MIDTERM 1 - B Flashcards
- River
A river is water on the surface of the earth moving downhill in a channel due to gravity.
- River Drainage Basin
The area of land drained by a river.
- (4) a river will always try to:
- flow downhill
- find the easiest way downhill
- reduce its gradient
- carry the maximum amount of sediment
- (4) things used to define a river:
- number, size and shape of the channel(s)
- discharge (Q)
- sediment load
- gradient
- What is the most important thing to remember about the four things used to define a river?
They are all interrelated.
- Dominant influence on controlling channel processes.
Climate which controls precipitation, which controls discharge
- Best way to describe the shape of a river and why
Shape is described using the ratio of width to depth. (w/d).
Because every river’s bottom is different. There are no smooth perfect river bottoms (that’s unnatural).
- How does water velocity vary within a river channel?
Meandering: Highest velocity by the cut bank, lowest by the point bar. At the cross-over point, the highest velocity is at the middle-top of the river (at surface or near surface of the water).
Braided: Higher velocity in the middle top of the channel and slower on the sides and bottoms.
9, Discharge
Volume of flow in a river that passes by a given point in a given unit of time. (cubic metres per second)
- Effective/Dominant discharge
The volume of water moving in the channel that does the most work. Work may be measured by how much sediment is moving.
- Bankfull discharge
the volume of water in the channel when the channel is full.
- Sediment load in a river
The sediments (solid particles and elements in solution) that are moving or may move.
- Base Level
level below which the stream cannot erode its valley. The lowest point that the river tries to reach.
- Weathering in a river
Solution weathering is important in many river environments.
Physical weathering is less important.
- How does erosion in a river?
THE IMPACT OF
- moving water
- sediment that is already moving
(Together these two processes are ABRASION)
- Hydraulic lift
- Relationship between grain size and threshold velocity
DRAW DIAGRAM
- The larger the grain, the more velocity is needed to set it in motion
- some small particles, such as clay minerals, have significant cohesion, which makes them stick together and harder to erode.
- How does transport occur in a River?
- Solution
- Suspension
- Saltation
- Traction
- How does deposition take place in a River
In a river the sediment stops moving when the velocity drops. (However, sediment in solution is deposited when precipitation occurs)
- Precipitation in a river
Water is removed by evaporation, thus concentration increases.
- How is the river different from the top to the bottom?
- the gradient is steeper : higher velocity at top
- size: smaller at top
- shape/ratio: higher at top
- sediment load: coarser sediment at the top
- discharge: more water at the bottom
- How does water velocity change from the top to the bottom of a drainage basin?
higher velocity at top
- How does discharge change from the top to the bottom of a drainage basin?
greater discharge at the bottom
- How does sediment load change from the top to the bottom of a drainage basin?
coarser sediment at the top
- How does size of a channel change from the top to the bottom of a drainage basin?
smaller at top
- How do the defining features of sediment change as sediment downhill in a river channel?
GRAIN SIZE: gets smaller. big stuff gets left behind because velocity drops because of gradient. breaking apart
SORTING: Gets better sorted
ROUNDNESS: They get more round
SPHERICITY: Get more spherical
COMPOSITION: More quartz because mafic minerals are heavier and get left behind. They are more easily eroded and more reactive, so they make them weaker.
Most common mineral at B: quartz: because they are lighter and more resistant.
- Assuming sediment load includes an abundance of mafic and felsic mineral grains, briefly explain why it is that the composition of the sediment load changes.
Because mafic minerals are heavier and get left behind. They are more easily eroded and are more reactive, therefore ‘weaker’.
- Bed roughness
The irregular shape of the channel, bottom and sides.
smooth bottom: little in the way of roughness
bumpy bottom: significant bed roughness
- Capacity of a river
A measure of the volume of sediment a river can carry.
- Competency of a river
A measure of the maximum grain size a river can carry.
- Turbulent flow
Water in the channel flows in irregular pathway with much vertical and lateral mixing. The most common type of flow and is caused by: friction.
- Laminar flow
The water in the channel moves in parallel layers with no vertical or lateral mixing.
(It is very rare. When it does exist it is only for a short period of time in a limited space.)
- Laminar Sublayer
The laminar sublayer is a layer of water, usually a very thin layer, adhering to the bottom of the channel.
This is a layer of water that has no velocity because of friction with the channel bottom
- Relationship between laminar sublayer and sediment erosion and transport.
