The main physical features are the geology, relief and drainage systems. They are examined generally and on regional basis.
The geology of Nigeria shows that the rocks found in the country range from the PreCambrian and Quaternary geological periods. The main rock types found in Nigeria can be broadly classified into basement complex, sedimentary and volcanic rocks. Their distribution in the country is shown in Fig 37.1.
The basement complex is another name for Pre-Cambrian rocks. They are the oldest and most stable exposed rocks in Nigeria. They are known to have been formed over 1, 500 million years ago. These rocks contain old igneous and metamorphic rocks such as granites and migmatites as well as remnants of ancient sedimentary rock series, which have been changed by heat and pressure that is, metamorphosed to form various metasediments such as quartzites, schists, marble, and so on. In some instances, the old sedimentary rocks were folded and broken up into mountain ranges.
Basement complex rocks cover half the land surface of Nigeria and underlie the sedimentary rocks in the remaining half. These rocks are found in four main areas:
- The areas that lie north of Niger and Benue Rivers. This area has the largest coverage of basement complex rocks, and includes parts of northwestern, north-central and the Jos Plateau areas, that is, the area generally referred to as the North-Central plateau region of Nigeria. This portion of the basement complex is linked to the Western highlands by a neck near Bussa, where there are falls and cataracts along the Niger River.
- The Western highlands or uplands of southwestern Nigeria. They cover a greater part of the region with the exception of a relatively narrow coastal zone south of Abeokuta.
- The third area is found in south-eastern Nigeria and it extends as far north as Yola.
- The fourth and smallest tract of the basement complex is found north of the Benue River in Taraba and Adamawa States of Nigeria. This is the area covered by Biu Plateau and the Mandara Mountain.
Sedimentary rocks are formed by the accumulation and consolidation of mineral and particulate matter deposited by the action of water or, less frequently, wind and glacial ice. The latter process is not known to have occurred in Nigeria, in recorded history. The oldest known sedimentary rocks were formed during the Cretaceous geological period and were
essentially of marine origin. Examples of sedimentary rocks found in Nigeria include sandstone, limestone, shale, coal, lignite and clay etc. Sedimentary rocks are found within sedimentary basins in Nigeria. These include the Sokoto-Rima basin, the Chad basin, the Niger-Benue trough (valley), the Niger delta and the coastal lowland region of the country.
Volcanic rocks are formed from the cooling of molten rock material, or magma, in the aftermath of volcanic eruptions or lava flows. Volcanic rocks are found in parts of Biu Plateau and Jos Plateau, which are known to have experienced active volcanic eruption in the past. These volcanoes are currently inactive and are classified as extinct volcano.
Relief refers to variations in the height or elevation and in the form or shape of the land surface. Nigeria has different types of relief and landforms. The country is dominated by plains which are generally less than 600m above sea level, but on the eastern border with the Cameroon are the Eastern highlands, which rise to about 2419m in the Chappal Waddi, the highest known point in Nigeria. In the north central region, the Jos Plateau rises sharply from below 600m at the margins of the north central plains, to over 1000m above sea level. The highest point on the Jos Plateau is the Shere Hills, at a height of 1, 781.6m.
The area of the country can be broadly classified into three main relief regions, namely:
- The plains.
- The highlands.
- The troughs and river valleys.
Relief Map of Nigeria
The Plains are the dominant relief features in Nigeria. The major plains in the country are the coastal lowlands, the Niger Delta, the flood plains of Rivers Niger and Benue, the Bornu plains, often referred to as the Chad basin, and the Sokoto plains. These lowlands or plains are made up of sedimentary rocks. They are generally below 300m above sea level.
Relief features classified under highlands include mountains, plateaus, ‘high plains’ and hills. The highest highlands are the Eastern Highlands and the Jos Plateau, where peaks stand over 2133m and 1524m, respectively. The Eastern Highlands consist of a range of highlands that run from the north-east to the south-eastern border region of Nigeria namely: Mandara Mountain, Shebshi Mountain, Adamawa Plateau, Obudu Plateau and Oban Hills.
Rivers Niger and Benue Troughs
There are two major troughs or depressions on the relief surface of Nigeria. These are the Niger trough and the Benue trough. Interestingly both troughs are drained by the two major rivers in Nigeria.
The Niger trough was down warped millions of years ago (Cretaceous geological age) and filled in recent times with sediments, mostly sandstones. River Niger cuts through a narrow strip of crystalline rocks which joins the basement complex of the western uplands and the high plains of North Central (Hausa plains) Nigeria. The strong and resistant nature of the rock at this point of the trough provided a strong base for the construction of the Kainji dam.
