# 3. River Features Flashcards

1
Q

What does the Hjulström Curve demonstrate?

A

The Hjulström Curve shows the relationship between river velocity and competence

2
Q

What is Competence?

A

The competence is the maximum size of load (maximum particle size) a river is able to carry at a particular point

It shows the velocities at which sediment of a given size will normally be eroded, transported and deposited

3
Q

What is Capacity?

A

Capacity is the total volume of material a river can transport

• A greater velocity is required to erode material compared to just transporting it
• Generally, higher velocities are needed to erode larger particles
• Deposition occurs when the velocity decreases - heaviest first
4
Q

What is the velocity curve?

A

The velocity curve is the minimum velocity needed for the river to erode and transfer material of a different size

5
Q

What is the setting velocity?

A

The setting velocity shows which size particles are dropped as the river velocity slows.

6
Q

How does discharge affect the size and amount of the river’s load?

A

During times of high discharge and high velocity, the size and amount of the river’s load will increase considerably causing increased erosion within the channel.

7
Q

Is coarser material more susceptible to deposition?

A

For coarser material, the boundary between transportation and deposition is narrow. A small drop in velocity is needed for deposition / sedimentation.

8
Q

What features does fine clay have and how do these affect the Hjulström Curve?

A

Fine clay has high cohesive (sticky) properties and therefore needs a velocity similar to that of pebbles to initiate movement.

However, many individual minerals in clay are soluble so it stays in transport for a long time.

9
Q

Describe the formation of a waterfall and gorge:

A

As water passes over alternating bands of soft and hard rock, it erodes the soft rock faster through abrasion and hydraulic action leading to the formation of a plunge pool and overhang.

Eventually, the overhand is eroded until it becomes unsupported and collapses, causing the waterfall to retreat and a steep-sided gorge to form. Attrition also causes the plunge pool to widen.

10
Q

Describe the formation of rapids:

A

As water passes over alternating bands of soft and hard rock, it erodes the soft rock faster through abrasion and hydraulic action.

The water flow becomes turbulent as erosion of soft rock creates an uneven channel bed.

11
Q

What are pools and riffles?

A

A pool is an area of deep water and greater erosion (energy build-up due to less friction).

A riffle is an area of shallow water created by deposition of coarse sediment.

12
Q

How are meanders formed?

A

Meanders are formed because the thalweg, or line of fastest flow, swings from side to side as it passes around bends.

This causes erosion on the outside of the bend where the current is fastest and has more energy, and deposition on the inside of the bend where the current is slower.

13
Q

Describe the formation of an oxbow lake:

A

Lateral erosion from the thalweg erodes the sides of the meander.

The meander continues to erode towards itself.

Eventually the meander erodes into itself forming a lake and a small island.

The change in the thalweg causes deposition to form land, sealing off the old meander and forming an oxbow lake.

14
Q

What is a flood plain?

A

A flood plain is an area of flat land adjacent to the main river which experiences flooding during times of intense discharge.

15
Q

What is a bluff?

A

A bluff is a steep, rounded slope formed by erosion from meanders. They usually define the edge of a flood plain.

16
Q

A

Levées are made from deposited material and alluvium left from flooding.

17
Q

How is a levée formed?

A

Flooding ocurrs and the river overflows its banks

As it does, the velocity of the water decreases rapidly due to friction between the bank and the water

This causes deposition on the river banks - largest sediment is deposited on the bank forming a levée whereas smaller settlement is deposited across the floodplain.

18
Q

What is an alluvial fan?

A

An alluvial fan is an accumulation of sediment that fans outwards from a concentrated source of sediments, such as emerging from a narrow canyon into an escapment.

19
Q

How are deltas formed?

A

Deltas are formed at the mouth of rivers, when material is deposited faster than the sea can remove it.

20
Q

What traits do straight channels often have?

A
• OR guided by a fault
• OR found in solid rock
• OR found where there is constant heavy discharge
21
Q

Why are straight channels rare?

A

Straight channels are rare because helicoidal flow dominates in most rivers (as opposed to laminar flow).

22
Q

Where do the largest meanders occur?

A
• River banks are soft
• Later erosion occurrs / has significant effects
• The river is very wide (length of the meander is ~10x the width)
• The river has a high discharge - increasing discharge increases width
23
Q

Where are braided channels found?

A

Braided channels are found where there is lots of load material in the water e.g. outwash plain of glacier or arid area, or delta in an area with lots of volcanic ash.

24
Q

What features are typical of braided channels?

A
• Lots of islands, bars and egots
• Lack of vegetation
25
Q

What is Laminar Flow?

A

Laminar flow is where water flows in a simple sheet with no eddies or meanders whatsoever

26
Q

What is Turbulent Flow?

A

Turbulent flow is water flow with eddies and turbulence

This ocurrs because:

• Water encounters sufficient friction with the banks and river bed to slow the water closest to the bed and banks
• This causes water in the middle of the channel to overtake the water on the edges, creating eddies / turbulence
27
Q

What is Helicoidal flow?

A

Helicoidal flow is where water spirals as it flows downwards (like a plughole)

This causes the thalweg to take a spiraling motion, concentrating erosion on one bank then the otherm resulting in the formation of regularly shaped meanders along the river’s course.

28
Q

What are the properties of river channels on the upper course of the river?

Cross-section, Valley Features, Gradient, Channel Features, Processes at work

A
• V-Shape valley / cross profile
• Steep sides
• Waterfalls
• Vertical erosion
29
Q

What are the properties of river channels on the middle course of the river?

Cross-section, Valley Features, Gradient, Channel Features, Processes at work

A
• U-Shape valley / cross profile
• Wide flood plain
• Meanders
• Vertical and lateral erosion
30
Q

What are the properties of river channels on the lower course of the river?

Cross-section, Valley Features, Gradient, Channel Features, Processes at work

A
• Wide cross profile
• Sand banks and deposited sediment