Rivers
The hydrological cycle
The hydrological cycle is also known as the water cycle.
Seas and oceans contain 97 per cent of the world's water, and ice holds 2 per cent. That leaves just 1 per cent of the world's water as fresh water on land or in the air. This water is recycled again and again through the process ofevaporation, condensation and water transfers such as surface run-off.
Terminology for the study of rivers
The hydrological cycle is also known as the water cycle.
Seas and oceans contain 97 per cent of the world's water, and ice holds 2 per cent. That leaves just 1 per cent of the world's water as fresh water on land or in the air. This water is recycled again and again through the process ofevaporation, condensation and water transfers such as surface run-off.
Terminology for the study of rivers
- Drainage basin - the area of land drained by a river.
- Catchment area - the area within the drainage basin.
- Watershed - the edge of highland surrounding a drainage basin. It marks the boundary between two drainage basins.
- Source - The beginning or start of a river.
- Confluence - the point at which two rivers or streams join.
- Tributary - a stream or smaller river which joins a larger stream or river.
- Mouth - the point where the river comes to the end, usually when entering a sea.
Long Profile of a River
The source of a river is often - but not always - in an upland area. Near the source, a river flows over steep slopes with an uneven surface. It often flows over a series of waterfalls and rapids. Highland areas are usually composed of hard igneous rocks, which are ideal for forming such features.
As a river flows down steep slopes the water performs vertical erosion. This form of erosion cuts down towards the river bed and carves out steep-sided V-shaped valleys.
As the river flows towards the mouth, the slopes become less steep. Eventually the river will flow over flat land as it approaches the sea.
The discharge (amount of water flowing) will increase as the river approaches the sea.
The source of a river is often - but not always - in an upland area. Near the source, a river flows over steep slopes with an uneven surface. It often flows over a series of waterfalls and rapids. Highland areas are usually composed of hard igneous rocks, which are ideal for forming such features.
As a river flows down steep slopes the water performs vertical erosion. This form of erosion cuts down towards the river bed and carves out steep-sided V-shaped valleys.
As the river flows towards the mouth, the slopes become less steep. Eventually the river will flow over flat land as it approaches the sea.
The discharge (amount of water flowing) will increase as the river approaches the sea.
Cross profiles of a river
Near the source of a river there is more vertical erosion as the river flows downhill, using its energy to overcome friction (A). As a result the channels are narrow and shallow and may contain large boulders and angular fragments eroded and weathered from the steep valley sides. The sediment in the river creates turbulence and friction.
As the river approaches the mouth, velocity and energy increase due to increased discharge. The river performs more lateral erosion making the channel wider, and smoother (B) and (C). As a result there is less turbulence and friction, making the flow of water more efficient.
Near the source of a river there is more vertical erosion as the river flows downhill, using its energy to overcome friction (A). As a result the channels are narrow and shallow and may contain large boulders and angular fragments eroded and weathered from the steep valley sides. The sediment in the river creates turbulence and friction.
As the river approaches the mouth, velocity and energy increase due to increased discharge. The river performs more lateral erosion making the channel wider, and smoother (B) and (C). As a result there is less turbulence and friction, making the flow of water more efficient.
River processes shape the land in different ways as the river moves from its source to its mouth.
ErosionErosion involves the wearing away of rock and soil found along the river bed and banks. Erosion also involves the breaking down of the rock particles being carried downstream by the river.
The four main forms of river erosion
Transport
Rivers pick up and carry material as they flow downstream.
The four different river transport processes
Transport of material in a river
Rivers need energy to transport material, and levels of energy change as the river moves from source to mouth.
Deposition
When a river loses energy, it will drop or deposit some of the material it is carrying.
ErosionErosion involves the wearing away of rock and soil found along the river bed and banks. Erosion also involves the breaking down of the rock particles being carried downstream by the river.
The four main forms of river erosion
- Hydraulic action - the force of the river against the banks can cause air to be trapped in cracks and crevices. The pressure weakens the banks and gradually wears it away.
- Abrasion - rocks carried along by the river wear down the river bed and banks.
- Attrition - rocks being carried by the river smash together and break into smaller, smoother and rounder particles.
- Solution - soluble particles are dissolved into the river.
Transport
Rivers pick up and carry material as they flow downstream.
The four different river transport processes
- Solution - minerals are dissolved in the water and carried along in solution.
- Suspension - fine light material is carried along in the water.
- Saltation - small pebbles and stones are bounced along the river bed.
- Traction - large boulders and rocks are rolled along the river bed.
Transport of material in a river
Rivers need energy to transport material, and levels of energy change as the river moves from source to mouth.
