1.3 River channel processes and landforms

Question 1

What are the different types of erosion?

Answer 1

Abrasion
Solution
Hydraulic action
Attrition

Question 2

What is abrasion?

Answer 2

• scraping and grinding of rocks along river channel causing soil and rock to wear away
• common where river is flowing with high velocity as it has enough energy to carry larger rocks

Question 3

What is solution?

Answer 3

• water dissolving parts of rock/soil that makes up the river channel
• most prominent when the river channel is made from water soluble rocks, such as chalk

Question 4

What is hydraulic action?

Answer 4

• When waves hit the cliffs or river banks, air trapped in cracks is compressed.
• The force from the compressed air can exert enough pressure to widen cracks and dislodge pieces of rock.

Question 5

What is cavitation?

Answer 5

explosion of air bubbles trapped in river banks by water action

Question 6

What is attrition?

Answer 6

• river erodes the load it is carrying by particles bumping into each other
• gradually become smoother and rounder

Question 7

What are the different types of transportation?

Answer 7

Traction
Saltation
Suspension
Solution

Question 8

What is traction?

Answer 8

Movement of larger rocks and pebbles through water rolling them along the river bed (too heavy to be carried)

Question 9

What is saltation?

Answer 9

Pebbles are bounced along the riverbed

Question 10

What is suspension?

Answer 10

Small pebbles and material are carried (suspended) sighing the water

Question 11

What is solution?

Answer 11

Soluble materials are carried within the water

Question 12

What is deposition and when will it occurs?

Answer 12

• river drops its load when the river no longer has enough energy to carry the load
• occurs when a river becomes shallower or when the volume of water decreases

Question 13

Why do clay particles need high velocities to be entrained ( picked up)?

Answer 13

• they flocculate (stick together) as a result of weak electrical bonding

Question 14

What is the general trend for material size and velocity needed to erode?

Answer 14

• the larger the material size the more energy needed to transport/erode
• therefore the higher the velocity needed

Question 15

What is laminar flow?

Answer 15

-flow of water in parallel streams
- no cross currents or swirls
- one directional, orderly flow
- found in smooth, shallow straight channel with a slow velocity
- lower sections of a river

Question 16

What is helicoidal flow?

Answer 16

• corkscrew movement
• occurs in bends in the river channel - meanders
• responsible for the erosion and deposition in a meander

Question 17

What is turbulent flow?

Answer 17

• disorderly flow
• changes in velocity
• can be caused by friction
• Eddie’s (swirling reverse currents) common in this flow

Question 18

What is the thalweg?

Answer 18

• line of fastest flow within a river
• also deepest point

Question 19

Where d you find straight channels?

Answer 19

• *upper course** of the river
• vertical erosion prevalent
• thalweg moves from side to side

Question 20

Where do you find meandering channels?

Answer 20

• middle and lower course
• series of bends and curves from side to side
• deposition on inside of bend
• erosion on outside of bend

Question 21

Where do you find braided channels?

Answer 21

• not usually in the upper course
• large deposits of sediment within the channel, causing multiple channels separated by islands of sediment
• river deposits when it loses energy

Question 22

How do waterfalls form?

Answer 22

• in an area where a river flows over a area of hard rock and soft rock, soft rock erodes more quickly
• soft rock erode away underneath hard rock, creating a step
• over time, soft rock continues to erode further, *undercutting the hard rock**
• leaves hard rock to overhang
• rotational movement of the after quickens erosion, creating a deep plunge pool
• gravity causes overhang to collapse and broken up rocks fall into plunge pool
• acts as tools for erosion and further deepens the plunge pool
• erosion continues to undercut hard rock, creating an overhang again
• continual process of overhang collapsing causes the waterfall to retreat upstream over time
• plunge pool continues to deepen

Question 23

How does a gorge form?

Answer 23

• it is a steep sided, narrow valley within river flowing through it
• formed when a waterfall continually retreat over time
• back of waterfall wall and channel floor is eroded, whereas valley sides are not, leaving a deep gorge

e.g tiger Leaping gorge china

Question 24

how do meanders form?

