Rivers Flashcards
What is erosion?
Erosion is the wearing down of a surface.
What are the four erosion processes that alter the shape of the river channel?
- Hydraulic action – The sheer force of water hitting against the riverbed and banks.
- Abrasion – When the materials carried by the river scrape away the riverbanks and bed.
- Attrition – When the material being carried by the river collides and hits each other, causing the pieces to become rounder and smaller.
- Corrosion (solution) – when rocks are dissolved by the slightly acidic pH of the water.
What is vertical erosion?
- Vertical erosion is dominant in the upper course of rivers. It increases the depth of the river and valley, as the river erodes downwards.
What is lateral erosion?
- Lateral erosion is dominant in the middle and lower course of rivers. It increases the width of the river and valley as it erodes sideways.
What are the four processes of transportation?
- Traction – occurs when larger rocks and material are rolled along the riverbed.
- Saltation – when smaller material bounces along the riverbed in a leapfrog motion.
- Suspension – lighter particles/material is carried within the river flow.
- Solution – when materials are dissolved in the river water.
What are the characteristics of the upper course?
*Shallow
*Steep valley sides
*Narrow
*Low velocity
*Large bedload
*Rough channel bed
*High levels of friction
*Vertical erosion
What are the characteristics of the middle course?
*Deeper than upper course channel
*Gentle valley sides
*Wider than upper course channel
*Greater velocity than upper course channel
*Material in river decreases in size
*Smoother channel bed
*Lower levels of friction than upper course channel
*Lateral erosion
What are the characteristics of the lower course?
*Deeper than middle course channel
*Flat floodplains
*Wider than middle course channel
*Greater velocity than the middle course channel (apart from as the river enters the mouth)
*Material carried mainly sediment and alluvium
*Smooth channel bed
*Lowest friction
*Deposition is dominant
What landforms are found in the upper course?
*Waterfalls
*Gorges
*V-shaped valleys
*Interlocking spurs
When do waterfalls form?
Waterfalls form where there is a drop in the river bed from one level to another. This drop tends to be because of changes in the hardness of the rock, where hard rock overlies soft rock.
How do waterfalls and gorges form?
*The soft rock erodes quicker due to hydraulic action and abrasion.
*This undercuts the hard rock and creates a plug pool.
*This leads to the development of an overhang of hard rock.
*The overhand of hard rock eventually collapses.
*The overhang falls into the plunger pool, increasing abrasion and making the plunge pool deeper.
*The process then begins again and the waterfall retreats upstream leaving a steep sided gorge.
How do V-shaped valleys form?
*V-shaped valleys are mainly formed by vertical erosion. This cuts down into the river bed and deepens the river channel.
*Weathering and mass movement also form V-shaped valleys since they cause the material from the valley sides to collapse into the river, creating a steeper v-shaped valley.
How do interlocking spurs form?
*Interlocking spurs form when the river, in the upper course starts to meander. - *Tends to be when there are areas of hard rock which are harder to erode, so the river will bend around it.
*This creates interlocking spurs.
What are features found in the lowland?
*Meanders
*Oxbow lakes
*Floodplains
*Levees
How do meanders form?
*Lateral erosion is dominant causing meanders to increase in size.
*The thalweg is on the outside of the river bends, leading to erosion.
*The erosion undercuts the riverbank, forming a river cliff.
*The riverbank collapses and the edge of the meander moves further out.
*The slowest flow is on the inside of the river bends, leading to deposition.
The deposits form a slip-off slope.
*Deposition on one side and erosion on the other leads to the meander migrating across the value.
How do oxbow lakes form?
*With distance downstream, the size of the meanders increase.
*The erosion on outside bends lead to the formation of a meander neck.
*At a time of the flood, the river lay cut through the neck of the meander. This forms a straighter course for the water.
*Flow of water at entry and exit from the meander will be slower, leading to deposition.
*Meander becomes cut off from the main river channel, forming an oxbow lake.
What are floodplains?
Floodplains are areas of land which are covered in water when a river bursts its banks.
How do floodplains form?
*Found in lower course.
*Formed by erosion and deposition.
*Meanders slowly move downstream.
High discharge may cause the river to overflow the banks.
*The water escapes onto the surrounding land (floodplain).
What are levees?
Levees are raised banks at the edge of the river channel.
How do levees form?
*When a river floods, more of the water is in contact with the land surface as the water spreads across the floodplain.
*The increased friction reduces velocity.
*This causes material to be deposited across the floodplain, thus gradually increasing the floodplain height.
*The heaviest material is deposited first nearer to the river channel, because they are heavier and the river has less energy to transport them further.
*Eventually , these larger sediments build up forming natural embankments called levees.
