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
- 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
3
Q
What is solution?
A
- 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
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
- river erodes the load it is carrying by particles bumping into each other
- gradually become smoother and rounder
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
Pebbles are bounced along the riverbed
10
Q
What is suspension?
A
Small pebbles and material are carried (suspended) sighing the water
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 weak electrical bonding
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
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
- occurs in bends in the river channel - meanders
- responsible for the erosion and deposition in a meander
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
- 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**:
- edge of floodplain, near Yarm in lower course of River Tees made of glacial till
29
what are riffle-pool sequence form?
- 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 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**
**Example**:
- upper course of river Tees near **Middleton-in-Teesdale** flows over stony bed from Whin sill and limestone geology
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
34
How do deltas form?
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
35
What are the 3 different kinds of deltas?
**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
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
