1.3. River Channel Processes and Landforms

Question 1

Load

Answer 1

The material transported by a river (includes rocks, stones, and other large particles.

Question 2

Capacity

Answer 2

The maximum volume of load that a river can carry at a specific point in its course

Question 3

Competence

Answer 3

The biggest sized particle that a river can carry at a specific point

Question 4

Traction

Answer 4

The large particles, such as boulders, are rolled along the river bed

Question 5

Saltation

Answer 5

Smaller particles, such as pebbles and gravel, bounce along the river bed

Question 6

Suspension

Answer 6

Fine particles float in the water

Question 7

Solution (transport)

Answer 7

Where particles, such as soluble rocks, are dissolved in the water

Question 8

Velocity

Answer 8

Refers to the speed of the river. Velocities increase in river that are deeper and when rivers are in flood.

Question 9

Discharge

Answer 9

This is the total volume of water flowing through a channel at any given point and is measured in cubic metres per second (cumecs).

Question 10

Wetted perimeter

Answer 10

The length of bed and bank in contact with water.

Question 11

Laminar flow

Answer 11

The horizontal movement of water, in sheets, parallel to the channel bed over a smooth surface, with no eddies or meanders.

Question 12

Turbulent flow

Answer 12

• Consists of a series of erratic eddies - vertical and horizontal in a downstream direction.
• Friction in the water from beds and banks slows the water nearest and water in the centre overtakes, creating turbulence.
• Water at the sides of the river begin to eddy towards the banks and water close to the bed of the river begins to eddy downwards.

Question 13

Helicoildal flow

Answer 13

• A continuous corkscrew motion of water as it flows along a river channel.
• The thalweg follows a spiralling path as the river moves downstream.
• The movement is constrained by the river channel - it moves from river bed to surface & bank to bank.
• The vertical movement of thalweg produces pools and riffles and bank to bank forms regularly spaced meanders.

Question 14

Floodplain

Answer 14

an area along a river that forms from sediments deposited when the river overflows its banks

Question 15

Pools

Answer 15

The deep-water areas of a meander where erosion is the main process.

Question 16

Riffles

Answer 16

shallow-water area found on the straighter sections of a meandering channel
- Riffles have more friction , causing the thalweg to be deflected to the pools.

Question 17

Waterfalls

Answer 17

an area where water flows over a vertical drop or a series of steep drops in the course of a stream or river

Question 18

Gorge

Answer 18

A deep narrow valley with very steep sides. It is currently occupied by a river or had a river in the past and is formed by a waterfall.

Question 19

Rapids

Answer 19

a fast-flowing and turbulent part of the course of a river.

Question 20

Abrasion

Answer 20

When a river uses its load of sediment to wear away its bed and banks( in uplands, pebbles get caught in hollows of the river bed - as they swirl, abrasion creates a pothole)

Question 21

Hydraulic Action

Answer 21

The force of water against the banks compressing air pockets into cracks, which expand and fracture the rock over time

Question 22

Attrition

Answer 22

Particles of sediment in the load of the river ( especially bedload) bump into each other and wear away each other. The river sediment will be smaller and more round.

Question 23

Solution (erosion)

Answer 23

Natural river water is often slightly acidic and it can dissolve rocks e.g. chalk and limestone

Question 24

Cavitation

Answer 24

Air bubbles trapped in the water get compressed into small spaces like cracks in the river’s banks. These bubbles eventually implode creating a small shockwave that weakens the rocks.

Question 25

Hjulstrom Graph

Answer 25

Shows what work a river will do (erosion, transportation, deposition) depending on its velocity and the size of material present.

Question 26

What scale does the Hjulstrom Graph use?

Answer 26

Double logarithmic scale

Question 27

Erosion velocity

Answer 27

The velocity required to entrain (pick up) particles from the stream’s bed and banks

Question 28

Settling velocity

Answer 28

The speed of river flow at which particles of specific size will be deposited

Question 29

Why do particles below 0.1mm have a higher erosion velocity than other particle sizes?

