Landforms Flashcards
How do potholes form?
- Pebbles carried in a high turbulence flow become trapped in small hollows in the stream bed.
- Vertical Eddies in the flow cause the pebbles to grind holes into the rock by abrasion.
Under what flow conditions do potholes form and where in the river are these conditions found?
Potholes form in turbulent, high velocity water that is loaded with pebbles.
These conditions are most often found in the upper to middle course of a river.
How are rapids and waterfalls formed?
Waterfalls and rapids are found where there is an increase in the rivers gradient.
This increase is normally more so than in the case of waterfalls.
What causes a gradient change in a river?
Gradient changes are only due to the river channel encountering bands of erosion-resistant rock.
How does a waterfall retreats upstream, forming a gorge?
- The river falls into a deep plunge pool.
- In this pool the soft surrounding rock is eroded more rapidly than the overlying erosion resistant rock.
- This deepens and widens the pool.
- This causes the pool to undercut the overlying rock until it collapses under the force of gravity.
What are meanders?
Meanders are bends in a river channel.
How are meanders formed?
- Alternating bars of sediment called riffles form on the bed of straight river channels in low flow conditions.
- As water flows around these alternating bars, seeking the most efficient flow path, centripetal force directs the water towards one of the banks.
- This results in high velocity flow, erosion and undercutting.
- This creates a concave outer bank.
- On the inside of the bend the velocity is slower and deposition occurs this results in an increase in river sinuosity.
What is a braided river?
Braided rivers are rivers which are split into several smaller channels that diverge and converge repeatedly as they flow around small islands.
What characteristics of sediment and discharge are associated with braided river formation?
Braided channels form when a river has fluctuating discharges and a sufficient supply of sediment to result in the periodic to occurrence of high sediment loads.
How are oxbow lakes formed?
- As water flows around the meander in a river the water flow is fastest on the outside of the bend this leads to a erosion. At the same time, deposition occurs on the inside of the meander.
- This leads to an increasingly sinuous river in which the gap between successive arms (neck) of the river gets narrower.
- During a period of high discharge, the river cuts through the neck of the meander. The new channel becomes the main channel, as it is a straighter, shorter route downstream.
- Eventually deposition cuts the old meander off from the main channel and the isolated meander becomes an oxbow lake.
What is a levee?
Levees are small raised banks of sediment found along the sides of the channel.
How does flooding lead to levee development?
- Rivers typically flood during times of high discharge.
- As the river overtops its banks, the velocity of the flow drops, resulting in deposition.
- As the coarsest material will be deposited first, this process results in the development of levees along the sides of the channel.
What is a floodplain?
The floodplain is a relatively flat area of land on either side of the river that is subject to flooding.
How does a floodplain form?
Floodplains are predominantly depositional features.
They are formed when a river overflows its banks during a flood event.
The resulting loss in velocity and increase in wetted perimeter combine to cause deposition of flat layers of alluvium (silts/clays).
What is a delta?
A delta is an area of deposited sediment located where a river enters a slow-moving body of water e.g. the sea or a lake.
