Running water is in fact the most prominent agent in the sculpturing of the earth’s surface except in the arid or frozen lands. A river is running water, and refers to a mass of water flowing over the land surface from its source and empties usually into the seas, lakes, swamps or depressions. Whenever rain falls, it is drained from the land surface into rivers and lakes, apart from that evaporated. A drainage basin is formed by this arrangement. An area which drains all the water that falls on it, not including that removed by evaporation, into a river or stream and finally to the sea or lake is termed a drainage basin. The water in a river is an energy system. Its energy depends mainly upon the volume and velocity, and together make up the term, discharge. Discharge is expressed in cubic metres per second (m’/sec). It is used by the river for erosion and the transportation of its own eroded materials and those brought into it by rain. As the discharge of a river increases, usually during flood, its ability to carry load increases at a greater rate than the discharge, but with very minute increase in the friction with the bottom and sides of the river. This results in more energy being left for transport. A rapid flowing river can carry more material and fragments of larger size than a slow one. The measure of the ability of a river to carry a load is called its competence. Erosion is accomplished through four processes, namely:
i. Hydraulic action
A river can carry loose materials and can break up solid rock when it enters into its cracks. Its turbulence and eddying, result in the undercutting of the banks on a curve termed bank caving. If bubbles of air or water vapour in a fast flowing river burst, shock waves are formed and help in eroding the banks, termed cavitations.
The river uses its load as the grinding tool for wearing away banks and beds. Corrasion may be vertical or lateral. It is vertical when the bed of the river is made deeper and lateral when the banks are made wider.
The fragments of rocks that a river carries are constantly colliding not only with the bed but also with each other thereby wearing themselves down. This action is known as attrition. The reduction in the size of the materials makes the transportation by the river easier.
This refers to chemical erosion and involves the solvent work of water as it flows over such rocks as limestone. The solid material in solution, carried by a river is termed:
Suspension: The very small particles carried by a river is done by suspension. The particles are swirled along by the river.
Saltation: Larger particles being transported by a river touch the bed at intervals in a series of hops. This is termed saltation.
Traction: For the very large particles such as pebbles, they are rolled along the river bed by gravity and the pushing power of the river. The big stones and boulders are moved mainly during flood.
Solution: Although the load transported in solution cannot be seen by the eyes, some good quantity is done so.
A river with materials in solution, suspension and the traction load
The erosional and depositional features of a river are best studied and understood through the study and the understanding of its course. The profile of a river is graded from source to mouth and divided into three pronounced sections. These are the upper course, middle course and the lower course.
The Upper or Mountain Course
The upper valley or course of a river is mainly by vertical corrosion and very little of lateral corrosion. By so doing, a steep-sided V-shaped valley is cut. The river here follows a winding course because of obstacles formed by various resistant rocks. Ordinarily, rivers follow the line of less resistant interlocking spurs.
River Gorges: These are commonly associated with the upper course of a river. They are relatively deep, elongated, steep-sided and hollow. They are usually found where vertical erosion by running water is more rapid than the forces of weathering can wear back and open up the sides. Examples include the Shiroro Gorge along River Kaduna (Nigeria), Indus Gorge in Kashmir and Aar Gorge near Meiringen in Switzerland. Gorges are sometimes formed in the desert climates by a river from another area whose volume is dependent on snow-melt on mountains away from the desert. Such gorges are called canyons in America. Examples include the Grand Canyon of the Colorado River, and those found on the Snake and the Yellow Stone River.
River Capture: This is one of the features found in the upper course of a river. it is sometimes called river piracy or river beheading. It takes place if the erosion power of a river is far greater than that of another river on the other side of a divide (watershed). With this, the stronger river expands its basin at the expense of the weaker one through back cutting or headward erosion. Where the divide is broken through, the weaker stream loses its head waters to the stronger one. Examples can be found in Ghana, which include the beheading of the Rivers Amisa Ochi and Nakwa by River Pra; and the capture of the Mansl by the Ofin. Other examples are the capture of the Upper Sittang by the Irrawaddy in Burma, and the beheading of the Blyth and Wanshock by the North Tynet in Northumberland, England.
Rapids, Cataracts and Waterfalls: These are also common features of the upper course even though they may be found all through the course of the river. Rapids are those outcrops of very resistant rocks on the beds of rivers. Since they do not make for a free flow of the river, navigation is usually difficult. Examples are the Bussa rapids and Awuru rapids in River Niger. Cataracts are rapids of greater dimensions while waterfalls refer to points along the course of a river where there is a plunging down from a reasonable height.
The Middle Course
Here, unlike in the upper course, lateral corrosion is in greater action. The valley becomes wider as a result and with greater volume of water that permits some deposition. The deposition slows down the river so that any swing in the river may turn out a meander.
With the formation of a meander, a cliff is created overhanging the river bank. Such a cliff is known as a river cliff Also, a sloping spur projecting from the opposite side is formed called a slip-off slope.
The Lower or Flood Plain Course
At this stage, deposition predominates the work of the river thereby having a series of sweeping meanders over a broad almost level valley. This is as a result of the near absence of vertical corrasion while lateral corrosion continues to erode the banks. Flood plains are the most common features here. They are formed when rivers in flood deposit their load on the adjoining low-lying areas. Flood plains are usually very thick and made up of finely divided silt. On the other hand, most mud is often deposited at the edge of the channel where the current is less active to form a natural embankment. Where the floods subside this embankment is known as a levee. Examples can be found in Mississippi.
Ox-bow lakes or cut-offs are also important features of the lower course. They are simply relics of former meanders. Often, such lakes end up as swamps through sitting during floods.
As a river enters the sea, it drops the remaining load in a fan shape. This forms the delta made up of alluvial deposits that make the river discharge through smaller channels known as tributaries. Examples are the Niger Delta in southern Nigeria and the Ganges Delta in west Malaysia.
Sometimes, after reaching its lowest course, a river may rejuvenate i.e. become young through renewed erosion, thereby developing fresh features. This action is termed rejuvenation. It produces terraces on either side of the channel by both vertical and lateral corrasion. Also produced here is the deep cutting of the channel into the surface with the plan of the original meanders maintained.