Answer 29

• These particles are silt and clay, and they need greater velocities than large particles
• This is because silt and clay have the ability to form cohesive bonds between particles in the form of flocculation
• And because of it, greater velocities are required to break the bonds and move these particles

Question 30

Thalweg

Answer 30

An imaginary line where river flow is fastest

Question 31

Straight Channels

Answer 31

• Rare in nature
• Where they do occur, the channel is usually controlled by a straight zone of weakness in the underlying rock
• Often man-made, such as channelization to reduce flooding or erosion problems. The idea is that a straight, smooth channel moves the water through faster, so less is available to spill out onto the floodplain

Question 32

Braided Channel

Answer 32

• Consists of a network of small channels separated by small islands of deposited material called eyots
• Occur in rivers with a large sediment load, unstable and easily eroded sediment, high slope, highly variable discharge

1) The water slows down in sections passing through flat, wide valleys with little downward slope allowing sediment to settle on the river bed.
2) The sediment starts to block the stream of water, causing lower velocity.
3) Until enough sediment settles on the bed, forming eyots that block the stream
4) The stream overflows its banks forming new channels. The stream is split into many streams causing the braided channel shape
5) The streams meet at a certain point only to split up again

Question 33

Formation of Waterfalls

Answer 33

1) Waterfalls form where a river flows over an area of hard rock( more resistant) followed by an area of soft rock(underneath/below)
2) Less resistant rock undercuts underneath - continues and undercuts hard rock above it- the river energy creates a plunge pool.
3) Less resistant rock beneath is eroded more rapidly by abrasion and hydraulic action. This creates a ledge which overhangs and collapses.

Question 34

Formation of Rapids

Answer 34

Form at areas of steepest gradient and river bed is rocky, resistant and irregular. Caused by a band/bands of hard rock in the river bed.

Question 35

Formation of Potholes

Answer 35

Holes carved by abrasion into rock bottom of the river channel with a whirlpool action of sediment caught in eddies.

Question 36

Formation of Meanders

Answer 36

1) Alternating pools (area of deep water) and riffles (shallower water) develop at equal spaces along the river due to the difference in river bed roughness leading to the thalweg flowing from side to side.
2) At pools, river water is more efficient so has greater erosive power and energy. Energy is lost as the river travels over a riffle as there’s friction
3) Pools and riffles now alter the speed of flow in different parts of the channel and cause the river to flow in a corkscrew pattern which spirals from bank to bank
5) The flow causes more erosion and deepening of the pools, forming river cliffs, and the eroded material is then deposited on the inside of the bend, forming slip-off slopes
6) The river continues to erode and deposit material. Eventually the curves the river become very close.
7) The river breaks through the thin barrier (during times of flooding) and the water no longer flows round the meander but straight along the new channel
8) Once the channel meander neck is breached an ox-bow lake forms

Question 37

Formation of Bluffs

Answer 37

• River bluffs are formed by the erosion of the valleyside along the floodplain.
• During flooding, the water erodes the base of the cliff / valley by abrasion and hydraulic action.
• Also, when meanders migrate across the floodplain, the erosion of the outer bends undercuts the valley side to form river bluffs

Question 38

Formation of Floodplains

Answer 38

Forms when the river floods

1) River discharge in river exceeds bankfull discharge and water spills over land also carrying sediment
2) Friction reduces water velocity so the deposition of fine sediments like silt and clay occur on the flat land, creating a layer of alluvium
3) Heaviest sediment is deposited first on the river channel and builds up over time to form natural levees. Finer material is carried further

Question 39

Formation of Deltas

Answer 39

1) As a river approaches the sea, its speed reduces and deposition takes place
2) The heaviest material is dropped first. These form islands and the river begins to braid around these islands
3) Over time, more and more sediment layers are deposited and build up above sea level. The delta now extends out to sea
4) As a delta forms, sediment is carried across the flat top surface, then deposited over the edge in steeply sloping fore-set beds
5) Finer meterial is carried further but as flat bottom-set beds. While the delta grows, the flat top adds some more sediment forming top-set beds

Question 40

Bluff Lines

Answer 40

The outer limits of the floodplain. The bluff line is basically the edge of the valley floor.

Question 41

Point bar

Answer 41

a crescent-shaped accumulation of sand and gravel deposited on the inside of a meander

Question 42

Knick Points

Answer 42

Part of river profile where a sudden break/ irregularity occurs in the gradient. Waterfalls, dams, river rejuvenation and rapids cause Knick Points

Question 43

How to calculate river discharge

Answer 43

cross-sectional area x river velocity