1.3 River channel processes and landforms

Question 1

List and define main 3 types of river processes

Answer 1

• 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

Question 2

List and define 4 types of erosion (upper and middle course)

Answer 2

• 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

Question 3

Factors affecting rate of erosion

Answer 3

• Load
• Velocity
• Gradient
• Geology
• pH
• Human impact

Question 4

Difference between erosion in the upper course and lower course

Answer 4

• 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

Question 5

List 4 types of transportation (middle course)

Answer 5

• Suspension
• Saltation
• Traction
• Solution
• 2 factors that determine whether a particle is transported or not: size of particle and velocity of water

Question 6

Define suspension

Answer 6

smallest particles such as silts and clays are carried as a suspended load

Question 7

Define saltation

Answer 7

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

Question 8

Define traction

Answer 8

large particles are shunted,dragged and rolled along the bed as a tracted load. Mainly happens during a period of high discharge

Question 9

Define solution (transportation)

Answer 9

In areas of calcerous rocks, materals are carried in solutionas a dissolved load

Question 10

Define capacity

Answer 10

The largest amount of debris that a stream can carry

Question 11

Define competence

Answer 11

The diameter of the largest particle that the river can carry at a given velocity

Question 12

Causes of deposition (middle and lower course)

Answer 12

• 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

Question 13

Factors affecting the energy available

Answer 13

• 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

Question 14

The long profile (upper, middle and lower course)

Answer 14

• 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.

Question 15

What does the Hjulstrom curve show?

Answer 15

Shows the relationship between the 3 river processes and the velocity for particles of different size

Question 16

Define entrainment

Answer 16

the process whereby sediment is picked up by the channel flow and is dependent upon the balance between the velocity and size of particle

Question 17

Define critical erosion velocity curve

Answer 17

Shows the relationship between particle size and velocity required to entrain the particle within the channel flow

Question 18

Define settling velocity curve

Answer 18

Shows the relationship between particle size and deposition. Below this line particles will be deposited

Question 19

Relationship between velocity and sediment size

Answer 19

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

Question 20

Weaknesses of the Hjulstrom curve

Answer 20

• 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

Question 21

List and define 3 types of flow

Answer 21

• 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

Question 22

Reasons for differences in flow patterns

Answer 22

• Flow is associated and dependent on velocity and energy
• Gradient of channel
• Discharge level
• Shape of channel
• Channel roughness

Question 23

Define thalweg

Answer 23

A line running along a river’s profile linking its deepest points. A river’s fastest flow normally runs along it

Question 24

Define sinuosity

Answer 24

The length of a stream channel expressed as a ratio of the valley length

Question 25

List and define 3 types of channel

Answer 25

• Straight channel: sinuosity value 1.5
• Braided channel: channel is divided by islands or bars. Islands are vegetated when bars aren’t

Question 26

How do braided channels form?

Answer 26

• A Braided Channel is a channel which is divided by islands or bars . They occur when a river does not have the capacity to transport its load in a single channel
• 3 factors: steep channel, variable discharge, large load that is easily eroded
• Begins with mid-channel bar which grows downstream, discharge decrease after precipitation => finer sediment is added to the bar
• During low discharge, vegetation can stabilise bars asit grows roots
• The river divides around these islands and rejoins

Question 27

How do meanders form?

Answer 27

• Helicoidal flow (cork screwing motion) means that a river tends to alternate between fast and slow water flow.
• This then creates riffles and pools as fast water erodes and slow water deposits
• Erosion occurs outside of bend and deposition inside of bend because there’s less friction inside of bend. Also on the outside, channel is deeper => faster flow of water => able to overcome friction

Question 28

How do oxbow lakes form?

Answer 28

Erosion occurs outside of the bend => outside bends move closer => meander neck is narrower => when there’s a flood (high discharge) river cuts across the neck and takes a straight course. Deposition seals off the old meander forming a new straighter river channel => horseshoe-shaped oxbow lake

Question 29

Definition of riffles

Answer 29

A short relatively shallow and coarse-bedded length of stream over which flows at slower velocity but higher turbulence

Question 30

Definition of pools

Answer 30

A long relatively deep and soft-bedded length of stream over which flows at higher velocity but less turbulence

Question 31

How do waterfalls form?

Answer 31

• A river flows over a band of hard and soft rock
• Abrasion erodes the softer rock away quicker leaving the hard rock exposed
• As the water falls over the top more erosion occurs (hydraulic and abrasion)
• A plunge pool is formed, cutting a cave below the hard rock
• Due to the undercutting, the overhang is exposed
• Eventually overhang collapses due to lack of support
• Process repeats and the waterfall retreat leaving a gorge behind

Question 32

Definition of levees and floodplain and how do they form?

Answer 32

• Levees are raised embankments on both sides of a channel, that acts to heighten the level of the river bank
• Floodplain: the area a river can flood when discharge level is exceeded
• They are both formed when the river bursts its bank for a long period of time, therefore, the water loses velocity and results as deposition of alluvial material near the channel edge to form embankments

Question 33

Define pointbar and its formation

Answer 33

This occurs on the inside bend of a meander when deposition accumulates on the slip of slope and it breaks through the surface of the water to form a bank of land.

Question 34

Definition of deltas and how do they form?

Answer 34

• Are wetlands formed when a river empties its sediment and water into another body of water such as oceans, lakes or another river
• Are formed when ther river water - less dense entering a denser body of water => slowly spreads out and results in the spread of alluvium
• The river must have a strong flow and the body of water must have a negligible flow
• The saline water of the body of water helps to adhere materials such as clay making deposition easier

Question 35

Definition of alluvial fans and cones and how do they form?

Answer 35

• Alluvial cones: are cone-shaped accumulation of coarse-grained material. Its slope angle goes up to 15 degrees due to rapid deposition
• Alluvial fans: are widepread accumulation of refined material. Its slope angle is about 1 degree due to slow depostion
• Formation: the water moves down from the mountain and enter the valley => stream’s gradient decreases => deposition of sediment. This reduces the capacity of the channel and gradually builds up alluvial fans and cones