River landforms of erosion and deposition * Flashcards
How are V-shaped valleys formed?
In the upper course of the river, the rivers volume and discharge are low meaning most energy is used to overcome friction. The remainder of this energy is used to deepen the channel using hydraulic action.
Freeze-thaw weathering gradually broadens out the valley giving it a V-shaped cross profile. This weakens the rock, meaning fragments break loose and further erodes the valley sides (this is scree)
How are interlocking spurs formed?
There are areas of soft rock and hard rock in the upper course of the river, and the river will take the easiest route, winding around areas of hard rock creating projections of land entering a V-shaped valley from alternate sides.
How are waterfalls and gorges formed?
In the upper course of the river, there are areas of hard rock and soft rock. The river will erode the soft rock faster, creating a step in the riverbed. The water undercuts the hard rock creating a plunge pool and the hard rock is left as an overhang, which eventually breaks off and deepens the plunge pool. The process is repeated over time and the gorge retreats further up the river.
How are meanders formed? (outside and inside of the river bend)
Outside of the river bend :
The water travels faster on the outside of the river bend as it gains momentum, meaning the water level is elevated as it is carrying more water which causes additional speed. This causes the river bed to be eroded making it deeper. Reduced friction from the river bend as it is eroded allows the river speed to increase.
Inside of the river bend:
Less water is travelling on the inside so the river has less energy and more friction from the river bed. Because the river has lost energy it will deposit its load. Over time the deposited load will build up to form a slipoff slope
Both of these processes causes the river bend to become more curved.
How are ox-bow lakes formed?
The narrow neck of a meander is constantly being eroded, and during a flood the river will take the shortest course through the neck of the meander. Deposition takes place, sealing off the old meander, and an ox-bow lake is left behind when the meander is completely cut off.
How are levees formed?
During times of heavy rain, the river will flood. When the water floods the river banks it carries the rivers load with it, and as it loses energy it will deposit the load. The heaviest rocks are deposited first because the river is unable to carry it. The finer materials are deposited after resulting in a gradual declining gradient levee.
The levee will increase in size after every flood.
How are flood plains formed?
Flood plains are areas surrounding a river that has burst its banks (due to heavy rainfall when the water level became higher than the river banks). When the river floods, it transports sediment onto the flood plain. When the flood ends, the sediment remains on the flood plain which causes the water to be higher after every flood.
What are estuaries and how are mudlfats formed on them?
An estuary is where the river meets the sea, the river is tidal and when the sea retreats the volume of water in the estuary is reduced. When there is less water, the river has little energy so it deposits silt, sand and mud to form mudflats
