FEATURES OF MIDDLE COURSE OF A RIVER
- Meander: This results when rivers are unable to flow straight because of low gravity, sluggish movement of water and heavy loads, also, the irregularities of the ground, force the river to swing in loops, thereby, forming meanders. It is a turn or sharp bend in a stream course. The winding course or curve that develops along the course of the river is called meander.
- Interlocking spur: This refers to the portion of the valley wall which projects from both sides of the concave bends of the river as it meanders. They usually obscure the view of the river valley.
- Wide V-Shaped valley: As a result of lateral corrosion which has become more dominant than vertical corrosion, the river banks widen to produce a wide V-shaped valley. The valley sides are less steep.
- River cliff: This is a face of a rock standing steeply on the bank of a river. Water moving along the surface of the river near the outer bank tends to do so at a greater speed than water moving against the inner bank. Thus, the outer bank gets eroded by the force and velocity of the fast moving water, resulting in the formation of a river cliff.
FEATURES OF LOWER COURSE OF A RIVER
- Flood plain: During annual or sporadic floods, materials carried by rivers are evenly spread over the low lying adjacent areas. Gradually, a fertile flood plain is built up from a layer of sediments deposited during the floods. Flood plains are very good fertile lands for agricultural activities.
- Levees: As the river flows, it continues to deposit sediments and materials on its bank when it is in flood, thereby, raising the bank upwards and higher than usual. These raised banks are called levees.
- Ox-bow Lake: It occurs in marshy areas. It has horse-shoe shape with a shallow convex bank and a deep concave bank. It is also called bayous or cut-offs. Examples of Ox-bow lakes are found in Mississippi and Mekong.
- Mode of formation: Ox-bow lake is formed in the lower course of a river where there is pronounced meanders. As the water flows, there is erosion on the concave side and deposition on the convex bank. As a result of this, narrow neck of land separates the two slopes. As time goes on, the narrow neck of land is cut through during a flood, leading to the straight flow of the river, thereby; abandoning the loop after the flood has subsided. There is also the deposition of sediments at the entrance of the loop resulting in the formation of Ox-bow Lake.
- Braided river: This is a river which builds up a complex tangle of channels through the deposition of sand bars on the floor of the river. The deposition causes the river to break into several channels which later rejoin to form a braided river.
- Delta: A delta is a fan-shaped alluvial plain formed as a result of deposition of fine materials by a river as it reaches or enters the sea or ocean. Delta is found in the lower course of a river. It is low-lying and occurs in swampy areas. It has a seaward extension of river’s flood plain and composed of alluvium. It has distributaries and is of various shapes – arcuate, bird’s foot, estuarine, etc. Examples are found in Niger and Nile rivers.
Conditions favourable for the formation of a Delta
- There must be active vertical and lateral erosion in the upper course of the river to provide the sediments to be deposited at the mouth of the river.
- The coast must be sheltered, preferably tide less.
- The sea adjoining the delta should be shallow to prevent the disappearance of sediments into deep waters.
- There should be no large lakes in the river course to remove the sediments.
- There should be no strong current running at right angles to the river mouth in order to prevent the washing away of the sediments.
Delta is formed by the deposition of materials or sediments carried from the upper part of the river to the mouth of the river. Owing to absence of strong current, the sediments block the main river which causes it to divide into several distributaries and these extend out into the sea, resulting in a sheltered coast with shallow water. The river then flows through these distributaries into the sea.
Importance of Delta to Man
- Delta provides a good agricultural land especially for cultivation of rice, oil palm and raffia palm e.g. River Niger.
- It is also good for fishing.
- It is equally good for petroleum mining.
Types of Delta
There are three types of Delta. These are:
- Arcuate Delta: This consists of both coarse and fine sediments and has the shape of an inverted cone. It is crossed by many distributaries. Examples are found in Niger, Nile, the Ganges, the Indus, the Mekong and Ho Deltas.
- Bird’s foot Delta: This delta consists of very fine materials called silt with several main distributaries like the foot of a bird extending into the sea. Examples are the Mississippi Delta and the Omo River delta in Ethiopia.
- Estuarine Delta: This delta is formed when it is partially submerged in coastal waters and there is a direct flow of water into the sea with little or no sediments e.g. Amazon, Ob (Poland), Vistula (Russia) and the Seine (France) deltas.
A river is said to be rejuvenated when it is given a new lease of life or being young again i.e. an extra power of erosion. This happens when the base level of the river falls, either because the land is uplifted or because the level of the sea falls.
The erosion of a rejuvenated river is mainly vertical into its valley floor and as a result, new types of features are produced e.g. gorges in the upper course of a river.
If a river on a flood plain is rejuvenated, the resulting vertical erosion by the river produces terraces that are paired i.e. terraces on one side of the river correspond in height with those on the other side. However, terraces formed by a migratory meanders are not paired. It sometimes happens that the point where the river crosses from the original flood plain to meet the new flood plain called, Knick point or rejuvenated head is usually visible and it is marked by rapids or waterfall.
If a meandering river is rejuvenated, the vertical erosion of the river causes the meanders to be cut deep into the valley floor. Such meanders are called incised or entrenched meanders.