Waterfalls are caused by fluvial erosion.
- Waterfalls form where a band of hard rock meets soft rock.
- The soft rock is eroded faster than the hard rock, causing a 'step' in the river bed
- The water flowing over the step speeds up due to a lack of friction as it drops over the step. This increase in speed gives the water greater erosive power.
- The harder rock is undercut, and gradually starts to collapse. A plunge pool is formed through abrasion at the foot of the waterfall.
- The waterfall gradually continues to retreat as more undercutting causes more collapse.
- A steep-sided gorge is formed due to this process
Potholes and Rapids
Potholes are small circular hollows in the river bed.
- Formed by abrasion as turbulence swirls a river's bedload round in a circular motion, causing it to rub and scrape out holes
Rapids are relatively steep sections or river with turbulent flow where there are several sections of hard rock.
Meanders are large, sweeping curves in a river's middle and lower stages formed by erosion and deposition
- Formed where alternating pools (deep water) and riffles (shallow water) develop at equally spaced intervals along a stretch of river. The distance between pools is 5-6 times the width of the river bed
- Because the river channel is deeper in pools it's more efficient - it has greater energy and more erosive power
- Energy is lost as the river flows over a riffle due to friction
- The spacing and distance between riffles and pools cause the river's flow to become uneven - maximum flow is concentrated on one side of the river
- Turbulence increases in and around pools as the water speeds up, so the flow of water begins to twist and coil
- This causes the helicoidal flow to occur, which spiral from bank to bank between pools
- The helicoidal flow causes more erosion and deepening of the pools
- The helicoidal flow causes eroded material to be deposited on the inside of the next bend, where the river loses energy.
Braiding and Flood Plains
Braiding occurs when rivers are carrying a vast amount of eroded sediment.
- If velocity drops or if the sediment load becomes too much for the river to carry, sediment is deposited in the channel
- This causes the river to divide into many small, winding channels that eventually rejoin to form a single channel
Flood plains occur where the river has overflowed its banks and floods the flat land either side of the river.
- This means there is an increase in the wetted perimeter and a reduction in hydraulic radius
- Friction is increased, reducing the velocity of the river and causing fine silt and sand to be deposited across the flood plain
Levees and Deltas
Levees are natural, raised embankments formed as a river overflows its banks.
- During a flood, material is deposited across the whole flood plain as the river loses velocity and energy due to increased friction
- The heaviest material e.g. sand and gravel is dropped first, closest to the river channel
- Over time this material builds up on the river bank, creating a levee.
A delta occurs where the river has to braid into several distributaries in order to reach the sea
- When a river reaches the sea or a lake, th energy of the river is absorbed by the slower moving water of the sea or lake
- This causes the river to deposit its load - deposits build up on the sea bed until the alluvium (deposited sediment) rises above sea level, partially blocking the river mouth.