Fluvial Geomorphology 3

Question 1

What is the difference between a channel and a valley?

Answer 1

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

Question 2

What is bedrock vs alluvium?

Answer 2

bedrock= in-situ material
alluvium= reworked river sediments

Question 3

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

What does planview mean?

Answer 3

sinuosity
width of floodplain

planview= birds eye view

Question 4

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

Answer 4

decrease

Question 5

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

Answer 5

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

Question 6

T/F

the surface water slope = valley slope

Answer 6

false
they are not equal

Question 7

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

Answer 7

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

Question 8

the width: depth ratio is approx equivalent to

Answer 8

the hydraulic radius
~wetted perimeter

Question 9

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

___ are easier to entrain

Answer 9

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

sands are easier to erode

Question 10

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

Answer 10

more

Question 11

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

Answer 11

high

upstream

Question 12

Why is the floodplain an important property of channels?

Answer 12

It confines the channel!
bedrock vs wide valleys

Question 13

What are the 4 types of channels?

Answer 13

1. straight
2. meandering
3. braided
4. anastomosing

Question 14

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

Answer 14

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

Question 15

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

Answer 15

• single channel
• highest sinuosity
• moderate confinement

Question 16

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

Answer 16

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

Question 17

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

Answer 17

• multiple channels
• moderate sinuosity
• high confinement

Question 18

Where might an anastomosing river be found?

Answer 18

at a river delta, where there is a low slope

Question 19

Why are there lots of braided rivers in the mountains?

Answer 19

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

Question 20

What type of channel is the north sask river?

Answer 20

meandering

Question 21

How does slope (gradient) influence sinuosity?

Answer 21

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

intermediate slope= high sinuosity

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

Question 22

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

Answer 22

decreases!

Question 23

steep slope= __ (high/low) discharge

Answer 23

high

and low slope= low discharge

Question 24

T/F

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

Answer 24

true!

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

Question 25

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?

Answer 25

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

Question 26

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

Answer 26

narrow deep high velocity

Question 27

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

Answer 27

shallow wide decreased velocity

Question 28

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

Answer 28

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

Question 29

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

Answer 29

more increase widens

Question 30

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

Answer 30

less decreasing narrows and deepens

Question 31

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

Answer 31

narrow and deep

Question 32

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

Answer 32

wide and shallow

Question 34

explain the w and d of a braided channel

Answer 34

wide and shallow

Question 35

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

Answer 35

cohesive fine sediment banks or flow through confinded bedrock

Question 36

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

Answer 36

downcut less colluvium

Question 37

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

Answer 37

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

Question 38

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

Answer 38

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

Question 39

Meadnering landscape features cut bank=

Answer 39

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

Question 40

Meadnering landscape features point bar=

Answer 40

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

Question 41

Meadnering landscape features Scroll bar=

Answer 41

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

Question 42

Meadnering landscape features ox bow=

Answer 42

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

Question 43

Meandering landscape features flood plane=

Answer 43

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

Question 44

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

Answer 44

sinuosity

Question 45

t/f meanders are inherit channel evolution of turbulent waters

Answer 45

true

Question 46

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

Answer 46

false fairly poorly preserved

Question 47

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

Answer 47

- 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

Question 48

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

Answer 48

braid bars

Question 49

Confluence scours in braided rivers=

Answer 49

where channel threads join together, producing erosional relief that may be considerably deeper than average channel depth. immediately downstream of braid bar= very deep

Question 50

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

Answer 50

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

Question 51

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

Answer 51

- 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!)

Question 52

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

Answer 52

levees wetlands (or lakes)

Question 53

avulsion=

Answer 53

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

Question 54

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

Answer 54

avulsions flooding breaches over levees = new channels can form

Question 55

T/F Levees are found on braided and anastomosing rivers

Answer 55

false only on anastomosing rivers, not braided rivers

Question 56

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

Answer 56

levees drier trees

Question 57

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

Answer 57

flooding high water table peat

Question 58

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

Answer 58

also at flood stage!

Question 59

describe crevasse splays in anastomosing rivers

Answer 59

=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

Question 60

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

Answer 60

wetland peat dunes

Question 61

How might a crevasse splay be "reactivated"?

Answer 61

If an avulsing channel erodes it

Question 62

Describe the sedimentary record associated with anastomosing rivers

Answer 62

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