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EAS 225 (Earth Surface/ Landforms) > Fluvial Geomorphology 3 > Flashcards

Fluvial Geomorphology 3 Flashcards

1
Q

What is the difference between a channel and a valley?

A

channel= the part of the valley that fills with water regularly (so it’s actively incising)
valley= the entire width: flood plain + alluvium reworked over time by lateral migration of channel

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2
Q

What is bedrock vs alluvium?

A

bedrock= in-situ material
alluvium= reworked river sediments

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3
Q

Planview allows you to see the ____ and ___ of the floodplain

What does planview mean?

A

sinuosity
width of floodplain

planview= birds eye view

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4
Q

River tend to change (eg move outward or incise) to ___ energy

A

decrease

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5
Q

What are some fundamental properties of a channel?
Brainstorm 3-5

A
  • slope (of landscape and channel)
  • width: depth ratio (hydraulic radius)
  • sinuosity
  • depositional vs erosional hot spots
  • # of channels
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6
Q

T/F

the surface water slope = valley slope

A

false
they are not equal

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7
Q

Do single or mutiple-channel systems have higher sediment load?

A

multi-channel systems have higher sediment load b/c they have so much to carry that they have to spread out

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8
Q

the width: depth ratio is approx equivalent to

A

the hydraulic radius
~wetted perimeter

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9
Q

Typically, clays and boulders are ___ to erode. Why?

___ are easier to entrain

A

difficult
b/c clays= cohesive, and boulders= heavy, so they’re both hard to entrain

sands are easier to erode

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10
Q

vegetation leads to ___ cohesion
(more/less) in a bank

A

more

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11
Q

____ sediment load = multiple branches. This reflects ____ bank composition

A

high

upstream

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12
Q

Why is the floodplain an important property of channels?

A

It confines the channel!
bedrock vs wide valleys

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13
Q

What are the 4 types of channels?

A
  1. straight
  2. meandering
  3. braided
  4. anastomosing
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14
Q

Define the following for straight channels
- # of channels
- degree of sinuosity
- channel confinement

A
  • single channel
  • low sinuosity
  • highly confined (by bedrock or clay banks)
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15
Q

Define the following for meandering channels
- # of channels
- degree of sinuosity
- channel confinement

A
  • single channel
  • highest sinuosity
  • moderate confinement
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16
Q

Define the following for braided channels
- # of channels
- degree of sinuosity
- channel confinement

A
  • multiple channels
  • low sinuosity
  • very low confinement (wide and shallow)
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17
Q

Define the following for anastomosing channels
- # of channels
- degree of sinuosity
- channel confinement

A
  • multiple channels
  • moderate sinuosity
  • high confinement
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18
Q

Where might an anastomosing river be found?

A

at a river delta, where there is a low slope

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19
Q

Why are there lots of braided rivers in the mountains?

A

because close to the headwaters in the mountains, so there are still lots of big sediments entrained= the river needs to widen out

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20
Q

What type of channel is the north sask river?

A

meandering

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21
Q

How does slope (gradient) influence sinuosity?

A

very low slope= no sinuosity (eg straight/ anastomosing channels- deltas)

intermediate slope= high sinuosity

steep slope= low/ no sinuosity (eg braided rivers at headwaters)

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22
Q

having high sinuosity ___(inc/dec) the energy of the channel

A

decreases!

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23
Q

steep slope= __ (high/low) discharge

A

high

and low slope= low discharge

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24
Q

T/F

width, depth, and velocity are all proportional to Q (discharge)

A

true!

Q=w d v, so it’s a power law relationship

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25
Q

Hydraulic geometry showed that Alberta rivers have the following constants compared to a midwest river:
- high width
- low depth
- lower velocity

Why would this be?

A

Alberta rivers have higher sediment loads of sands and other coarser materials, so they need to be wider to carry it. This is because they’re closer to (or in) the mountains, so the banks are made up of coarser materials

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26
Q

what would happen to the width, depth, and velocity of a river with more fine sediment?

A

narrow
deep
high velocity

27
Q

what would happen to the width, depth, and velocity of a river with less fine sediment?

A

shallow
wide
decreased velocity

28
Q

to engineer a channel that erodes less, you would want to add ___ (finer or coarser) material
why?

