River Processes - erosion, transport and deposition
The course of a river - you should know the main changes in both river channel and valley as it passes from source to mouth - cross profile and long profile
River Features:- describe and explain the characteristics / formation of each - state the processes involved.
Upper Course of the River - v-shaped valleys and waterfalls
Middle Course of the River - meanders and ox-bow lakes
Lower Course of the River - floodplains, levees, deltas
Erosion key words:
abrasion / corrasion
Solution / corrosion
Transportation key words:
2004 Question 4 With the aid of a diagram(s) explain the formation of a waterfall (7)
2001 Q1d With the help of an annotated diagram, explain how a waterfall is formed (5)
Level 3 answer
Waterfalls are formed where the river meets a band of softer, less resistant rock after flowing over a relatively hard, resistant rock. Initially a series of rapids may form
The softer rock is worn away more quickly by the processes of hydraulic action (the sheer force of the water eroding the soft rock) and abrasion (the swirling action of the river and its load which also erode the softer rock). The harder band of rock is undercut. In time the overlying harder rock (overhang) will become unsupported and will collapse.
At the foot of the waterfall is a deep plunge pool formed through abrasion. The water and rock debris from the overhang will help to deepen the plunge pool. The back wall is further eroded through the process of hydraulic action.
The whole process is repeated many times and will cause the waterfall to retreat upstream forming a gorge
less friction – more energy
More friction – less energy
2003 P1 Q7 b.
Explain the formation of an ox bow lake. You may use diagrams to help you. (6 marks)
Ox bow lakes are crescent shaped lakes formed on the floodplains of a river in its mature stage.
In a meander the fastest current is on the outside bank. Here the river bank will erode due to the sheer force of the water hitting the bank (hydraulic action) and from the water carrying pebbles (abrasion) to form a river cliff.
On the inside bank the river flow is slower and deposition occurs to form a slip off slope. Lateral erosion of the outer bank and deposition of the inner bank increase the amplitude (bendiness) of the meander
Over time a meander loop may become so pronounced that only a narrow strip of land is left separating two meander loops.
During times of flood the river will break through this and assume a new straighter and shorter course.
Water in the cut off meander is calmer. Deposition continues and eventually separates the oxbow lake from the main river channel. In time the ox bow lake is eventually filled in by marsh plants and peat and is left as a meander scar