Explain the formation of a waterfall
- Occurs where there is a band of hard rock that meets a band of softer rock
- Softer rock is eroded more than the harder rock causing a ‘step’ in the river
- Water flowing over step speeds up due to lack of friction as it drops over the step, water now has greater erosive power, causes further erosion of soft rock. Now there is undercutting of hard rock
- Hard rock is undercut, can collapse. Plunge pool is carved out by abrasion at foot of waterfall as bits of collapsed rock are swirled by turbulence
- Over time, more undercutting so more collapse, waterfall retreats, steep sided gorge left behind.
What are the 3 river landforms caused by fluvial erosion?
Waterfalls
Rapids
Potholes
What are Rapids?
Relatively steep sections of river with turbulent flow where there are several sections of hard rock, like mini-waterfalls
Why are potholes?
Small circular hollows in the river bed
How are potholes formed?
Formed by abrasion as turbulence swirls a rivers bed load round in a circular motion, causing it to rub and scrape out holes
What is the river landform caused by a combination or erosion and deposition?
Meanders
Explain the formation of a meander
- Form where alternating pools (deep water) and riffle (shallow water) develop at equally spaced intervals along a stretch of river
- As river channel is deeper in pools, more efficient, has more energy, more erosive. Energy lost over riffles because of friction.
- Distance between riffles and pools causes rivers flow to be uneven and maximum flow to be concentrated on one side of river
- Turbulence increases around pools as water speeds up, flow of water begins to twist and coil
- Cause currents in river called helicoidal flow which spiral from bank to bank between pools
- Helicoidal flow causes more erosion and deepening of pools. Causes eroded material to be deposited on the inside of the next bend where energy is lost
- Erosion and deposition exaggerates the bends until large meanders are formed. Combined processed also create the meanders distinctive asymmetric cross-section
- Oxbow lakes can be formed - neck of loop of meander is broken through, during flooding. Deposition dams off the loop
What are the 4 river landforms created by fluvial deposition?
Braiding rivers
Flood plains
Levees
Deltas
Explain the formation of braiding river
Occurs when rivers are carrying a vast amount of eroded sediment
- When the rain is heavy, streams start moving in higher velocity
- High velocity carries large amounts of sand and small stones
- The water flow slows down in the sections passing through wide valleys with little downward slope allowing sediment to settle on the river bed
- Sediment starts to block the stream of water, causing lower velocity
- The stream overflows its banks forming new channels
- Stream is split into many streams causing the braided channel shape
- Streams meet up again at a certain point to then split apart again
Explain the formation of flood plains
- Meander migration causes lateral erosion which widens the valley flood
- When river floods in the middle corse, they cover an area of land known as floodplain
- When they flood, velocity is slowed and deposition of any rocks being transported is encouraged
- Deposition leaves a layer of sediment across the whole flood plain
- After several floods there are several layers of sediment deep on the flood plain
- Over time the sediment builds up on the floodplain creating a very vast area of flat land
What is a braided river?
A channel that consists of a network of small channels separated by small and often temporary islands called braid bars or eyots.
What is a flood plain?
A relatively flat area of land either side of the river forming the valley floor, most common depositional feature of a river
What are levees?
Are the high banks of silt either side of the river channel
How are levees formed?
- River is on flood plain with a very low gradient
- River is carrying large amounts of silt
- When river floods, river slows, loss of energy, so a lot of silt is deposited on the river banks and much less on the floodplain
- The boulders are deposited closer to the banks as the river doesn’t have enough energy to carry them further
- Frequent flooding continues to build up
- When river is flowing normally it deposits in the river bed
- This raises the river above the flood plain
- These raises in the river banks are levees
What are deltas?
Depositional landforms that are created from the loading of sediment onto the land as the river capacity to carry that sediment is reduced
How are deltas formed?
- When a river reaches the sea, the energy of the river is absorbed by the slower moving water of the sea
- Causes the river to deposit it’s load, these deposits build up on the sea bed until the deposited sediment rises above sea level, partially blocking the mouth of the river
- The river has to braid into several distributaries in order to reach the sea, forming a delta
What does rejuvenation allow for?
A river to resume vertical erosion
How can we know a river is rejuvenated?
If it’s base level is lowered
What causes a river to be rejuvenated?
Occurs when the river has increased energy, as a result of a change in river height relative to base level, caused by fall in sea level or land uplift
Explain the process of rejuvenation
The drop in base level gives the river greater potential energy, increasing its vertical erosion potential, long profile of river is extended and a knock point (waterfall) will mark the junction between the original long profile and the new one.
What are the landforms associated with rejuvenation?
River terraces
Incised meanders
Explain river terraces
They are former flood plains which have been left above the level of present-day flooding following increased vertical erosion
Explain incised meanders
Formed when a river keeps its meandering course as vertical erosion increases
Result is a deep, winding valley with steep sides
River is left far below the level of the former flood plain
