Running water can be defined as the natural movement of water in a certain direction. Running water is one of the most important agents of denudation. Rivers are involved in erosion, transportation and deposition of materials.
A river can be defined as a large natural stream of water flowing in a channel to the sea, a lake or another such stream.
VELOCITY OF A RIVER
The velocity of a river refers to the rate of water movement, often measured in metres per second. River velocity is determined by the efficiency of the river in overcoming friction with the bed and banks.
TERMS ASSOCIATED WITH RIVERS
- Source of a river: The source of a river refers to where a river starts or begins, usually around highlands.
- Course of a river: This refers to the path or channel through which the river flows.
- Mouth of a river: This refers to where the river ends or where it enters into the sea, ocean or lake.
- River basin or Catchment area: This refers to all the areas drained by a river and its tributaries.
- Water shed or water divide: This refers to the highland area which separates two or more rivers or two river basins. It is from the watershed that rivers take their sources.
- River regime: This refers to the seasonal changes in the volume of water in a river in a year. It could be single regime where there is one period of high volume and one period of low volume and the double regime where there are two distinct periods of high volume in a year. Knowledge of a river regime is important to man in controlling possible floods, storing up water for irrigation and human consumption and for planning Hydro-Electric Power production.
- Confluence of a river: This refers to the meeting point of two different rivers.
- Tributaries: These are smaller rivers or streams that join together to form a larger one.
- Distributaries: These are channels formed by the division of a river as it flows into the sea. They are usually found in the delta region of a river.
- River energy: This refers to the velocity of a river. The efficiency of a river to erode and transport the eroded materials depends very much on its velocity.
FACTORS AFFECTING THE VELOCITY OF A RIVER
- The higher the volume of water released by a river, the higher the velocity. However, as the volume decreases during the dry season, the velocity also decreases.
- The steeper the slope, the higher the velocity of the water in the river.
- A river uses more energy to flow through a flat – wide valley than through a narrow – deep valley because the former has a large surface area.
- The greater the speed of a river, the greater the materials or loads it can carry or move. A river uses its energy to:
- Overcome friction along its channels.
- To transport its load.
- To erode the river valley. It is this energy that is trapped by man to generate electricity (H.E.P).
PROCESSES OF RIVER TRANSPORTATION
The load of a river is carried or transported along four main processes, they are:
- Solution: Some rock materials dissolve in water, and are carried in a solution from the upper to lower course of the river.
- Suspension: The lighter particles of solid materials are carried in suspension as the water flows.
- Saltation: In this process, larger particles are moved in series of hops and jumps along the stream bed.
- Traction: This involves very large fragments of materials being rolled or pushed along in the river.
PROCESSES OF RIVER EROSION
The load or materials carried by a river are the main agents of erosion, but the erosive work of a river consists of four interacting processes. These are:
- Hydraulic action: In this processes, fast flowing water forces itself into cracks and cavities within the valley under pressure and enlarges the cracks.
- Corrasion: Corrasion is the wearing away of the sides and floor of the river valley with the aid of sand, pebbles, silts and boulders which are being transported. These materials eventually widen and deepen the river valley.
- Attrition: This is the wearing down of the load as they collide with one another and with the floor and sides of the valley. Large boulders are broken down into small pieces like pebbles.
- Solution: This refers to the chemical action of water on materials it comes in contact with while flowing. Here, rock salts are dissolved and carried away in solution.
STAGES OF A RIVER
The entire length, valley or course of a river is divided into three main parts or stages. These are:
- The upper or mountain course (Youthful stage).
- The middle or valley course (Mature stage).
- The lower or plain course (Old stage).
- THE UPPER COURSE OF A RIVER
- It marks the beginning or source of a river.
- It is found around highland areas.
- It has steep sides.
- The dominant river flows swiftly down the steep slope.
- Its dominant work is vertical corrosion or erosion.
- THE MIDDLE COURSE OF A RIVER
- Lateral erosion is dominant over vertical erosion, resulting in widening of the valley.
- It has wide V-shaped valleys.
- There is the presence of bluff; river cliffs, meanders, terraces etc.
- There is increase in the volume of water due to addition of more water from tributaries.
- There is increase in the load of the river.
- The work of the river is mainly transportation with little deposition.
- THE LOWER COURSE OF A RIVER
- The main work of the river is deposition of materials.
- There is active lateral erosion.
- Increase in volume of the river due to additional tributaries.
- There is the lowering of the gradient of the valley floor.
- There is drastic reduction in the speed of the river.