Lecture 11: More on fluvial systems Flashcards
(16 cards)
Why are streams often not straight?
Straight channels are relatively rare in
nature.
Small variations in channel depth cause
the main channel to meander back and
forth
Thalweg - Deepest part of the channel
We often find a regular sequence of
shallow riffles and deeper pools in stream
channels
Riffle and pool sequences in straight
channels tend to evolve into meanders
What are the characteristics of a meandering stream?
Meandering streams have numerous bends or curves, known as meanders.
They commonly form in low-gradient landscapes where the slope is gentle.
These streams typically have a single channel with high sinuosity (curviness) and high discharge relative to sediment load.
Describe erosion and deposition in a meandering stream
In a meandering stream, erosion occurs on the outer curve (cut bank), where flow velocity is faster.
Deposition happens on the inner curve (point bar), where the velocity slows and sediments settle.
This combination of erosion and deposition causes meanders to expand and migrate downstream over time.
What are some common features of a meandering stream?
Meandering streams often have features like point bars, cut banks, oxbow lakes, floodplains, crevasse splays, and swamps.
How do Oxbow lakes form?
As meanders become more pronounced, the curves may eventually cut off, leaving behind a crescent-shaped oxbow lake. This occurs when the stream cuts a new, straighter path during a high-flow event, abandoning the older meander loop.
What are the characteristics of braided rivers?
Braided rivers contain multiple, actively shifting and converging channels separated by narrow sand and gravel bars or islands. These rivers form in areas with a high sediment load and where bedload sediment transport dominates.
Where Do Braided Rivers Form?
Braided rivers develop in regions with high sediment loads and in places where sediment is easily eroded.
Common environments include areas of intense erosion or glacial outwash regions, which are rich in sediment. These rivers often form in areas well above the graded stream profile elevation.
What are the features of a braided river?
Sinuosity:
Low (braided rivers tend to have a more direct path than meandering rivers).
Gradient:
High (the slope is steeper than in meandering rivers).
Discharge:
Low relative to sediment load (the river’s discharge is not sufficient to carry the high sediment load).
Longitudinal bars: These bars run parallel to the flow direction and form along the main channel.
Transverse bars: These bars are oriented perpendicular to the river’s flow and often divide the river into multiple channels.
What is alluvium?
Alluvium – solid particles dropped from
suspension in water
What are the three ways sediment load is transported?
- Dissolved Load
- Suspended Load
- Bed Load (Traction Load)
Describe suspended load
Formation of eddies in turbulent water and fast flow velocities keeps fine particles in suspension
Describe bed load transport
Bed load:
Traction transport:
-Rolling
-Sliding
-Impact & Creep
-Sometimes briefly lifted by turbulent
eddies (saltation)
What is the difference between capacity and competence in a stream?
Capacity:
The total amount of particles of all sizes that a stream can carry.
Competence:
The largest particle that can be moved.
Flowing water exerts force on the
stream bed
Increasing shear stress will cause
particles to move
Largest particle will dictate what
forces or velocity of water that is
acting on bed load
What is the base level of a stream?
The limiting level below which a stream
cannot erode the land is the base level of
the stream.
What is a graded stream?
A graded stream is a stream in equilibrium.
Any change in base level requires that the stream readjust its graded profile.
Any change in the headwaters of a stream (such as tectonic uplift) will require the stream to readjust to regain a graded
profile.
Natural rivers can only approach true equilibrium.
What is an aggrading stream vs a Degrading stream?
Aggrading streams – too much load,
so deposition will occur
Degrading (incising) streams
– small sediment load, so erosion will occur
