1.3 River channel processes and landforms Flashcards
1
Q
What are the different types of erosion?
A
Abrasion
Solution
Hydraulic action
Attrition
2
Q
What is abrasion?
A
- rocks/sediment scrape against the bedrock or other surfaces, causing the surface to wear away and the particles to become rounded and smoothed
- common where river is flowing with high velocity as it has enough energy to carry larger rocks
3
Q
What is solution?
A
- soluble minerals in rocks, such as limestone or chalk, are dissolved by acidic water, often rainwater containing carbonic acid
- most prominent when the river channel is made from water soluble rocks, such as chalk
4
Q
What is hydraulic action?
A
- 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.
5
Q
What is cavitation?
A
explosion of air bubbles trapped in river banks by water action
6
Q
What is attrition?
A
- rock particles and sediment carried by water, wind, or ice collide with each other, gradually wearing down and rounding the particles over time.
7
Q
What are the different types of transportation?
A
Traction
Saltation
Suspension
Solution
8
Q
What is traction?
A
Movement of larger rocks and pebbles through water rolling them along the river bed (too heavy to be carried)
9
Q
What is saltation?
A
lighter sediment/Pebbles are bounced along the riverbed
10
Q
What is suspension?
A
Small pebbles and material are carried (suspended) in the waterflow without touching the river bed due to velocity
11
Q
What is solution?
A
Soluble materials are carried within the water
12
Q
What is deposition and when will it occurs?
A
- 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
13
Q
Why do clay particles need high velocities to be entrained ( picked up)?
A
- they flocculate (stick together) as a result of electrostatic forces
14
Q
What is the general trend for material size and velocity needed to erode?
A
- the larger the material size the more energy needed to transport/erode
- therefore the higher the velocity needed
- except clay: flocculation
15
Q
What is laminar flow?
A
-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
16
Q
What is helicoidal flow?
A
- corkscrew movement, asymmetric
- occurs in bends in the river channel - meanders
- responsible for the erosion and deposition in a meander as carries sediment from outer bank to inner bank
17
Q
What is turbulent flow?
A
- disorderly flow
- changes in velocity
- can be caused by friction
- Eddie’s (swirling reverse currents) common in this flow
18
Q
What is the thalweg?
A
- line of fastest flow within a river
- also deepest point
19
Q
Where d you find straight channels?
A
- *upper course** of the river
- vertical erosion prevalent
- thalweg moves from side to side
20
Q
Where do you find meandering channels?
A
- middle and lower course velocity slower and gradient less steep
- series of bends and curves from side to side
- deposition on inside of bend
- erosion on outside of bend
21
Q
Where do you find braided channels?
A
- 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
22
Q
How do waterfalls form?
A
- 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
Example:
- high force waterfall, River Tees
- formed when river tees flows over hard, resistant Whin Sill, which lies on top of softer limestone/shale
23
Q
How does a gorge form?
A
- 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 gorge just downstream of high force waterfall on river Tees:
- whin sill, shale and carboniferous limestone
24
Q
how do meanders form?
A
- start with development of pools and riffles in a straight channel and thalweg begins to flow side to side
- 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
Example: Meanders on River Tees at Yarm, North yorkshire:
- alluvium, glacial till and soft sedimentary bedrock key to development of meander
25
How does a river cliff form?
- Created when the bank is undercut by erosion on the outside of the bend
- example: **Yarm on river tees**
- outer curve of meander made up of alluvium and glacial till
26
What is a slip-off slope?
- 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
27
How does an oxbow lake form?
- 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
28
How do bluffs form?
- 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
**Example**:
- **mississipi river bluffs - sandstone/shale eroded leaving behind limestone bluffs**
29
what are riffle-pool sequence ?
- are alternating patterns of shallow and deeper after in a river channel
- common in rivers with **gentle slopes**
-
30
How do riffle-pool sequences form?
- Riffles are formed in **shallow, fast-flowing areas** where water flows over rocks or gravel, creating turbulence and erosion and sediment carried downstrea,.
- Pools form in **deeper, slower-moving sections** where the water has lower velocity, allowing sediment to settle.
-
- e.g Tees gorge area - throug the Pennines. boulders/rocks = riffles, deeper section with sand/silt form pools
31
How do riffles and pools affect the river?
- 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
32
How do floodplains form?
- 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**
**Example**:
- River Tees floodplain in **darlington and Yarm** middle to lower course, alluvium and glacial till
33
How do levees form?
- 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 material**deposited 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
- natural levees along the nile river, particularly near the **nile delta**
34
How do deltas form?
1. As rivers near the sea **they contain high sediment loads**, velocity decreases as the river flows into the sea
2. **flocculation** causes clay sediment to be deposited on the sea bed forming the **bottom set bed** - *finest sediment travel furthest before being deposited*
3. **bedload** is then deposited above the bottom set layer to form the **foreset layer**
4. above the foreset layer, during times of river flood, sediment is deposited above the foreset layer to form the **topset layer** - *larger, heavier sediment are deposited first*
6. If sediment is **coarse grained arcuate** deltas form. If it is **fine grained then birds foot deltas** can be created (e.g. Mississippi).
35
What are the 3 different kinds of deltas?
1. **Bird’s foot delta** - a powerful river with a **large sediment load** entering a sea with **low tidal currents**
2. **Cuspate delta** – when rivers deposit sediment in areas with **strong wave action** from two different directions
3. **Arcuate delta** – is able to form when the **tidal range is low** and the currents remove the sediment, forming a smooth fan shaped delta
36
Why is the deposition of the river's load increased when it enters salt water?
- **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
37
example of a delta
**Nile Delta, Egypt**:
- multiple distributaries: Rosetta and Damietta branches
- rich in alluvial soil: sand, silt and clay
- under threat from rising sea levels = coastal erosion of **100 metres a year** as annual deposits from Nile floods don't reach the delta
- home to two thirds of Egypts population and 60% of food supply
38
conditions needed for a delta to form and type of material deposited
Conditions:
- heavily laden river, e.g Nile and a standing body of water with negligible currents, e.g Mediterranean
Type of material:
- bottomset beds: very fine material
- foreset beds: inclined layers
- topset beds: fine material
39
how do rapids form?
- river flows over **alternating bands of hard and soft rock**
- soft rock erodes faster through abrasion and hydraulic action, creating an uneven riverbed
- causes the water to become **turbulent** as it flows over the sudden changes in gradient, forming fast-flowing, white-water sections known as rapids
40
how does an eyot form?
- small island that forms in a braided channel
- form in braided channels when there is low discharge
