Types of Rivers Flashcards
Types of Rivers
BRAIDED
MEANDERING
Braided Channel
- Number of channels:more than 1
- Size of channel:smaller
- Shape of Channel (w/d ratio):wide and shallow
- Discharge:variable
- Sediment Load:coarse, poorly sorted sediment sometimes moving as bed-load
- Gradient:Steep
Meandering
- Number of channels:1
- Size of channel:larger
- Shape of Channel (w/d ratio)wide and Deep
- Discharge:consistent and stable
- Sediment loadfiner, better sorted sediment constantly moving in suspension
- Gradient
shallower, lower
Bedload
Includes sediment that moves by:
SALTATION
TRACTION
Saltation
Sediment bounces along the bottom.
Traction
Sediment rolls or slides along the bottom.
Sediment load in a Braided River
coarse, poorly sorted sediment sometimes moving as bedload.
Sediment load in a Meandering River
finer, better sorted sediment constantly moving in suspension.
Braided Rivers start…
Higher up in the drainage basin
Definition of a braided river
Braided rivers are defined by the presence of multiple channels separated by piles of sediment called channel bars.
Channel bars found in braided rivers
- Temporary piles of sediment that form during brief periods of increased dischargee.
- large quantities of coarse, poorly sorted sediment is eroded, transported and deposited over short distances and times.
- These bars often have a massive structure because deposition was rapid.
- may be within the channel or attached to the channel sides.
PICTURE —> O A
At A, water turbulence increases in the down-current or lee side of the channel bar which causes water velocity to drop
As a result, finer grains are sometimes deposited as a tail on the lee side of the bar.
The bar can also be reworked to some degree as water level drops and during low flow conditions.
Permeability
Measure of how well connected the pores are in sediment
Anastomosing Rivers
Very much like the braided river in that it has multiple channels. However the channel bars that separate flow are more permanent. because they have been stabilized by vegetation growth.
Thalweg
the line that defines where the deepest, fastest moving water is.
Cut bank
Erosion takes place at the outer bank to create a steeper cut bank because here is where the fastest water is.
Secondary flow caused by the cut bank?
There is more water pressing up against the outer bank which raises the water level here.
The surface from the cut bank to the point bar is inclined.
As a result there is a weaker secondary
Secondary flow caused by the cut bank?
There is more water pressing up against the outer bank which raises the water level here.
The surface from the cut bank to the point bar is inclined.
As a result there is a weaker secondary flow of water along the bottom from the cut bank to the point bar.
Sediment layers in a point bar cross bed
As water moving from the cut bank to a point bar slows down deposition takes place, creating a layer of sediment .
- (bottom) sediment at the base of the point bar is the coarsest sediment.
- medium grain sediment
- finer grain sediment.
Cross bed
layer of sediment deposited on the point bar at an angle
When does a river move most of its sediment?
During a flood when there is faster moving water in the channel. (not common)
Internal structure of a point bar
- series of super-imposed cross beds.
- horizontal layers within (“fining upwards” sequence of coarse, medium, fine grain sizes)
how does a meandering river migrate?
- Lateral Migration (deposition on the point par and erosion on the cut bank) , the channel migrates laterally (sideways).
- Downstream Migration (focusing of energy (the fastest water) further down stream at point E)
Where does sediment eroded from the cut bank end up?
Sediment eroded at A does not end up at B (the point directly across from it). The cross channel flow is much weaker than the downstream flow, so it ends up at a point bar further downstream.
Cross over point
The place between meander loops where the thalweg is in the center of the channel as it moves from one side to the other.
- more symmetrical because processes are more evenly distributed. flow converges at the surface.
- the channel is shallower here than at the point bar/cut bank. (here it is often called a pool.)
flood plain
A flat, low-lying area along a stream channel, created by and subject to recurrent flooding
Cross over point Vs. Point Bar/Cut Bank Pool
INSERT IMAGE
This channel and its various shapes are the result of ….
periods of increased discharge