Landforms of Fluvial Erosion + Deposition Flashcards
Waterfall
- change in lithology (granite to limestone), causing differential erosion
- weaker rock erodes faster –> sudden drop, increased height difference.
- plunge pool formed as water flows over waterfall, splashback undercuts resistant rock, eventually collapses.
- retreats upstream.
When do rapids occur?
sections of a river where the gradient of river bed is relatively steep.
- increasing turbulence and velocity
- gradient of river steep, bed made up of mainly hard rocks.
Potholes
- cylindrical holes drilled into rocky bed of a river by turbulent high-velocity water loaded with pebbles.
- pebbels trapped in slight hollows, water strong enough to drill
Braided Channel
when the river is forced to split into several channels separated by islands.
Featured in rivers that are supplied with large loads of sand and gravel.
When is a braided channel most likely to occur?
When a river has variable discharges. e.g. Glacial Streams with variable annual discharge.
What is the issue with banks formed from sand and gravel?
Easily eroded and generally unstable.
What is a consequence of sand and gravel composed banks?
Channel becomes very wide relative to its depth.
River can become choked, with several sandbars and channels that are constantly changing their locations
What is a meander?
a bend in a river that form as river sinuosity increases.
What does sinuosity mean?
is a measurement of how much a river varies from a straight line.
Its a ratio between the channel length and displacement (straight line distance) between two points in the river’s course.
Sinuosity formula
Sinuosity= Channel Length/Displacement
1= perfectly straight river, greater than 1 means there are meanders.
How does a meander develop?
When alternating riffles and pools form along a river channel.
What is a riffle?
A shallow section of a channel.
What is a pool?
a deep section of a channel.
Where do riffles and pools develop?
At equal points along the river channel with each pool about 5x the length of the channel.
What is the difference between a pool and a riffle?
In a pool the channel is more efficient.
Causes the flow of water to become irregular and the maximum flow is concentrated on one side of the river.
Therefore increases erosion on that side and deposition on the other, causing the channel to bend.
What is an Oxbow Lake
an evolution of a meander that underwent extensive deposition and erosion.
How is an Oxbow Lake formed?
Strong erosion on the outside bend, strong deposition on inside.
Neck of meander narrows –> in times of extremely high discharge its more efficient for a river to flow across the neck.
When discharge returns to normal levels, river follows new course.
Deposition separates the cut-off from main channel.
What is a Levee?
natural embankments produced when a river floods.
When a river floods, it deposits its load over the flood plain due to dramatic drop in the river’s velocity as friction increases greatly.
How are raised mounds formed? (Levee)
Largest and heaviest load is deposited first and closest to the river bank, often at the very edge, causing raised mounds.
What happens to finer material? (Levee)
Deposited further away from banks causing mounds to appear and taper off.
Repeated floods case mounds to build up and form levees.
Are Levees permanent structures?
No, once a river’s discharge exceeds its bankful discharge, the levees can be burst by the high pressure of the water.
Can Levees increase the height of the river channel?
Yes so bankful discharge is increased and it becomes more difficult for the river to flood.
What is a floodplain?
The area of land around a river channel which is formed during times of flood when the amount of water in a river EXCEEDS its channel capacity and DEPOSITION of rich silt occurs.
What is the function of a floodplain?
Is the area the river floods onto when experiencing high discharge.
collects alluvium from load deposition during floods due to reduced efficiency from increased friction.