The Benue trough has a broad flood plain which extends from Lokoja to Nigeria’s border with the Cameroon. These flood plains are fertile and extensively cultivated by the people of the Benue trough, the majority of who are traditional farmers. Minor rivers found within the Benue trough include River Katsina Ala and River Gongola.
Rivers Niger and Benue are the two major rivers and there are hundreds of rivers and streams in the country. Generally, rivers in Nigeria are classified into five drainage systems: the Niger system, the Benue system, the Chad system (an inland drainage), the Cross system and the Atlantic system. Excluding the Chad system, which is an inland drainage, almost all the river systems in Nigeria drain into the sea. This is shown in Fig. 37.3.
The Jos Plateau and the surrounding high plains constitute the major hydrographic centre of Nigeria as most of the rivers flowing into the Chad Basin, and the Niger and Benue valleys from the northern part of the country originate from there.
The Niger River and its major tributary (Benue River) and minor tributaries have significantly helped to shape the physical landscape of the country and influenced the culture and the nature of economic activities in many parts of the country. From the north-west point of entry into Nigeria, the Niger River flows for 1, 271 km before entering the sea in a series of distributaries. The deposition of materials brought down by both Rivers Niger and Benue has developed a huge Niger Delta. The Atlantic drainage system has short rivers, which follow a north-south course. Nigeria’s important rivers and their lengths are shown in Table 37.1.
Uses of Nigerian Rivers
Nigerian rivers are of social and economic importance. Specifically, these rivers are:
- Sources of water supply for both domestic and industrial use.
- Sources of water for irrigation especially when they are dammed.
- Tourism and recreation resources, for example, the Arugungu Fishing Festival on River Sokoto and Patigi Regatta on River Niger.
- Means of transportation of goods and people, particularly, among riverine communities.
- Channels for generating hydro-electric power (HEP), e.g. Kainji, Jebba and Shiroro.
- The sites for freshwater fishing to meet the animal protein needs of the nation.
Limitations to the Use of Nigerian Rivers
Nigerian rivers are yet to be fully exploited due to a number of limitations. These include:
- Lack of capital to develop the hydroelectricity and irrigation potentials on the rivers.
- Fluctuation in the volumes of most rivers in the country is a problem – the river flow is usually very low during the dry season thereby restricting navigation and so their use for transportation.
- The river mouths experience high levels of deposition or silting by eroded materials. This is particularly true of coastal rivers which thereby do not provide easy passage for big ships and boats.
- The smooth flow of the majority of the rivers is interrupted by falls and rapids. e.g. River Niger at New Bussa and Kurra falls on river Kurra where it flows down from the Jos Plateau.
- Aquatic weeds such as water hyacinth grow extensively on many river channels and lagoons in the country. Water hyacinth limits the use of the rivers for transportation and fishing.
River Basin Development in Nigeria
A river or drainage basin is defined as an area drained by a given river and its tributaries. Most river basins in Nigeria are of immense importance to the development of the country. It is for this reason that the River Basin Development Authority (NRBDA) system was established in 1976, with the aim of integrating and using the abundant water resources in the country for agricultural and rural development.
The NRBDA decree demarcated the country into eleven river basin development areas namely:
- Sokoto-Rima basin.
- Hadejia -Jama’are basin.
- Lake Chad basin.
- Upper Benue basin
- Lower Benue basin.
- Cross river basin.
- Anambra-Imo basin.
- Niger basin.
- Ogun-Osun basin.
- Benin basin.
- Niger Delta basin.
The RBDA system has contributed immensely to the development of the water resources and agricultural sector in Nigeria, though, it can not be said to have fully met its set target. The main areas of contribution and success include:
- Increasing the land under cultivation in the country, through land clearing and reclamation. Farmers under this technique are able to acquire large farm holdings on which mechanized farming can be practised.
- The river basin authorities encourage ‘fadama’ farming and irrigation agriculture. This facilitates farming all year round which has helped to increase agricultural production in the basins and in the country at large.
- The river basin authorities through their research department have been able to provide improved breeds of livestock such as cattle and poultry. These products and innovation usually reach the farmers through the river basin authority extension service.
- Credit and loan schemes are put in place by the river basin authorities for farmers to source for capital for the purchase of fertilizers and inputs for enhanced agricultural output.
- The river basins help to control the flow of rivers for enhanced river transportation and also for the development of hydro-electric power.
- The river basin development authorities have built multi-purpose water development projects that provide urban water supplies among other benefits.