River Processes and Lanforms

Question 1

What factors does how much work a river does depend on?

Answer 1

• Velocity of water - faster = can carry bigger pieces of material
• Volume of water - greater volume of water = greater load
• Bedrock type - hard rock = hard to erode

Question 2

Give and example of hard and soft rock.

Answer 2

• Hard:
  
  • Granite
• Soft:
  
  • Shale

Question 3

What can cause deposition to occur?

Answer 3

• A decrease in gradient
• A decrease in discharge as water drains away after heavy rain
• River meets sea or lake
• River flows more slowly on the inside of bends

Question 4

What is discharge?

Answer 4

The volume of water flowing down the river at any one time.

Question 5

How do you measure discharge of a river?

Answer 5

Measured in cubic metres per second (a.k.a. cumecs).

Question 6

What can cause a variable annual discharge?

Answer 6

• Climates with wet and dry seasons
• Melting glacial ice in spring

Question 7

What are the three main shapes of river channel?

Answer 7

• Braided (river splits into channels and flows around banks of material it has deposited)
• Sinuous
• Straight

Question 8

What are the characteristics of the river in its upper course?

Answer 8

Velocity

Low

Turbulence

High

Depth

Shallow

Breadth

Narrow

Process

Vertical erosion

Gradient

High

Question 9

How do potholes form?

Answer 9

1. Stone trapped in a hollow in the river bed.
2. Eddies in the water swirl stone around, causing corrasion.
3. Hollow becomes deeper and wider.
4. Hollows join together to make a pothole.

Question 10

How do rapids form?

Answer 10

1. Band of soft rock eroded morethan band of hard rock.
2. Creates a changing gradient.
3. Changing gradient creates turbulence.

Question 11

How does a waterfall form?

Answer 11

1. Waterfalls can develop from rapids.
2. Horizontal layer of hard rock lies on top of a layer of soft rock in a river valley.
3. Soft rock eroded faster. Plunge pool develops.
4. Splashing from water (hydraulic action) and stones (corrasion) undercuts cliff.
5. Cliff cannot support itself and eventually collapses.
6. Waterfall retreats upstream over time, leaving behind a gorge (steep sided valley) and escarpment (cliff).

Question 12

What can waterfalls be used for?

Answer 12

• Tourism
• Hydroelectricity
• Rapids good for water sport

Question 13

What problems do waterfalls and rapids bring?

Answer 13

Navigational problems.

Question 14

What are interlocking spurs, and how do they form?

Answer 14

• Water avoids hard rock and takes path that uses least energy.
• Water therefore forced to wind around spurs.

Question 15

What are the characteristics of the river in its middle course?

Answer 15

Velocity

Higher

Turbulence

Lower

Depth

Deeper

Breadth

Wider

Process

Transportation and lateral erosion

Gradient

Lower

Question 16

How do meanders and oxbow lakes form?

Answer 16

1. Water flow is naturally helical (corkscrew).
2. Water meanders as it erodes soft rock and flows around hard rock.
3. Helical flow sends energy laterally. Fastest flow is forced to outer bend (creating a river cliff where erosion takes place) and slowest flow is in the inner bend (creating a slip-off slope where deposition takes place).
4. Continued erosion creates a narrow neck between two meanders.
5. Faster flow around the neck means that the neck is cut through.
6. Old meander becomes an oxbow lake when deposition (at the sides in a straight river) seals off the ends.
7. Eventually dries up to become a marsh and then meander scar.

Question 17

What is a flood plain?

Answer 17

Area of flat land next to a river that is liable to flooding, enclosed by a bluff. Often marshland that is poorly drained.

Question 18

How does a flood plain form?

Answer 18

1. Deposition occurs at slip-off slopes at the inner bend of a meander. As meanders change over time, these deposits spread across the valley.
2. Gravel is deposited on the river bed.
3. During floods, the river bursts its banks and deposits fine alluvium on the plain itself, far away from the river because suspended load is lightest.

Question 19

How do levées form?

Answer 19

1. River floods.
2. Lightest load deposited far away from the river.
3. Heaviest load deposited on the banks of the river.
4. This build up levées on either side of the channel.

Question 20

Why is the river bed higher than the flood plain?

Answer 20

• During normal flows, the river deposits its material from its load onto the river bed within the channel.
• This happens more often than floods.

Question 21

What is a delta?

Answer 21

• An area of flat, low-lying, marshy land where the river meets the sea.
• Often contains lakes and lagoons.
• River channel has divided into distributaries (braided channel).

Question 22

How does a delta form?

Answer 22

1. River carrying its load meets the slow, still water of the sea or lake.
2. River loses velocity.
3. River deposits load.
4. Load builds up over time to form a delta.
5. Continued deposition blocks the main river channel, causing the development of distributaries.

Question 23

What is a long profile?

Answer 23

A line drawn from the source to the mouth of a river to show how gradient changes through its course.

Question 24

Why live on a floodplain or delta?

Answer 24

• Flat land is easy to build settlements and roads on.
• Fertile alluvium soils good for farming.
• River valleys often natural route ways.
• Rivers good for transportation.
• Rivers used for drinking.
• Rivers used for tourism and recreation.

Question 25

Why is it a bad idea to live on a floodplain?

Answer 25

• Frequent flooding
• Diseases carried by insects (applicable to the tropics)

Question 26

What factors cause flooding?

Answer 26

• Physical:
  
  • High rainfall or monsoon rain - high amount of antecedent rainfall makes soil saturated. Further rain is just surface runoff.
  • Glacial melt or heavy continual rain increases the discharge suddenly.
  • High relief means that rainwater flows over surface before it can infiltrate
  • Low temperatures reduce evaportation and transpiration rates.
  • Long periods of extreme cold can lead to frozen ground which does not allow water to infiltrate.
  • Permeable or porous rocks allow water to infiltrate or store water. This decreases surface runoff.
• Human:
  
  • Impermeable surfaces such as concrete or naturally hard rock (i.e. building on a floodplain) increases surface runoff, or enters drains which enter rivers.
  • Deforestation decreases interception, so rain reaches ground faster.
  • Non-contour ploughing creates channels which allows water to reach rivers faster.
  • Faulty sewage networks increases sediment in river

Question 27

How does water get from rain into the river?

Answer 27

Movement downwards:

1. Rain is intercepted by branches.
2. Water infiltrates through soil.
3. Water percolates through permeable or porous rock as groundwater.

Movement across:

1. Water flows on the surface of saturated or hard soil as surface runoff.
2. Water flows through soil as throughflow.
3. Water fows through rock slowly as groundwater flow.

Storage:

1. Water is stored in lakes etc. as surface store.
2. Water is stored underground as groundwater.

Question 29

What is a hydrograph?

Answer 29

A graph showing how a river responds to a particular storm.

Question 30

What is the lag time?

Answer 30

Difference between peak rainfall and peak discharge.

Question 31

Give an example of a permeable rock and a porous rock.

Answer 31

1. Permeable: limestone
2. Porous: chalk