A

coarser= less erosion

b/c when you have less fine sediment, the channel is more shallow, wider, and has lower velocity

29
Q

coarser banks= ___ erodible = ___ in w:d ratio= channel ___ (widens or narrows)

A

more
increase
widens

30
Q

finer banks= __ erodible= ___ w:d ratio= channel ___ (widens or narrows)and ____ (shallows or deepends)

A

less
decreasing
narrows and deepens

31
Q

explain the w and d of a straight channel with abundant vegetation

A

narrow and deep

32
Q

explain the w and d of an anastomosing channel with abundant vegetation

A

wide and shallow

33
Q
A
34
Q

explain the w and d of a braided channel

A

wide and shallow

35
Q

What kind of banks might a straight river have (2 types)

A

cohesive fine sediment banks
or
flow through confinded bedrock

36
Q

Because straight rivers are confined, they mainly ____ and are __ preserved in the sedimentary record (unless the channel is filled with ___- mass wasting sediments)

A

downcut
less
colluvium

37
Q

____ channels have a single channel with a single thalweg that migrates from cut banks to cut bank

What is a thalweg?

A

meandering

thalweg= zone of highest velocity in the river (hugs the cut bank)

38
Q

meadnering rivers form at __ slopes and have what kind of sediment load?

A

moderate slopes
mixed load of sediment (sand, gravel, up to cobbles)

39
Q

Meadnering landscape features

cut bank=

A

part that’s actively eroding (outside bank of a meander)

40
Q

Meadnering landscape features

point bar=

A

part that’s actively depositing (inside bank of a meander)

41
Q

Meadnering landscape features
Scroll bar=

A

previous depositional boundaries from high flood events (old high stands of water)

42
Q

Meadnering landscape features

ox bow=

A

a meander that has become seperated from the flow of water
- form through pinching of cut bank + channel ovulsion

43
Q

Meandering landscape features
flood plane=

A

entire reigon at low elevation: will fill with water during a flood, accumulates fine sediment

44
Q

high ____ decreases the work done by lengthening the channel relative to the channel length

A

sinuosity

45
Q

t/f
meanders are inherit channel evolution of turbulent waters

A

true

46
Q

t/f
meandering river deposits are well preserved in the sedimentary record

A

false
fairly poorly preserved

47
Q

Give some characteristics of bradided rivers
- # channels
- w and d
- gradients
- bank stability
- grain size

A
  • multiple channels
  • very wide (can be many km!)
  • shallow
  • form at steep gradients
  • unstable banks + rapidly shifting channels b/c the banks are highly erodible
  • cobbles, boulders, sand, minor silt= coarsest sediment load of the river types because at headwaters= sediment hasn’t broken down yet
48
Q

Braiding is caused by the interaction b/w flow and coarse sediments which form ___ ____ in channels

A

braid bars

49
Q

Confluence scours in braided rivers=

A

where channel threads join together, producing erosional relief that may be considerably deeper than average channel depth.

immediately downstream of braid bar= very deep

50
Q

How can you tell anastomosing rivers from braided rivers in planview?

A

anastomosing rivers= no braid bars, and lots of vegetation (surrounded by veg and wetlands!). Also presence of levees

51
Q

Explain the following characteristics of anastomosing rivers
- # channels
- bank stability and cohesion
- grain size
- w and d
- gradient formed at

A
  • multiple channels
  • stable and cohesive banks and channels
  • high proportion of fines (clays and silts)– high sed load!
  • narrow and deep
  • form at very low gradients (<20cm/km!)
52
Q

Around anastomosing rivers, there are channel ___sounded by ___, which are at a lower stage than the river itself! (very prone to flooding)

A

levees
wetlands (or lakes)

53
Q

avulsion=

A

the rapid abandonment of a river channel (cut off by erosion) and the formation of a new river channel

54
Q

Anastomosing rivers migrate through channel ____. They tend to be aggrading and experience ___ often. Flooding is key to avulsions and new channel formation. How?

A

avulsions

flooding

breaches over levees = new channels can form

55
Q

T/F
Levees are found on braided and anastomosing rivers

A

false
only on anastomosing rivers, not braided rivers

56
Q

Anastomosing channels are bordered by ___, which stand above the flood plain level. These are ___ environments that support vegetation like __

A

levees
drier
trees

57
Q

The anastomosing stream flood plain is subject to ___ and a __(high or low) water table. Organic wetlands will accumulate ___

A

flooding

high water table

peat

58
Q

Levees are breached at flood stage, but when are they built?

A

also at flood stage!

59
Q

describe crevasse splays in anastomosing rivers

A

=sedimentary fluvial deposit which forms when a stream breaks its natural or artificial levees and deposits sediment on a floodplain

  • crevasse may heal with time or develop into an avulsion
60
Q

Abandoned channels will be covered by ___ ___ as the floodplain aggrades. ____ might form on top of sandy bars left behind

A

wetland peat

dunes

61
Q

How might a crevasse splay be “reactivated”?

A

If an avulsing channel erodes it

62
Q

Describe the sedimentary record associated with anastomosing rivers

A

they’re well preserved in the sedimentary record because they build up over time

63
Q
A

EAS 225 (Earth Surface/ Landforms) (13 decks)

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