River Processes

Question 1

What does the river energy depend on?

3

Answer 1

1. Water mass - more mass, more energy - gravity.
2. The height of the river above its base level - gives it a source of potential energy.
   The higher the source, the more energy it has.
3. Channel steepness- controls speed of the water - determines how much kinetic energy there is.

Question 2

What is the wetted perimeter?

Answer 2

Overall length of the bed and banks that the river is in contact with.

Question 3

How can river energy be lost?

2

Answer 3

Internally - through turbulence flow within the flow of the river.
Externally - through contact with bed and banks

Question 4

What is abrasion?

Answer 4

Eroded pieces of rock in the water scrape and rub against the river beds, removing the material.

Question 5

What is hydraulic action?

Answer 5

Power of moving water.

The pressure of the water breaks rock particles away from the bed and banks.

Question 6

What is corrosion?

Answer 6

The minerals in the rock are dissolved by weak acids (caused by CO2) in the water and are carried away in solution.

Question 7

What types of rocks does corrosion affect?

Answer 7

Rocks that contain carbonates e.g limestone and chalk.

Question 8

What is attrition?

Answer 8

Eroded rocks smash into each other and break into smaller fragments.
Smooth and rounded rock edges.

Question 9

What is cavitation?

Answer 9

Air bubbles in turbulent waters implode causing shockwaves that break pieces of rock off the bed and banks.

Question 10

What is the information for vertical erosion?

3

Answer 10

1. Upper course - the river is attempting to cut down to its base level, which is usually sea level.
2. Causes steep-sided valleys.
3. Mainly abrasion and hydraulic action.

Question 11

What is the information for lateral erosion?

4

Answer 11

1. Middle to lower course.
2. Valley floor lies closer to sea level.
3. Lots of energy here - used to laterally widen the valley as the river meanders.
4. Mainly hydraulic action.

Question 12

What is traction?

Answer 12

Large boulders are rolled along the river bed by water moving downstream.

Question 13

When does traction occur?

+ EXAMPLE

Answer 13

Times of high discharge (and high energy levels).

EXAMPLE: Boscastle flood August 2004 - River Valency.

Question 14

What is saltation?

Answer 14

Small stones bounce along the river bed.

Question 15

What is suspension?

Answer 15

Very small particles (sand/silt) are carried along by the flow of the water.
Material also picked up, as well as carried.

Question 16

What is solution? (transport)

Answer 16

Dissolved minerals are transported within the mass of the moving water.

Question 17

What does river capacity mean?

Answer 17

The amount of material it can carry - the total volume of the load.

Question 18

How is river capacity related to river velocity?

Answer 18

A river’s capacity increases according to a third power of its velocity.

Question 19

What does river competence mean?

Answer 19

The diameter of the largest particle that the river can carry for a given velocity.

Question 20

When does deposition occur?

5

Answer 20

1. River gradient reduction e.g enters a lake.
2. Reduced discharge e,g during a dry spell.
3. Shallow water e.g inside of a meander.
   4, Increase in load size.
4. The river floods - reduced velocity on the floodplain.

Question 21

What does the Hjulstrom Curve show?

Answer 21

The link between river velocity and river competence.

Question 22

What is the first relationship that the Hjulstrom Curve shows?
3

Answer 22

1. Sand particles are picked up by lower critical erosion velocities than silt or clays.
2. The smaller silt and clay are harder to pick up (entrain) as they stick together.
3. They lie on the river bed and offer less resistance to water flow than larger particles.

Question 23

What is the second relationship that the Hjulstrom Curve shows?
2

Answer 23

1. Once picked up, particles can be carried at lower velocities than those required to pick them up.
2. Larger particles - only a small difference between the critical erosion velocity and the settling velocity - deposited very soon after being picked up.

Question 24

What is the third relationship that the Hjulstrom Curve shows?

Answer 24

Clays and silts are only deposited at very low velocities.

Some clay particles may never be deposited on the river bed.

Question 25

What is flocculation?

Answer 25

Fresh river water meets saltwater at a river estuary.
Causes chemical settling of clays and silts and causes extensive areas of mudflats.
Coagulation (clustering) of the clay/silt particles causes them to sink more rapidly.

Question 26

Name the different types of river load in size order?

BIGGEST to SMALLEST - 5.

Answer 26

Boulders and cobbles.
Pebbles.
Gravel.
Sand.
Silt.

Question 27

What happens to the river load as you get further downstream?

Answer 27

The particles making up the load get smaller - attrition.

Question 28

Where can big river loads be located in the world?

+ STATS.

Answer 28

The Mississippi River has a vast drainage basin - 1/3 of the USA.
Transports 136 million tonnes in solution.
340 million tonnes in suspension.
40 million tonnes by saltation.

Question 29

Name the factors that influence the spatial variation in load.
5

Answer 29

1. Size of the drainage basin.
2. Rock type.
3. Relief.
4. Precipitation.
5. Human activity.

Question 30

Explain how the size of the drainage basin influences the spatial variations in load.

Answer 30

Larger drainage basins have many tributaries - more sediment transported.

Question 31

Explain how the rock type influences the spatial variations in load.

Answer 31

Soft rock areas - sand/clay particles.
Hard rock areas - limestone - soluble transportation.
The sediment yield in soft rock area is higher than in hard rock area - soft rock is easily eroded.

Question 32

Explain how relief influences the spatial variations in load.

Answer 32

Low relief - low loads.

Limited energy for erosion/transport - only a small difference in altitude between the source and base level.

Question 33

Explain how precipitation influences the spatial variations in load.

Answer 33

Low loads are found in drainage basins with low precipitation rates.
Seasonal differences in sediment yield.

Question 34

Explain how human activity influences the spatial variations in load.

Answer 34

Can increase/decrease sediment yields.
Deforestation - load increase - soil erosion.
Farmers use nitrates/phosphates as chemical fertilisers. These substances enter the river by throughflow and overland flow - transported in solution.
Dams created e.g Hoover dam, Colorado River.
Dams trap sediment - lower the sediment yields.

Question 35

How does a large load result in landforms being created?

Answer 35

A fast-flowing river has the competence to carry a large load.
The particles erode the river by abrasion.
Potholes, waterfalls and gorges created.

Question 36

How does deposition of river load result in landforms being created?

Answer 36

Water volume falls quickly - fall in competence - deposited.

Depositional features e.g. levees, floodplains and deltas created.

Question 37

Where does both erosion and deposition occur to create a landform?

Answer 37

Meander bend,
Carried suspended load erodes the outside by abrasion.
Deposited load on the inside - forms a point bar.