. 50