1.3 River channel processes and landforms Flashcards
List and define main 3 types of river processes
- Erosion: involves the wearing away of rock and soil found along the river bed and banks. Erosion also involves the breaking down of the rock particles being carried downstream by the river.
- Transportation: the process where sediments are are entrained and moved by the water
- Depostion: the geological process in which sediments, soil and rocks are added to a landform or land mass - These processes require energy
List and define 4 types of erosion (upper and middle course)
- Abrasion: rocks carried along by the river wear down the river bed and banks. Mechanical impact
- Hydraulic action: the force of the river against the banks causes air to be trapped in cracks and crevices (cavitation). The pressure weakens the banks and gradually wears it away
- Attrition: rocks carried by the river smash together and break into smaller smoother particles
- Solution: soluble particles are dissolved into the river
Factors affecting rate of erosion
- Load
- Velocity
- Gradient
- Geology
- pH
- Human impact
Difference between erosion in the upper course and lower course
- Upper course: tends to be more vertical, mainly abrasion and hydraulic action - Lower course: more lateral and more frequent, happens outside of the bend, mainly abrasion and attrition
List 4 types of transportation (middle course)
- Suspension
- Saltation
- Traction
- Solution
- 2 factors that determine whether a particle is transported or not: size of particle and velocity of water
Define suspension
smallest particles such as silts and clays are carried as a suspended load
Define saltation
sands, gravel and small stones are being transported by a series of hop. Small particles are thrust up from river bed and fall back to the bottom when they dislodge other particles upwards => causes more bouncing
Define traction
large particles are shunted,dragged and rolled along the bed as a tracted load. Mainly happens during a period of high discharge
Define solution (transportation)
In areas of calcerous rocks, materals are carried in solutionas a dissolved load
Define capacity
The largest amount of debris that a stream can carry
Define competence
The diameter of the largest particle that the river can carry at a given velocity
Causes of deposition (middle and lower course)
- a shallowing gradient which decreases velocity and energy
- an increase in friction between water and channel
- a decrease in the volume of water in channel
- an increase in size of loads
- when river floods
Factors affecting the energy available
- Weight and velocity of water/ Discharge
- Gradient - height above sea level (greater potential energy)
- Channel roughness: amount/size of rocks and vegetation
- Shape of river (Meander/straight)
- Hydraulic radius (cross section/wetted perimerter) the higher the ratio the more effective a river is
The long profile (upper, middle and lower course)
- Upper: Vertical erosion (hydraulic and attrition), transportation (mostly traction)
- Middle: Lateral erosion (abrasion and attrition), transportation (suspension and traction), deposition on slip off slopes.
- Lower: Transportation (suspension and solution), mostly small particles deposited. Levées and slip off slopes formed.
What does the Hjulstrom curve show?
Shows the relationship between the 3 river processes and the velocity for particles of different size
Define entrainment
the process whereby sediment is picked up by the channel flow and is dependent upon the balance between the velocity and size of particle
Define critical erosion velocity curve
Shows the relationship between particle size and velocity required to entrain the particle within the channel flow
Define settling velocity curve
Shows the relationship between particle size and deposition. Below this line particles will be deposited
Relationship between velocity and sediment size
They are not neccessarily directly proportional. For example, even though sand has a bigger diameter, it requires lower velocity to move because it is less cohesive
Weaknesses of the Hjulstrom curve
- Graph relates to smooth channels, most channels aren’t
- It can be argued that velocity isn’t the only key factor for entrainment but the drag exerted on particles - depend on variables such as water depth
- The shape of the particles are composed are variables that the graph does not take into account
List and define 3 types of flow
- Laminar flow: for a smooth, straight channel with low velocity required. Allows water to flow in laminae, parallel to the channel bed. Horizontal movement. Occurs in lower reaches, groundwater and glaciers
- Turbulent flow: higher velocity is required as there is a complex channel morphology such as meanders. Creates eddies
- Helicoidal flow: horizontal turbulence that works in a corkscrewing motion. Associated with the presence of alternating pools and riffles
Reasons for differences in flow patterns
- Flow is associated and dependent on velocity and energy
- Gradient of channel
- Discharge level
- Shape of channel
- Channel roughness
Define thalweg
A line running along a river’s profile linking its deepest points. A river’s fastest flow normally runs along it
Define sinuosity
The length of a stream channel expressed as a ratio of the valley length

