Underground water is the water that sinks on to the ground, i.e. the water that enters the rocks. When it rains, part of the rain that falls evaporate back to the air, part of it flows as run off to surface water bodies while the rest infiltrates into the ground as GROUND WATER.
Water can enter a rock in two ways:
- Through the space called pores which normally separates the individual grain of rock
- Through the joints or faults in a rock.
- Through Pore spaces/Porous Rock
A rock which has a pore space into which water can infiltrate is called a porous rock such as sandstone. Water is easily absorbed by such rocks and may be stored in the pore spaces. Thus, most porous rocks are also permeable. However, some rocks are porous but impermeable e.g. clay.
RIVER ACTION AND THE FEATURES IT PRODUCES
Rivers are most important agents of denudation. They are involved in erosion, transportation and deposition of materials. They carve out channels as they flow and they transport and deposit materials for very great distance often hundreds or thousands of kilometres.
Factors affecting the velocity of a River
- The volume of water released
- Slope of the river valley
- Shape of the river valley
- Amount and size of materials.
- The Volume of Water Released
The higher the volume of water released by a river, the higher the velocity. As the volume decreases during the dry season, the velocity also decreases.
- The Slope of the River Valley
The steeper the slope, the higher the velocity of the water on the river
- The Shape of the River Valley
A river uses more energy to flow through a fat – wide valley than through a narrow – deep valley because the former has a large surface area.
- Amount and Size of Materials
The greater the stepped of a river, the greater the materials/loads it can carry or move.
ACTIONS OF RIVERS
Rivers perform three types of work:
- Carving and eroding its valley
- Transporting the eroded materials; ad
- Depositing the transported loads
In general, the effectiveness of river action depends on a number of factors like the energy of the river, the nature of the load or materials carried the nature of the channel of the river upon which friction depends.
Mechanics of River Erosion: the main process of river include:
- Hydraulic action
- Corrosion or Abrasion
- Hydraulic Action: this is a purely mechanical process whereby the physical impact of the on – rushing forces of fast-moving water attacks the river banks tearing away weak rock piece from the valley sides and bottom. This can also refer to the force of the flowing water on the river which exerts dragging effect upon the bed of the river and erodes poorly consolidated materials, such as sand, silt and clay. This process accounts to some extent for the hollows known as plunge-pools below water falls.
- Corrasion or Abrasion: This is wearing away of the sides and floor of rivers with the aid of the boulders, pebbles, sand and silt which are being carried or transported. In this case, the rock pieces that had earlier been torn off the banks are beds of the river valley are used as tools or chisels to further wear away the bottom and banks of the valley through frictional grinding action. As these rock materials are dragged along and halted against both valley bottom aids sides, their constant scrapping effectslead to the widening and deepening of valleys to cause ‘pot-holes’.
- Solution or Solvent Action: this is the solvent and chemical action of river water on the materials with which it comes into contact. For example, calcium carbonate in limestone. This takes place in form of dissolution as the river water dissolves adjacent rocks such as limestone and rock silt which are soluble. Corrosion can be likened to chemical weathering.
- Attrition: this is the wearing down of the load itself because of contact with the floor and banks of the stream and the other material being transported. It can also be seen as the process by which through constant collision of eroded rock pieces against one another and against the river banks and bed, the rock pieces become worn down, reduced and rounded.
RIVER TRANSPORTATION/TRANSPORTATION WORK OF RIVER
The load carried by a river is moved down stream in various ways: solution, saltation, suspension and traction. By far, the amount of river load transported through saltation and traction is lesser than those transported through solution and suspension.
- Solution: certain rock pieces are dissolved in water and carried away in solution by the water. This is what makes river water to be muddy especially during floods.
- Suspension: this is the process whereby tight particles of rock materials are suspended within the river water and transported along in that way.
- Saltation: some rock materials in transit within the rivers are too large for the river to carry in suspension. These larger particles are therefore moved downstream in a series of hops or jumps touching the river beds and sizes at intervals as they bounce along.
- Traction: this is made up of the biggest rock materials like stones and boulders that a river is capable of carrying. These materials are dragged and rolled along the river bed as traction load or bed load by the pushing power of the river.
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