The Hydrological Cycle
Features of a 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
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.
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 outside of 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 is deposited, 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.
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.
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 rich alluvium 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 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:
MEDC case study: causes and effects of flooding in
- Over 60 mm of rainfall (typically a month's rainfall) fell in two hours.
- The ground was already saturated due to the previous two weeks of above average rainfall.
- The drainage basin has many steep slopes, and has areas of impermeable slate causing rapid surface run-off.
- Boscastle is at the confluence (where tributaries meet) of three rivers - Valency, Jordan, and Paradise. A large quantity of water all arrived within a short space of time causing the rivers to overflow.
- The flooding coincided with a high tide, making the impact worse.
- Homes, businesses and cars belonging to more than 1,000 people were swept away.
- Income from tourism was lost. This had an impact on livelihoods and the local economy.
- There were vast numbers of subsequent insurance claims.
- No lives were lost, partly due to the rapid response of the emergency services.
LEDC case study: causes and effects of flooding in
The Mozambique floods of 2000 show that what happens in one country can very often affect another.
The flooding was triggered by exceptionally heavy rain in South Africa, lasting for five weeks in early 2000. Botswana was particularly badly hit, receiving 75 per cent of its yearly rainfall in three days. On 22 February, Cyclone Eline hit, bringing more heavy rainfall. The rain from Botswana and other Southern African countries ran into the Limpopo, Zambezi and other rivers which flow through Mozambique to the sea. These rivers eventually burst their banks, causing severe flooding in Mozambique.
In addition, the loss of grassland and draining of marshland for farms contributed to more rapid surface run-off.
The results were disastrous: services were cut off and many people were stranded, homeless or had died through drowning or disease. Urbanisation in South Africa may have contributed to the large quantities of surface water run-off swelling the rivers.