Answer 24

• erosion stronger on outside of a bend, as it’s is where the river is at its highest velocity. River channel is at its deepest so less friction
• deposition more prominent on inside of bend, as where river has lowest velocity
• less energy to carry material and instead deposits it
• river channel at its slowest here, causing after to be slower due to friction
• over time, erosion causes upside bends to get closer, until only small area of land remains

Question 25

How does a river cliff form?

Answer 25

Created when the bank is undercut by erosion on the outside of the bend

Question 26

What is a slip-off slope?

Answer 26

• a deposit of fine sediment, e.g sand, on the river bank inside of a meander bend
• usually changes position as the meander changes shape, leaving a mark where it once was

Question 27

How does an oxbow lake form?

Answer 27

• when river is in flood and it breaks its banks, the river will flow along the **shortest available course **
• this can be across the neck, cutting off the meander
• eventually the meander bend becomes fully cutoff due too deposition = oxbow lake

Question 28

How do bluffs form?

Answer 28

• they are large, rounded cliff on the side of a river
• formed on outside bend of a meander, where erosion is dominant
• water erodes the lower section of the river bank, causing the upper section o the river bank to become unsupported and therefore unstable
• eventually, the unsupported upper part of the bank collapses, causing a steep bluff to be left
• also occur when water in the outside bend of a meander erodes a steep valley wall

Question 29

what are riffle-pool sequence form?

Answer 29

• are alternating patterns of shallow and deeper after in a river channel
• ## common in rivers with *gentle slopes**

Question 30

How do riffle-pool sequences form?

Answer 30

• riffles form due to the deposition of sand and gravel on the bed of a river channel causing the bed to be shallow and tough over which water flows in. turbulent flow
• pool is a deeper area in which water flows in a calmer, smother laminar flow

Question 31

How do riffles and pools affect the river?

Answer 31

• causes the river **current to swing from side to side in its channel **
• encourage the development of a meandering river channel
• the deeper pools tend to be found wher erosion is greatest, such as on the outside of developing meander bends, while the shallower riffles are regularly paced along the **straighter parts of the channel ** and are expatriated by the deeper pools

Question 32

How do floodplains form?

Answer 32

• when river reaches bankfull level, any water that is added to the river will then become overbank flo and floods surrounding land
• quickly loses energy and deposit its load
• regular flooding will result in the building up of layers of alluvium (silt) which form a flat and fertile floodplain

Question 33

How do levees form?

Answer 33

• natural embankments, create raised edges on the river and, making it less likely for the river t break its banks
• when river floods, sediment being transported in the river floods onto floodplain
• heavy, coarse materialdeposited first as requires most energy to be transported
• finer light sediment requires les energy to be transported
• so is deposited further way from the channel

Question 34

How do deltas form?

Answer 34

1. As rivers near the sea they contain high sediment loads and the rivers start to spread out laterally. Ocean water entering these streams can reduce velocities as well.
2. This lateral movement of water reduces hydraulic radius and increases wetted perimeter.
3. This causes sediment to be deposited, as does flocculation where clay sediments join together, gain in mass and sink.
4. This sedimentation builds up over time and can create small islands which split the channel, similar to braided streams.
5. This can happen again and again until the river consists of a number of smaller streams separated by islands.
6. If sediment is coarse grained arcuate deltas form. If it is fine grained then birds foot deltas can be created (e.g. Mississippi).
7. Sediment is often deposited on the sea ward side, which builds the delta outwards into the sea. These areas are called fore beds, and are highly unstable

Question 35

What are the 3 different kinds of deltas?

Answer 35

Arcuate: have many distributaries,** fan shaped** and build out radially in uniform shape

Digital (or birds foot): found where the river has vasts amounts of load and spits into two or more channels which deposit their load along their sides and extend out into the sea/lake

Cuspate: form a pointed delta where there is a dominant channel shaped by tidal and long shore currents

Question 36

Why is the deposition of the river’s load increased when it enters salt water?

Answer 36

• flocculation
• causes the salts in the sea water to join with the fine clays brought down by the river in a chemical reaction
• fine clay particles join together to form larger heavier particles which will then fall to the sea bed