Drainage Basin Networks and Sources to Sink

Question 1

How does topography determine drainage direction? Give an example

Answer 1

Mountainous regions act as barriers and slopes that guides the flow of water within them. In the USA three rivers are guided by the Rocky mountains, Appalachian Mountains and post-glacier landscape to flow south in to the Mississippi river

Question 2

What is discharge?

Answer 2

The measure of flow rate within rivers. The amount of water that passes by time unit thorugh a section of river.

Question 3

What unit is used to measure discharge and how is it calculated?

Answer 3

Cumecs (m^3S^-1) = Area of River (Height x Width) x velocity (ms^-1).

Question 4

What is bankful discharge?

Answer 4

the maximum volume that a channel can hols without flooding

Question 5

What are the three determinants of river velocity?

Answer 5

Slope, Depth, Roughness

Question 6

How do you calculate river velocity using its determinants?

Answer 6

Roughness x square root of (depth x slope)

Question 7

What is river capture?

Answer 7

When the headward erosion of a river leads in to another channel. It captures that river within it.

Question 8

What is an example of river capture?

Answer 8

Indus River used to flow through the Ganges but is now flowing through the Indus Fan.

Question 9

What is river splitting?

Answer 9

This is when a flat plane develops in to two drainage basins following changes of the previously uniform rainfall pattern

Question 10

Explain how river splitting happens

Answer 10

Initially there is an area which experiences uniform rainfall and tectonic uplift. Changes to the uniform rainfall over an area cause part of a plane to receive more than the other. This means that the same part will experience greater denudation. This will cause the area to rise at different rates creating a ridge of which either side there will be different sloped basins.

Question 11

What is the most dominant and everyday determinant of changes within drainage basins?

Answer 11

Hydrological Cycle

Question 12

What does the Hjulstrom curve represent?

Answer 12

The different flow velocities required to erode, transport and deposit different sized sediments along a logarithmic graph.

Question 13

Explain clay within the Hjulstrom curve?

Answer 13

It initially requires a high velocity to be eroded due to it being so compact and coagulated. However, once release it is very fine and so forms part of the channel and has a very very low deposition velocity so much so that it cannot be represented on the curve

Question 14

What is transport capacity?

Answer 14

Total amount of sediment that a body of flowing water can hold.

Question 15

What is the transport rate?

Answer 15

The velocity at which the flow of water can carry the maximum transport capacity

Question 16

What is the sediment delivery ratio?

Answer 16

the fraction of sediment eroded from the slope that reaches the drainage basin outlet

Question 17

What tends to happen to sediment delivery ratio as you move downstream and why?

Answer 17

Sediment delivery ratio decreases downstream because there are more places within the channel that can trap and store sediment.

Question 18

What are some of the parts of the channel that can trap and store sediment?

Answer 18

Floodplains, Braided Channels, Deltas, Meander Bars

Question 19

How do you calculate sediment delivery ratio?

Answer 19

Sediment Output from Basin / Sediment eroded from slope

Question 20

What is the sediment to sink concept?

Answer 20

The notion that as water and sediment travels downstream it goes through a ‘jerky conveyor belt’ system. This means that it gets stopped at different point within the system for different durations until it finally finds its way to the mouth. For example, in braided rivers it is only stored for a short period whereas in floodplains it can be stored for ever possibly.

Question 21

What is Sediment Accommodation Space?

Answer 21

The space available to store sediment.

Question 22

How does sediment accommodation space vary across the long profile and why?

Answer 22

Upstream there is very little because it is so steep and volatile. Downstream there is a greater accumulation of sediment and water that can carve space.

Question 23

How can you increase sediment accommodation space upstream artificially?

Answer 23

Dams

Question 24

What is the main site of study for sediment accommodation space?

Answer 24

Downstream

Question 25

What are the two determinants of sediment accommodation space downstream?

Answer 25

Sea Level Rise

Tectonic Uplift

Question 26

How does sea level rise affect sediment accommodation space?

Answer 26

If sea level rises then there is more space available for the sediment to be stored underneath the surface on a continental shelf

Question 27

How does tectonic uplift affect sediment accommodation space?

Answer 27

In the event of uplift this causes the elevation of the system to rise which increases potential energy increasing erosion which forces downstream to create more space so it can still function

Question 28

What is the Lane Diagram?

Answer 28

A representation of how changes in the slope of the stream and sediment size and the balance between them can cause either degradation and aggradation

Question 29

What is the proportional relationship between Qs (sediment discharge) and D (Sediment Diameter) with Q (water discharge) and S (slope) relating to the Lane Diagram?

Answer 29

Qs x D proportional to Q x S

Question 30

What are the controls on river long profile?

Answer 30

Discharge: Increases downstream
Sediment Load: Increases downstream but at reduced rate due to increased sediment storage
Sediment Size: Decreases in Size

Question 31

How do rarely occurring straight rivers create meanders?

Answer 31

They deposit material on alternating side of the channel in bars creating an oscillating pattern. this diverts flow to channel sides and increases erosion there leading to a meander

Question 32

What impact do tropical forests have on the shape of meanders?

Answer 32

Vegetation stabilises flows and causes the winding to bunch up and become more dense in a given area.

Question 33

How do braided rivers form?

Answer 33

When a channel receives large amount of sediment that it covers up the channel vegetation reducing its depth. Small changes in this huge constant supply cause the channel to produce multiple mini channels.

Question 34

What sort of river systems are produced in post-glacial landscapes?

Answer 34

Similar to braided rivers due to the lack of vegetation

Question 35

What are the controls on channel patterns?

Answer 35

Slope - This affects the rate of vertical erosion braider rivers rely upon it to erode and transport the massive amounts of material it receives
Bank Strength - If the banks are strong then this limits lateral erosion and therefore lateral sediment supply.

Question 36

What are determinants on the bank strength of channels?

Answer 36

Composition - It may be composed of clay which is compact and difficult to erode
Vegetation - This can act as a binder increasing strength

Question 37

How does long profile aggradation happen?

Answer 37

If aggradation occurs within the channel then this also occurs within the floodplain as it overflows during flooding events where it is then deposited.

Question 38

How can long profile aggradation downstream lead to channel migration?

Answer 38

The aggradation causes overflowing of the channel and its sediment which creates an alluvial ridge (levee if you will). When the channel floods, some of the water cannot overcome the alluvial ridge and so is forced to form a new channel somewhere else through incision in the floodplain of the original river.

Question 39

What is avulsion?

Answer 39

Overflowing of a channel’s sediment on to its floodplain that can create an alluvial ridge

Question 40

Give an example of where a river has migrated due to avulsion?

Answer 40

Kosi Fan since 1736