River Processes Flashcards
What can erosion affect?
The length, depth and width of a river.
What can be eroded?
Bed and banks
Makes river longer, deeper and wider.
What form of erosion makes a river longer? Where does it happen?
Headward erosion
Happens near a river’s source.
What form of erosion makes a river channel deeper? Where does it happen?
Vertical erosion
Happens in the upper stages of a river.
What form of erosion makes a river wider? Where does it happen?
Lateral erosion
Happens in middle and lower stages of a river.
What are the 5 main ways in which river erosion happens?
Hydraulic action Abrasion (corrasion) Attrition Cavitation Corrosion (solution)
What is hydraulic action?
Pressure of the water breaking rock particles away from the bed and banks.
What is abrasion (corrasion)?
Eroded pieces of rock in water scrape and rub against the bed and banks - removing material.
What is attrition?
Eroded rocks smash into each other and break into smaller fragments. Edges get rounder as they rub together
Doesn’t erode, just makes particles smaller.
What is cavitation?
Air bubbles in turbulent stretches of water implode causing shockwaves that break pieces of rock off river bank and bed.
What is corrosion (solution)?
Dissolving of Rick by chemical processes.
CO2 dissolves in water to form weak acid which reacts with rocks like limestone and chalk - breaking them down.
What is transportation?
The process of eroded material being carried in a river.
How are particles transported?
The velocity of a river provides the energy needed for it to transport eroded material.
What is the eroded material being carried by a river called?
Load
What are the 4 ways that the load can be transported?
Solution
Suspension
Saltation
Traction
What is solution?
Substances that can dissolve are carried along IN the water - limestone dissolves if water is acidic.
What is suspension?
Very fine material (silt + clay) is taken up by turbulence and carried along in the water.
Most eroded material is transported in this way.
What is saltation?
Larger particles (pebbles or gravel) are too heavy to be carried in suspension, instead, force of water bounces them along the river bed.
What is traction?
Very large particles (boulders) are pushed along the river bed by force if water.
Define bedload.
Material transported by traction or saltation.
What is deposition?
The process of dropping eroded material.
When does deposition occur?
When the river loses its energy
When it slows down, it loses energy and drops some of its load.
What are the 5 ways that speed and energy can be reduced?
Reduced rainfall Increased evaporation Friction Reduced velocity The sea
How does reduced rainfall reduce speed and energy of a river?
Causes lower discharge, means the river slows down, has less energy.
How does increased evaporation reduce speed and energy?
Slows down river discharge.
How does friction reduce speed and energy?
Shallow areas of the river and close to the bake, reduced speed of river, reducing its energy.
How does reduced velocity reduce speed and energy?
E.g. narrow section of river, loses energy.
How does the sea reduce speed and energy?
The sea absorbs the energy, energy is lost.
What is the capacity of a river?
Total load a river can transport at a given point.
What is the capacity of a river measured in?
Volume, weight or mass
How is the load of the river divided?
Can divided into categories of partial size.
Sizes range from silt and clay (0.1mm) to big boulders.
Define competence.
Maximum particle size that a river is capable of transporting at a given point.
What does the Hjulström curve show?
The link between river velocity and competence.
Shows how processes of erosion, deposition and transportation vary with the river velocity.
What affects competence?
The amount of energy it has (relates to velocity)
What does the critical erosion velocity curve show?
The MINIMUM velocity needed for the river to pick up and transport particles of different sizes.
What does the mean settling velocity curve show?
The velocities at which particles of different particle sizes are deposited.
Shows the competence of the river at different velocities.
What does the graph show about different particle size and the speed they are eroded? Why does this happen?
Particles of sand can be eroded quicker at lower velocities than clay and silt.
Because silt and clay particles stick together more than sand - harder to dislodge - requires more energy to erode them.
What causes erosion?
Energy of a river flowing downhill.