- When energy levels are very high, large rocks and boulders can be transported. Energy levels are usually higher near a river's source, when its course is steep and its valley narrow. Energy levels rise even higher in times of flood.
- When energy levels are low, only small particles can be transported (if any). Energy levels are lowest when velocity drops as a river enters a lake or sea (at the mouth)
Deposition
When a river loses energy, it will drop or deposit some of the material it is carrying.
- Deposition may take place when a river enters an area of shallow water or when the volume of water decreases - for example, after a flood or during times of drought.
- Deposition is common towards the end of a river's journey, at the mouth.
- Deposition at the mouth of a river can form deltas - for example, theMississippi Delta.
River landforms
Upper-course river features include steep-sided V-shaped valleys, interlocking spurs, rapids, waterfalls and gorges.
Middle-course river features include wider, shallower valleys, meanders, and oxbow lakes.
Lower-course river features include wide flat-bottomed valleys, floodplains and deltas.
Upper course features
Upper-course river features include steep-sided V-shaped valleys, interlocking spurs, rapids, waterfalls and gorges.
Middle-course river features include wider, shallower valleys, meanders, and oxbow lakes.
Lower-course river features include wide flat-bottomed valleys, floodplains and deltas.
Upper course features
As the river moves through the upper course it cuts downwards. The gradient here is steep and the river channel is narrow. Vertical erosion in this highland part of the river helps to create steep-sided V-shaped valleys, interlocking spurs, rapids, waterfalls and gorges.
Interlocking spurs on a tributary of the Yangtse
Interlocking spurs on a tributary of the Yangtse
- As the river erodes the landscape in the upper course, it winds and bends to avoid areas of hard rock. This createsinterlocking spurs, which look a bit like the interlocking parts of a zip.
- When a river runs over alternating layers of hard and soft rock, rapids andwaterfalls may form.
Middle course features
In the middle course the river has more energy and a high volume of water. The gradient here is gentle and lateral (sideways) erosion has widened the river channel. The river channel has also deepened. A larger river channel means there is less friction, so the water flows faster:
In the middle course the river has more energy and a high volume of water. The gradient here is gentle and lateral (sideways) erosion has widened the river channel. The river channel has also deepened. A larger river channel means there is less friction, so the water flows faster:
- As the river erodes laterally, to the right side then the left side, it forms large bends, and then horseshoe-like loops called meanders.
- The formation of meanders is due to both deposition and erosion and meanders gradually migrate downstream.
- The force of the water erodes and undercuts the river bank on the outsideof the bend where water flow has most energy due to decreased friction.
- On the inside of the bend, where the river flow is slower, material isdeposited, as there is more friction.
- Over time the horseshoe become tighter, until the ends become very close together. As the river breaks through, eg during a flood when the river has a higher discharge and more energy, and the ends join, the loop is cut-off from the main channel. The cut-off loop is called an oxbow lake.
Lower course features
In the lower course, the river has a high volume and a large discharge. The river channel is now deep and wide and the landscape around it is flat. However, as a river reaches the end of its journey, energy levels are low and deposition takes place.
Floodplains
The river now has a wide floodplain. A floodplain is the area around a river that is covered in times of flood. A floodplain is a very fertile area due to the richalluvium deposited by floodwaters. This makes floodplains a good place for agriculture. A build up of alluvium on the banks of a river can create levees, which raise the river bank.
Deltas
Deltas are found at the mouth of large rivers - for example, the Mississippi. A delta is formed when the river deposits its material faster than the sea can remove it. There are three main types of delta, named after the shape they create:
Three main types of delta
In the lower course, the river has a high volume and a large discharge. The river channel is now deep and wide and the landscape around it is flat. However, as a river reaches the end of its journey, energy levels are low and deposition takes place.
Floodplains
The river now has a wide floodplain. A floodplain is the area around a river that is covered in times of flood. A floodplain is a very fertile area due to the richalluvium deposited by floodwaters. This makes floodplains a good place for agriculture. A build up of alluvium on the banks of a river can create levees, which raise the river bank.
Deltas
Deltas are found at the mouth of large rivers - for example, the Mississippi. A delta is formed when the river deposits its material faster than the sea can remove it. There are three main types of delta, named after the shape they create:
Three main types of delta
Type of delta:
Arcuate or fan-shaped- the land around the river mouth arches out into the sea and the river splits many times on the way to the sea, creating a fan effect.
Arcuate or fan-shaped- the land around the river mouth arches out into the sea and the river splits many times on the way to the sea, creating a fan effect.
Cuspate - the land around the mouth of the river juts out arrow-like into the sea.
Bird's foot - the river splits on the way to the sea, each part of the river juts out into the sea, rather like a bird's foot.