Significant sediment loads are also carried by the Osse (or 'Ovia') river that runs from the Yoruba Uplands into the Benin Estuary, and by the smaller Imo river system.
4.4.2 FRESHWATER - 'BLACK-WATER'
However, because of the very high rainfall, a number of river systems (primarily the New Calabar, Sombreiro and Warri systems) rise within the Niger Delta itself. They discharge 'black-water', so called because of the organic acids it carries in solution. The water is darkened but remains clear, as it carries little or no sediment.
In contrast to the white-water bearing rivers that arise in the North and drain into the Delta, there is a much less pronounced seasonal peak flow in the locally fed blackwater rivers. This is because the freshwater ecosystems of the Delta act as a sponge, regulating flow by soaking up water in the wet season and releasing it slowly through the dryer months.
Where black-water discharges into a white-water river, there may be a visually dramatic contrast, as where the local Okomu river flows into the Osse.
4.4.3 BRACKISH WATER
A major factor in the hydrology of the Niger Delta is the intrusion of seawater, making water in much of the area slightly salty, or brackish. The influence of seawater extends up to 60km inland, from northeast of Sapele (on the Benin river), northeast of Warri, and as far as Choba on the New Calabar river. It extends rather less far along the coastline, for just under 20 km between the Sangana and the Forcardos.
As freshwater comes down the river to meet seawater, there is a gradient of salinity. The concentration of salts is in a constant state of flux; both daily, according to the tides, and seasonally, according to levels of freshwater discharge. At the peak of the white-water discharge in October, freshwater extends out into the Atlantic; flying over the Niger Delta at that time of year, the silt-laden waters are an impressive and slightly disturbing sight. However in January, with low volumes of freshwater in the rivers, brackish water briefly intrudes into freshwater systems. During this time, tidal movements have a greater influence on water flow than the river currents.
4.5 SOILS
It is no coincidence that the world's first great civilisations developed where there were especially fertile soils; soils are the basis of human economic activity. An understanding of soils goes a long way towards understanding human ecology.
4.5.1 SOIL PROFILES
Soils can be described in terms of their 'profile'; that is, a description of a vertical section from the surface down to the parent material. This can variously be achieved by simply digging a hole, by taking out a plug sample with an auger, or as is often the case, by looking at the sides of a borrow-pit. Usually a series of layers can be seen. They are known as 'horizons'.
The figure 5 and the explanation below describe a generalised soil in terms of four horizons, working down from the surface. Many of the Niger Delta soils are far from 'typical', but here are the relevant terms nonetheless.
