River Processes - Erosion, Transportation, Deposition, Capacity & Competence, and the Hjulström Curve

Question 1

What are the types of load?

Answer 1

Types of Load in relation to the Hjulström Curve:

• Bed load - Large material e.g. boulders on river bed only transported in flood events.
• Dissolved load - invisible small particles held in solution.
• Supsended load - e.g. mud, sand, silt held in main body of water.

Examples of load from the Hjulström Curve -

The load of a river can be divided into different categories according to the calibre (particle size). The particle sizes range from fine silt and clay (less than 0.1mm in diameter) to big boulders.

Order: Clay, Silt, Sand, Gravel, Pebbles, Cobbles, Boulders

Question 2

1. Briefly explain erosion.
2. Describe the types of erosion.
3. Explain the ways in which river erosion happens.

Answer 2

1. The energy of a river flowing downhill causes erosion. The bed (the bottom of the river channel) and banks (the sides of the river channel) of a river can be eroded - making the river longer, deeper and wider:
2. Erosion can affect the length, depth and width of a river:

• Lateral Erosion - Lateral erosion makes the river wider. It happens in the middle and lower stages of a river, where the valley floor lies closer to sea level.
• **Vertical Erosion **- Vertical erosion deepens river channels. It happens in the upper stages of a river.
• **Headward erosion **- Headward erosion makes the river longer. It happens near a river’s source as throughflow and surface runoff causes erosion at the point the water enters the river channel (the valley head).

3. There are 4 main ways in which river erosion happens:

• **Hydraulic action **- the pressure of the water breaks rock particles away from the bed and banks. It’s strongest in rapids and waterfalls, and during floods.
• Abrasion (corrasion) - eroded pieces or rock in the water scrape and rub against the bed and banks, removing material. Most erosion of river beds and banks happens by abrasion.
• **Attrition **- eroded rocks smash into each other and break into smaller fragments. Their edges also get rounded off as they rub together. Attrition doesn’t erode the bed and banks - it just makes the particles of rock in the river smaller and more rounded.
• Corrosion (solution) - the dissolving of rock by chemical processes. Carbon dioxide dissolves in water to form a weak acid, which reacts with rocks like limestone and chalk, breaking them down.

Question 3

1. Briefly explain transportation.
2. State and explain the ways in which the river load is carried.

Answer 3

1. Transportation is the process of eroded material being carried in a river. The velocity of a river provides the energy needed for it to transport eroded material.
2. The eroded material carried in a river is called its load, and it can be carried in four ways:

• **Solution **- substances that can dissolve are carried along in the water. E.g. limestone is dissolved into river water that’s slightly acidic.
• **Suspension **- very fine material, like silt and clay particles, is whipped up by turbulence (erratic swirling of water) and carried along in the water. Most eroded material is transported this way.
• **Saltation **- larger particles, like pebbles or gravel, are too heavy to be carried in suspension. Instead, the force of the water causes them to bounce along the river bed.
• **Traction **- very large particles, e.g. boulders, are pushed along the river bed by the force of the water.

Material transported by traction or saltation is called the river’s bedload.

Question 4

1. Briefly explain Depostion.
2. Explain the reasons behind why deposition may occur.

Answer 4

1. Deposition is the process of dropping eroded material. A river deposits when, owing to a decrease in its level of energy, it is no longer competent to transport its load.
2. The speed and energy of a river can be reduced in many ways:

• Reduced rainfall causes lower discharge, which means the river slows down and has less energy.
• Increased evaporation or abstraction (taking water out of a river for human use) also causes lower discharge.
• Friction, e.g. in shallow areas of the river and close to the banks, reduces the speed of the river, reducing its energy.
• When the river is forced to slow down, e.g. before a narrow section of the channel, it loses energy.
• A lot of energy is lost when the river meets the sea (the sea absorbs the energy).

Question 5

Explain the Capacity of a river.

Explain the Competence of a river.

Answer 5

1. The capacity of a river is the total amount of load (measured in volume, weight or mass) that a river can transport/carry at a given point. Research has found that a river’s capacity increases according to the third power of its velocity. For example, if a river’s velocity doubles, then its capacity increases by eight times (23).
2. The competence of a river describes the maxium particle size (the diameter of the largest particle) that it can transport/carry for a given velocity. Again, research has shown that a river’s competence increases according to the sixth power of its velocity. For example, if a river’s velocity doulbes, then its competence increases 64 times (26). This is because fast-flowing rivers have greater turbulence and are therefore better able to lift particles from the river bed.