Mountains Lecture 5- Deciphering the fingerprints: how erosional processes shape mountain range form

Question 1

What is a hillslope?

Answer 1

‘A soil surface that lies at an angle to the horizontal’
‘Side or slope of a hill’
‘Surface which connects a ridgeline to it’s nearest stream’
‘Surface which delivers water and sediment to the fluvial network’

Question 2

What is a channel?

Answer 2

‘Where flow is concentrated’
‘A landform where water and/or sediment converges and moves downslope or through a valley’
‘Where a narrow body of water is situated’

Question 3

Name 6 ways moutain landscape morphology can be investigated.

Answer 3

Historical maps and data
Topographic maps
Satellite imagery and data
DEM
Surveying
Field data collection

Question 4

What is one way to study sediment routing systems?

Answer 4

Model landscapes- ‘mountain in a box’
At set time intervals, telemetric lasers will measure the changing elevation of the sand in the box.
E.g. Badlands in Southern Spain has been modelled.

Question 5

What can model experiments show us?

Answer 5

The effects of varying different parameters.
For example, increase rock uplift rates lead to steeper, higher and rougher topography.

Question 6

What can we divide mountainous landscapes into?

Answer 6

Channels and hillslopes.

Question 7

Describe hillslopes.

Answer 7

Have low upslope contributing areas and a range of gradients

Question 8

Describe channels

Answer 8

Start at a given contributing area and their gradients decrease downstream

Question 9

What happens to drainage areas as you move through a valley?

Answer 9

It gets bigger.

Question 10

How can we analyse slope change with distance downstream?

Answer 10

Using longitudinal profiles of a river, which follows a drop of water (or sediment grain) from the drainage dicide to the river mouth.

Question 11

What happens on the hilltop (mountain topography)?

Answer 11

At the headwaters (top of valley, near the ridgeline), the elevations drop rapidly.
The slope of the hillslope is nearly uniform.

Question 12

What happens in the river channel (mountainous landscapes)?

Answer 12

Downstream, the elevation drops more gradually - low and always decreasing slope.

Question 13

Do channels and hillslopes show different patterns in slope?

Answer 13

Yes

Question 14

What happens to channel gradient with increasing drainage area?

Answer 14

Decreases

Question 15

What happens to hillslope gradient with increasing drainage area?

Answer 15

Increases/remains high (moving from ridgeline downslope)

Question 16

What does the sediment routing system describe?

Answer 16

The pathways that sediment travels from the mountain peaks to the river/ocean basin.
The processes of transport vary through a sediment grain’s journey.

Question 17

What processes move sediment change through hillslopes?

Answer 17

1. Sediment production and diffusive sediment transport
2. Landslides

Question 18

What processes move sediment change through channels?

Answer 18

3. Debris flows
4. Fluvial flows

Question 19

What is hillslope evolution controlled by?

Answer 19

Thickness of the soil and the rate at which it is produced by weathering bedrock.
This typically happens at 10s to 100s microns per year.

Question 20

How does sediment production and diffusive transport work?

Answer 20

Rock interacts with water, air and life in the near surface. Physical processes cause rock to fracture and fragment. Chemical processes cause chemical alteration to rock.

Question 21

What happens when soil production is rapid?

Answer 21

The landscape is covered or ‘mantled’ by soil - smooth, round hillslopes.

Question 22

What is the rate of sediment (soil) transport dependent on?

Answer 22

Slope and happens grain-by-grain (diffusive sediment transport).

Question 23

In soil mantles landscapes, what is sediment transport often influenced by?

Answer 23

Biogenic processes.

Question 24

What may be the main cause of sediment transport in some landscapes?

Answer 24

Gophers.

Question 25

What is a landslide?

Answer 25

A landslide is the movements of masses of rock, earth or debris down a slope under the effect of gravity (Cruden & Varnes, 1996)

Question 26

Are there different types of landslides and what do they depend on?

Answer 26

There are different types of landslides depending on the material they move (rock, debris, mud) and the type of failure mechanism (e.g. slide, fall, flow).

Question 27

How do landslides work?

Answer 27

As hillslopes get steeper, loose sediment is transported in discrete shallow landslides rather than grain-by-grain. This produced hillslopes that are much more planar.

Question 28

What are shallow landslides very efficient at?

Answer 28

Removing material from the hillslope - the transport rate (sediment flux) increases very quickly with increasing hillslope gradient.

Question 29

What is the threshold slope called?

Answer 29

The angle of repose.

Question 30

What does shallow landsliding require?

Answer 30

A loose layer of soil at the surface. If the soil and other loose material (regolith) is removed (by landsliding), we are left with a bedrock landscape.

Question 31

What do bedrock hillslopes look like?

Answer 31

May have a thin or discontibuous regolith cover, but not a smooth, rounded landscape.

Question 32

How do bedrock hillslopes evolve?

Answer 32

Via deep-seated/bedrock landslides, which can involve entire hillslopes. Produces planar (no rounded) hillslopes.

Question 33

What do stronger rocks or less landsliding lead to in bedrock hillslopes?

Answer 33

high relief and steeper gradients. E.g. Karakoram Highway in Himalayas.

Question 34

Are soil-mantles, high relief landscapes rare or common in mountain settings?

Answer 34

Rare, hence why we see lots of steep, bedrock mountain peaks.

Question 35

What is threshold slope?

Answer 35

When the slope gets very steep, particles start to slide downslope. This transport gets very fast as the slope approaches a critical value.

Question 36

What are debris flows?

Answer 36

Many steep channels (slopes > 10%) in mountainous regions are dominated by debris flows. Mixtures of sediment and water that behave as a slurry or semisolid mass, rather than as a simple fluid.

Question 37

What happens if sediment cannot be transported within a channel (debris flow)?

Answer 37

The bed builds up and the channel aggrades.

Question 38

What would happen without debris flows?

Answer 38

Sediment would be trapped on hillslopes (at least until the next large landslide) instead of being delivered to the channel network.

Question 39

What are debris flows a way of?

Answer 39

Hillslopes 'handing off' sediment from hillslopes to larger channels. E.g. 2008 Wenchuan earthquake hazard cascade.

Question 40

What are characteristics of debris flows?

Answer 40

Debris flows are a significant hazard for mountain communities due to their ability to rapidly transport large boulder.

Question 41

What are fluvial channels?

Answer 41

Channels which transport sediment and incise bedrock through the action of flowing water. Much more efficient at transporting water than hillslopes (once there is water in the channel).

Question 42

Are sediment transport rates in channels orders of magnitudes greater than those on hillslopes?

Answer 42

Yes

Question 43

What do geomorphic processes in the channel network depend on?

Answer 43

Gradient of the river Distance from the divide/ridgeline Presence/absence of sediment Nature of the sediment

Question 44

What general downstream trends do many mountain rivers show?

Answer 44

1. decreasing slope 2. decreasing exposure of bedrock 3. increasing sediment cover 4. decreasing grain size

Question 45

What is topographic relief?

Answer 45

The difference in elevation between the highest and lowest point of a valley, landscape or mountain range.

Question 46

As we transition from the hillslope to the river channels, how does the relationship between slope and drainage area change?

Answer 46

Move down the hillslope (away from the ridgeline), the slope increases with drainage area. We move into the river channel, where the slope then starts to decrease with increasing drainage area.

Question 47

What is the ‘angle of response’?

Answer 47

The critical hillslope gradient for landsliding to occur.

Question 48

What are debris flows made of?

Answer 48

Sediment and water that behave as a slurry or semi-solid mass.

Question 49

How does grain size of sediment generally change with distance downstream in mountain environments?

Answer 49

Decreases.