Fluvial Geomorphology 3 Flashcards
What is the difference between a channel and a valley?
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
What is bedrock vs alluvium?
bedrock= in-situ material
alluvium= reworked river sediments
Planview allows you to see the ____ and ___ of the floodplain
What does planview mean?
sinuosity
width of floodplain
planview= birds eye view
River tend to change (eg move outward or incise) to ___ energy
decrease
What are some fundamental properties of a channel?
Brainstorm 3-5
- slope (of landscape and channel)
- width: depth ratio (hydraulic radius)
- sinuosity
- depositional vs erosional hot spots
- # of channels
T/F
the surface water slope = valley slope
false
they are not equal
Do single or mutiple-channel systems have higher sediment load?
multi-channel systems have higher sediment load b/c they have so much to carry that they have to spread out
the width: depth ratio is approx equivalent to
the hydraulic radius
~wetted perimeter
Typically, clays and boulders are ___ to erode. Why?
___ are easier to entrain
difficult
b/c clays= cohesive, and boulders= heavy, so they’re both hard to entrain
sands are easier to erode
vegetation leads to ___ cohesion
(more/less) in a bank
more
____ sediment load = multiple branches. This reflects ____ bank composition
high
upstream
Why is the floodplain an important property of channels?
It confines the channel!
bedrock vs wide valleys
What are the 4 types of channels?
- straight
- meandering
- braided
- anastomosing
Define the following for straight channels
- # of channels
- degree of sinuosity
- channel confinement
- single channel
- low sinuosity
- highly confined (by bedrock or clay banks)
Define the following for meandering channels
- # of channels
- degree of sinuosity
- channel confinement
- single channel
- highest sinuosity
- moderate confinement
Define the following for braided channels
- # of channels
- degree of sinuosity
- channel confinement
- multiple channels
- low sinuosity
- very low confinement (wide and shallow)
Define the following for anastomosing channels
- # of channels
- degree of sinuosity
- channel confinement
- multiple channels
- moderate sinuosity
- high confinement
Where might an anastomosing river be found?
at a river delta, where there is a low slope
Why are there lots of braided rivers in the mountains?
because close to the headwaters in the mountains, so there are still lots of big sediments entrained= the river needs to widen out
What type of channel is the north sask river?
meandering
How does slope (gradient) influence sinuosity?
very low slope= no sinuosity (eg straight/ anastomosing channels- deltas)
intermediate slope= high sinuosity
steep slope= low/ no sinuosity (eg braided rivers at headwaters)
having high sinuosity ___(inc/dec) the energy of the channel
decreases!
steep slope= __ (high/low) discharge
high
and low slope= low discharge
T/F
width, depth, and velocity are all proportional to Q (discharge)
true!
Q=w d v, so it’s a power law relationship
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?
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
what would happen to the width, depth, and velocity of a river with more fine sediment?
narrow
deep
high velocity
what would happen to the width, depth, and velocity of a river with less fine sediment?
shallow
wide
decreased velocity
to engineer a channel that erodes less, you would want to add ___ (finer or coarser) material
why?
coarser= less erosion
b/c when you have less fine sediment, the channel is more shallow, wider, and has lower velocity
coarser banks= ___ erodible = ___ in w:d ratio= channel ___ (widens or narrows)
more
increase
widens
finer banks= __ erodible= ___ w:d ratio= channel ___ (widens or narrows)and ____ (shallows or deepends)
less
decreasing
narrows and deepens
explain the w and d of a straight channel with abundant vegetation
narrow and deep
explain the w and d of an anastomosing channel with abundant vegetation
wide and shallow
explain the w and d of a braided channel
wide and shallow
What kind of banks might a straight river have (2 types)
cohesive fine sediment banks
or
flow through confinded bedrock
Because straight rivers are confined, they mainly ____ and are __ preserved in the sedimentary record (unless the channel is filled with ___- mass wasting sediments)
downcut
less
colluvium
____ channels have a single channel with a single thalweg that migrates from cut banks to cut bank
What is a thalweg?
meandering
thalweg= zone of highest velocity in the river (hugs the cut bank)
meadnering rivers form at __ slopes and have what kind of sediment load?
moderate slopes
mixed load of sediment (sand, gravel, up to cobbles)
Meadnering landscape features
cut bank=
part that’s actively eroding (outside bank of a meander)
Meadnering landscape features
point bar=
part that’s actively depositing (inside bank of a meander)
Meadnering landscape features
Scroll bar=
previous depositional boundaries from high flood events (old high stands of water)
Meadnering landscape features
ox bow=
a meander that has become seperated from the flow of water
- form through pinching of cut bank + channel ovulsion
Meandering landscape features
flood plane=
entire reigon at low elevation: will fill with water during a flood, accumulates fine sediment
high ____ decreases the work done by lengthening the channel relative to the channel length
sinuosity
t/f
meanders are inherit channel evolution of turbulent waters
true
t/f
meandering river deposits are well preserved in the sedimentary record
false
fairly poorly preserved
Give some characteristics of bradided rivers
- # channels
- w and d
- gradients
- bank stability
- grain size
- 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
Braiding is caused by the interaction b/w flow and coarse sediments which form ___ ____ in channels
braid bars
Confluence scours in braided rivers=
where channel threads join together, producing erosional relief that may be considerably deeper than average channel depth.
immediately downstream of braid bar= very deep
How can you tell anastomosing rivers from braided rivers in planview?
anastomosing rivers= no braid bars, and lots of vegetation (surrounded by veg and wetlands!). Also presence of levees
Explain the following characteristics of anastomosing rivers
- # channels
- bank stability and cohesion
- grain size
- w and d
- gradient formed at
- 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!)
Around anastomosing rivers, there are channel ___sounded by ___, which are at a lower stage than the river itself! (very prone to flooding)
levees
wetlands (or lakes)
avulsion=
the rapid abandonment of a river channel (cut off by erosion) and the formation of a new river channel
Anastomosing rivers migrate through channel ____. They tend to be aggrading and experience ___ often. Flooding is key to avulsions and new channel formation. How?
avulsions
flooding
breaches over levees = new channels can form
T/F
Levees are found on braided and anastomosing rivers
false
only on anastomosing rivers, not braided rivers
Anastomosing channels are bordered by ___, which stand above the flood plain level. These are ___ environments that support vegetation like __
levees
drier
trees
The anastomosing stream flood plain is subject to ___ and a __(high or low) water table. Organic wetlands will accumulate ___
flooding
high water table
peat
Levees are breached at flood stage, but when are they built?
also at flood stage!
describe crevasse splays in anastomosing rivers
=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
Abandoned channels will be covered by ___ ___ as the floodplain aggrades. ____ might form on top of sandy bars left behind
wetland peat
dunes
How might a crevasse splay be “reactivated”?
If an avulsing channel erodes it
Describe the sedimentary record associated with anastomosing rivers
they’re well preserved in the sedimentary record because they build up over time
