Fluvial processes Flashcards
What is a drainage basin
A drainage basin is an area of land which is drained by a river.
Draw the different types of transportation
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What is a tributary
A small river that joins the main river channel
What is a confluence
The point at which two rivers meet
What is a main river channel
The feature in which the river flows
What is a water shed
The imaginary line which surrounds a drainage basin
(an area of highland which divides two drainage basins)
Any rain falling outside the watershed, will flow into another river basin
What is the Hjulstrom curve
It is a graph that shows the relationship between sediment size and the velocity needed to erode, transport or deposit
What are the types of transportation
Traction
Saltation
Suspension
Solution
Describe the process of traction
This is when, heavy pebbles are rolled along the river bed. This is most common near the source of a river, as here the load is larger.
Describe the process of suspension
This is when lighter sediment is suspended (carried) within the water, most commonly near the mouth of the river
Describe the process of saltation
This is when pebble-sized particles are bounced along the river bed, by the force of the water.
Most commonly near the source
Describe the process of solution
This is when soluble materials (e.g. limestone) dissolve in the water and are carried along
State some reasons why rivers slow down and deposit material
The volume of the water falls
The amount of eroded material increases
The water is shallower e.g. on the inside of a bend
The river reaches its mouth
What is deposition
Deposition is when a river drops the material it is transported
When does deposition occur
Deposition occurs when a river loses velocity and energy
Explain how particle size affects how far material is transported and where in a river’s course it’s deposited
Smaller particles are transported further and deposited closer to the river’s mouth
State some formation of landforms resulting from erosion
Interlocking spurs, waterfalls and gorges
Describe the formation of a waterfall (+ gorge)
The river may flow over an area of hard rock with softer more easily eroded rock underneath
Over time, the softer rock is eroded more rapidly than the hard rock by processes of hydraulic action and abrasion. This creates a ‘step’ in the river.
As the water flows over the step, it erodes more and more of softer rock
A steep drop is eventually created, which is called a waterfall
The hard rock is eventually undercut by erosion. It becomes unsupported and collapses.
The collapsed rocks are swirled around at the foot of the waterfall where they erode the softer rock by abrasion. This creates a deep plunge pool.
Over time, more undercutting causes more collapses. The waterfall retreats upstream, leaving behind a steep-sided gorge
What is a gorge
Over time, because of erosion and deposition,
meanders gradually change shape and move across the floodplain and
migrate downstream.
Describe the formation of interlocking spurs
In the upper course of a river most of the erosion is vertically downwards.
This creates steep-sided, V-shaped valleys
The rivers lack the power to erode laterally (sideways), so they have to wind around the high hillsides that stick out into their paths on either side
The hillsides that interlock with each other as the river winds around them are called interlocking spurs
State the formations of landforms resulting from erosion and deposition
Meanders and ox-bow lakes
Where can meanders be found
In the middle and lower courses
Describe how meanders are formed
The current is faster on the outside of the bend because the river channel is deeper and there is less friction (to slow the water down).
Therefore more lateral erosion (through abrasion and hydraulic action) takes place on the outside of the bend, which undercuts the banks forming steep-sided river cliffs
The current is slower on the inside of the bend because the river channel is shallower and there is more friction, (which slows the water down)
Therefore more eroded material gets deposited on the inside of the bend, (because there is more friction), forming slip-off slopes
Over time, because of erosion and deposition,
meanders gradually change shape and move across the floodplain and
migrate downstream.
What is a meander
A meander is a bend in the river
Explain why a waterfall is only a temporary feature on a river’s course (4 marks)
The river will try to develop a smooth long profile. A waterfall is formed where the river flows over a band of more resistant rock (1).
This represents an irregularity in the river’s long profile that it will try to remove. Where the more resistant rock overlays less resistant rock, different rates of erosion take place as the less resistant rock is eroded faster than the overlying harder rock by abrasion and hydraulic action (1).
Over time the less resistant rock is removed leaving a shelf of more resistant rock overhanging. Eventually this overhang will collapse and the waterfall will retreat further upstream (1).
This process continues until all the more resistant rock has been removed, the waterfall disappears and the river’s long profile is smoothed out, therefore a waterfall is only a temporary feature (1).
Describe how ox-bow lakes are formed
Ox-bow lakes are formed from Menaders
Erosion (by abrasion and hydraulic action) causes the outside bends of meanders to get closer until there is only a small bit of land between the bends (called the neck)
The river breaks through this land, usually during a flood and the river flows along the shortest course
Deposition eventually cuts off the meander forming an oxbow-lake.
Examples of formations of landforms resulting from deposition
levées, flood plains
and estuaries.
Describe how flood plains are formed
The flood plain is the wide valley floor on either side of a river which ocassionally floods
When rivers flood, the water slows down, loses energy and deposits the material that its transporting. This builds up the flood plain
Meanders widen as they migrate (move) across floodplains laterally,
Over time, meanders also migrate downstream.
The deposition that happens on the slip-off slopes of meanders also builds up the flood plain
Describe how levees are fromed
Levees are natural embankments (raised banks) along the edges of a river channel
During flood, eroded material is deposited over the whole flood plain
The heaviest material is deposited closest to the river channel, because it is dropped first when the river slows down and loses energy
Over time, the deposited material builds up, creating levees along the channel
Describe how Estuaries are formed
Estuaries are found at river mouths.
The land is close to sea level and the river valley is at its widest.
The water here is tidal - the river level rises and falls each day
When the water floods over the banks of the river, it carries silt and sand onto the valley floor
As the tide reaches its highest point, the water moves slowly and has little energy, so it deposits sediments
Over time, more mud builds up, creating large mudflats
At low tide, the wide, muddy banks are exposed
A levee could become a ______
A levee could become a floodplain
