7: Fluvial Systems Flashcards
What does Fluvial, Drainage basin, and Gradient mean?
Fluvial: process associated with streams and rivers
Drainage basin (or watershed): The movement of fluid from a higher elevation to a lower elevation through a converging network of rivers
Gradient: Elevation over distance
how do you calculate gradient?
It’s really just slope, but we usually find it in:
- m/km
Easy to visualize how many m you up up/down per km as opposed to a decimal
What is Velocity, Area and Discharge?
Velocity: The time it takes a particle of water to travel a certain distance
- Depending on position in the stream (Highest in surface in center of channel, lowest on bottom of channel, think 215!)
Area: Depth x width
Discharge: The amount of water flowing in a stream at a particular location at an instant in time
- Q = A x V
As the amount of water in a stream changes, stream velocity and / or cross-sectional area adjusts to maintain a balance!
How does flooding relate to discharge?
Flooding occurs when the discharge of the stream becomes too high to be accommodated in the normal stream channel
Resulting in the stream widening its channel and flooding the low-lying areas around the stream
How are sediments eroded?
They’re eroded through:
- dissolution
- entrainment
- abrasion
What are the 3 main ways sediment is transported by water? What is the total amount of sediment a stream carries called?
Three main ways:
- bed load: rolling, sliding, of saltation (jumping) off bed-loaded particles
- suspended load
- dissolved load transport
Total amount of sediment = load
What are the 3 main ways sediment is transported by water? What is the total amount of sediment a stream carries called?
Three main ways:
- bed load: rolling, sliding, of saltation (jumping) off bed-loaded particles
- suspended load
- dissolved load transport
Total amount of sediment = load
How do you tell what type of river channel a stream will evolve into?
Rivers erode the valley in which they flow over long and periods of time. Depending on parameters such as channel gradient, base level, discharge, and sediment, the river will evolve into a braided stream or a meandering stream.
We know when sediment is eroded and picked up, but when is sediment deposited?
Sediment is deposited when the energy level (proportional to the stream’s velocity) of the stream devreases.
The channel type reflects the energy in the system
What is channel form determined by?
- channel gradient
- type of material that the channel is incising (cutting)
- type and amount of sediment that is being transported
The terrain through which rivers flow have a significant impact on the type and amount of sediment being transported
What are the 2 types of channels?
Braided streams
- streams that flow in broad, shallow channels and consist of multiple subchannels which are separated by islands or bars
Meandering streams
- follow a sinuous course
- channels tend to be narrow and deep
- degree of meandering is given by the sinuosity = ratio of channel length to valley length
When do streams follow a braided pattern or a meandering pattern?
Braided pattern
- forms when the stream is unable to transport all the available bed load consistently
- a very dynamic system where the discharge varies
- Commonly form in regions where the elevation loss is high, this leads to high sediment supply
- Coarse sediments are transported and deposited as bars, locally dividing the channel into two or more channels (braids)
Meandering Patterns
- Common in areas where elevation change i slow
- Sediment supply is low (large amount of cohesive sediments)
- Primary transport mechanism is suspended load
- There is no room left for downward erosion
What does a high velocity and low velocity mean in a meandering stream?
High velocities: cause band erosion as it’s quickly crashing into the wall
Low velocities: mean the energy level is low enough for the sediment to get desposited, forms a point bar.
This digging into the bank as well as deposition can cause a meandering stream to burrow straight through and make an oxbow lake (n shaped part of the stream closed off)
What happens when part of a meandering stream is cut off?
The gradient is increased locally and can lead to development of a braided stream/path
What are Deltas and Alluvial Fans?
Alluvial Fans:
Formed when streams enter a flat valley floor, causing a loss of energy and sediment deposition
Deltas:
Formed when streams enter a body of standing water such as a lake or an ocean
Sediment deposition occurs because the stream current slows, dissipates, and loses energy.
