Fluvial (river) processes = processes relating to erosion, transport and deposition by a river → help shape
the river’s channel and valley
● Fluvial erosion = process by which a river wears away the land → ability to erode depends on velocity
Vertical/lateral erosion:
→ Rivers erode in 2 directions:
● Vertical erosion = the deepening of the river bed, mostly by
hydraulic action
○ Most evident in upper course of river
○ Uses the little energy the river has, after overcoming
friction, to deepen the channel
○ Higher the velocity , increase in erosion
● Lateral erosion = “ sideways ” erosion, wears away the banks
of a river
○ Most evident in the lower/middle course of river
○ Widens the width of the river
Corrasion (abrasion)
○ Small boulders/stones scratch and
scrape their way down river during
transport
■ They wear down the river
banks/bed
○ Stones which have recently fallen
into channel will be sharp/jagged →
effective tools of abrasion
○ Ongoing abrasion causes vertical
and lateral erosion of the channel
● Attrition
○ When stones first enter a river
they’re sharp/jagged
○ They are transported downstream →
collide with each other and the river’s
banks/bed
○ Gradually knocks off the stone’s
jagged edges so they become
smooth/rounded
○ Some collisions may cause a stone
to smash into many smaller stones
● Solution
○ Solution = the dissolving of rocks (eg/
chalk, limestone)
○ Rivers travelling over these rocks will
erode
● Hydraulic action
○ Force of fast-flowing water hits the
river banks and beds
○ Forces the water into cracks and
compresses air in the cracks
○ Repeated changes in air pressure
weaken the channel
○ Responsible for vertical erosion in
the upper course of a river
○ Lateral erosion in the lower course
(outside bend of a meander)
River transport processes:
● Fluvial transport = process by which a river carries its load
● Load = name for eroded material in a river (larger the load, greater the energy needed to move it)
○ Differs in size:
■ Large, angular boulders in upper course
■ Fine, suspended silt in lower course
● Solution
○ Minerals are dissolved in water → chemical changes affect rocks such as chalk/limestone
○ Load transported in this way is called solute load
● Suspension
○ Fine, light material is held up and carried within the river’s flow → called suspended load
Saltation
○ Small pebbles/stones are bounced along riverbed
○ Load is alternately lifted and dropped
○ There is a local rise and fall in velocity of the water
● Traction
○ Large boulders/rocks are rolled over along the river bed
○ Called bedload
Deposition of sediment:
● Deposition = process by which a river drops its load
● Sediment = material deposited by a river
○ The bigger the load particles , the greater the velocity needed to transport it
○ When velocity falls , large boulders are the first to be deposited
○ Finest particles are deposited last
● Along its course, the river will deposit its load wherever the velocity falls
○ eg/ at the base of a waterfall
○ eg/ the inside bend of a meander
○ eg/ where the river enters a sea/lake
■ Friction of water with bed/banks causes energy to be lost → load is deposited in upper
course
● Will deposit more load in a period of drought , when the discharge is low = low velocity
○ Discharge = the quantity of water that passes a given point on a stream in a given period of time