Chapter 10 - part 1

Question 1

1. What is a drainage basin?

Answer 1

The area of land drained by a river or river system.

Question 2

2. How does a typical drainage basin vary from its highest point to its lowest point?

Answer 2

At the highest point,

• the gradient is steeper - higher velocity
• channel size: smaller
• shape/ratio: higher width to depth ratio at top
• sediment load: coarser sediment at the top
• discharge: more water at the bottom

Question 3

How is sediment created in a river?

Answer 3

Weathering creates sediment.

Question 4

How does EROSION occur in a river?

Answer 4

because of the impact of:

MOVING WATER
IMPACT of other moving sediment
HYDRAULIC LIFT

Question 5

How does TRANSPORT occur in a river?

Answer 5

SOLUTION

SUSPENSION

SALTATION

TRACTION

Question 6

How does DEPOSITION occur in a river?

Answer 6

Occurs when water velocity drops.

Solution deposition occurs with precipitation.

Question 7

4. How does a river erode, transport and deposit sediment?

Answer 7

Basic processes:

1. WEATHERING creates sediment.
2. EROSION in a river occurs because of the impact of moving water, impact of other moving sediment and hydraulic lift
3. TRANSPORT (through solution, suspension, saltation and traction)
4. DEPOSITION occurs when water velocity drops (solution deposition occurs with precipitation)

Question 8

5. When does a river erode, transport and deposit most of its sediment load?

Answer 8

Most sediment movement takes place during rare, brief episodes of increased discharge (a flood).

Question 9

6. BRAIDED RIVER

Answer 9

1. Number of channels:more than 1
2. Size of channel:smaller
3. Shape of Channelhigher w/d ratio
4. Discharge:variable
5. Sediment Load:poorly sorted coarse or large moving as bedload
6. Gradient:Steep

Question 10

6. MEANDERING RIVER

Answer 10

1. Number of channels:1
2. Size of channel:larger
3. Shape of ChannelLower w/d ratio
4. Discharge:consistent and stable (uniform)
5. Sediment loadwell sorted, fine or small moving in suspension
6. Gradient

shallower

Question 11

Braided Rivers are found ____ in the drainage basin

Answer 11

higher up in the drainage basin

Question 12

Meandering Rivers are found ____ in the drainage basin

Answer 12

lower down in the drainage basin

Question 13

7. DISCHARGE

Answer 13

The volume of water moving in a river in a certain amount of time.

Question 14

8. BANKFULL DISCHARGE

Answer 14

The maximum amount of water in the channel. (Often the dominant discharge)

Question 15

9. DOMINANT DISCHARGE

Answer 15

The volume of water in the channel that does the most work.

Work is measured by the amount of sediment that is moved.

Question 16

9. DOMINANT DISCHARGE

Answer 16

The volume of water in the channel that does the most work.

Work is measured by the amount of sediment that is moved.

This is often bankfull discharge.

Question 17

11. FLOOD

Answer 17

A flood is an even when there is more discharge than the channel can accommodate.

(this extra water leaves the channel and goes onto the flood plain; assuming there is one.)

Question 18

12. FLOODPLAIN

Answer 18

A flat, low-lying area along a stream channel. The area of land next to the river channel used to store excess water (temporarily) and sediment (longterm)

Question 19

14. How often do rivers flood?

Answer 19

Rarely.

Question 20

15. Which river, braided or meandering is most likely to produce a catastrophic flood?

Answer 20

Both rivers are capable of creating a catastrophic flooding event.

While a flood in a braided river may be more violent (faster water), it will be shorter in duration.

A flood in a meandering system may be less violent (slower water) but last longer.

Also, a flood in a braided system will arrive sooner, whereas in a meandering system it may arrive later after the precipitation event.

Question 21

16. Flooding in a braided system (see photo)

Answer 21

The excess water of a flood will enter the channel very soon after the rainfall event.

Discharge will increase quickly and reach a maximum value quickly

Just as quickly, discharge will drop as this excess water passes through the river.

Potentially a lot of sediment will be moved.

Braided rivers usually lack a flood plain so water and sediment continue to move during a flood.

Question 22

16. Flooding in a braided system (see photo)

Answer 22

The excess water of a flood will enter the channel very soon after the rainfall event.

Discharge will increase quickly and reach a maximum value quickly

Just as quickly, discharge will drop as this excess water passes through the river.

Potentially a lot of sediment will be moved.

Braided rivers usually lack a flood plain so water and sediment continue to move during a flood.

Question 23

16. Flooding in a meandering system (see photo)

Answer 23

Excess water from precipitation will enter the system more slowly.

This means that discharge will increase starting later after precipitation, reach a lower maximum value over a longer period of time, and then decrease slow over time.

In a meandering river, less sediment may be removed from the system. More might be deposited or simply moved within the system.

Question 24

18. Explain the effects that saturated ground has on flooding

Answer 24

Where the ground is saturated with water, any extra water will remain on the surface.

As a result it will travel quicker into the river contributing to a more rapid increase in discharge.

Question 25

19. Hydrograph

Answer 25

A hydrograph is a chart that compares discharge (maximum Q) over time.

Question 26

21. Urban flood hydrograph PHOTO

Answer 26

An urban setting is less permeable: - There are many hard surfaces that prevent water from infiltrating into the ground. - There is also artificial drainage (i.e. sewers) that will channel water quickly into a river. These both contribute to sudden increases and decreases in discharge.

Question 27

21. Rural flood hydrograph PHOTO

Answer 27

In a rural setting, there are a lot of permeable surfaces (fields, forests etc. ) that allow water to infiltrate into the ground. There is also little or no artificial drainage that will move quickly. There is also lots of vegetation that can intercept and absorb water, slowing water movement over the surface.

Question 28

22. Explain the unique circumstances surrounding the behaviour of a north flowing river in the Northern Hemisphere when it floods. Give an example.

Answer 28

In Canada, many rivers flow north. As they flow north the landscape changes; permafrost is encountered. Permafrost makes the ground impermeable. Precipitation is therefore quickly channeled into the river adding to a faster increase in Q. Also, if there is flooding, this excess water will remain on the surface, making the flood worse. During the winter, most rivers in Canada are frozen over. They behave like a pipe in that the water moving in them is confined to a fixed channel site. In spring, North flowing rivers in Canada thaw first in the south. Here the extra water from snow melt eventually flows into a channel to the North that is still sealed with ice. Here water velocity increases. At the same time, water pressure under the ice increases eventually causing the ice to break, sometimes suddenly and violently.

Question 29

22. Explain the unique circumstances surrounding the behaviour of a north flowing river in the Northern Hemisphere when it floods. Give an example.

Answer 29

In Canada, many rivers flow north. As they flow north the landscape changes; permafrost is encountered. Permafrost makes the ground impermeable. Precipitation is therefore quickly channeled into the river adding to a faster increase in Q. Also, if there is flooding, this excess water will remain on the surface, making the flood worse. During the winter, most rivers in Canada are frozen over. They behave like a pipe in that the water moving in them is confined to a fixed channel site. In spring, North flowing rivers in Canada thaw first in the south. Here the extra water from snow melt eventually flows into a channel to the North that is still sealed with ice. Here water velocity increases. At the same time, water pressure under the ice increases eventually causing the ice to break, sometimes suddenly and violently.

Question 30

17. Explain the effects that saturated ground has on flooding.

Answer 30

Permafrost/frozen ground makes the ground impermeable. Precipitation is therefore quickly channeled into the river adding to a faster increase in Q. If there is flooding, this excess water will remain on the surface, making the flood worse.