Depending on the thickness of this layer and the size of sediment, sediment on the bottom may or may not move.
- Why is it that turbulent flow is the most common type of flow?
Because of friction. Friction with the bottom that is affected by bed roughness and friction within the water itself.
- Bedrock channel
- Defined by the local bedrock
- Little to no clastic sediment in the channel
- Found: higher in the drainage basin
- No sediment load
- transport capacity: Ability of the discharge to remove sediment is greater than the supply of sediment.
- No flood plain (lacks the space to store sediment)
- Alluvial Channel
- Defined by the clastic sediment in the river
- In this channel there is abundant sediment
- Found: lower in the drainage basin
- Transport capacity: Sediment load exceeds the ability of the discharge to remove it.
- Flood plain (room to store sediment)
- Relationship between erosion of a bedrock channel and sediment load
Sediment load in a bedrock channel is often absent. However, it is the presence of moving sediment in the channel that is most effective at eroding the channel.
If there is too little sediment the channel is not eroded.
If there is too much sediment then some of it will remain in the channel forming a deposit that shields the bedrock preventing erosion - what is needed is the right volume of sediment so that it can all move.
- What happens to an alluvial stream when capacity and competency INCREASE?
Erosion and transport increase, and the channel becomes wider and deeper. gradient may be less(shallower) slow velocity capacity and competency slows it down
- What happens to an alluvial stream when capacity and competency DECREASE?
Deposition will take place. Sediment accumulates in the channel A reduction in width and depth, increase gradient, width and depth decrease as sediment accumulates in the channel as the gradient increases.
- Graded Stream
A graded stream is one where the processes that affect it (ie: gradient, discharge) have reached an equilibrium, they are more or less balanced…(temporary at best)
- how do channel bars form in a braided river?
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.
- Describe water flow in a meandering river?
Turbulent.
Aside from that, there is a weaker secondary flow.
- How does a point bar form in a meandering river?
The point bar is a product of deposition.
As water moving from the cut bank to a point bar slows down deposition takes place, creating a layer of sediment .
- How does a cut bank form in a meandering river?
The cut bank is a product of erosion.
Erosion takes place at the outer bank to create a steeper cut bank because here is where the fastest water is.
- Thalweg
The place in the river where the deepest, fastest water is..
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.
Secondary Flow
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.
- A typical sequence of alluvial channel reaches includes;
Cascade, Step-Pool, Plane-Bed, Poo-Riffle, Dune-ripple
- What is a channel reach?
This is a section of river channel that displays certain characteristics.
- What is a colluvial reach?
A section of channel characterised by a narrow channel width, steep slope, lack of flood plain and steep slopes next to the channel.
- Why does grain size increase downstream in a colluvial reach?
Because of the work of the river removing the finer grains and transporting them further downstream, leaving the coarser grains behind.
- What is an alluvial reach?
An alluvial reach is a section of channel dominated by sediment in the channel that is moved within the channel. Unlike the colluvial channel, very little sediment comes from external sources.
- Briefly explain the transition that takes place in this sequence (sequence of alluvial channel reaches).
?
- What is a floodplain?
the area of land next to the river. It is next to the channel and part of the river system. It is only rarely affected by the active channel, which is during a flood.
- What is a floodplain used for?
A floodplain is used to store water (short term) and sediment (long term).
- What is a delta?
The deposition of sediment that forms what a river reaches base level.
- how does a Delta form?
In a typical delta there are distinct layers or crossbeds. These form as the river brings “surges” of sediment into lake/ocean. During periods of increased discharge, as multiple episodes of sediment or brought by the river, multiple crossbeds are deposited.There is a coarse layer on top and a finer layer on the bottom crossbeds and this coarsening upwards sequence define a delta.
- a number of longer, linear extensions grow as multiple channels transport and deposit sediment.
Base Level
Base level is where the water in a river stops(ocean or sometimes lake)Sea level is ultimate base level.
47.
- The shape of a meandering channel is created during those brief, rare episodes of increased discharge.
- during the intervening periods of lower discharge, not much changes.
- this same shape is maintained by subsequent periods of increased discharge when sediment is moved and work is done.
SOLUTION
The way in which minerals dissolve in water.
SUSPENSION
The finest/smallest solid particles that are carried in the water. (This sediment rarely makes contact with the channel bottom.)
SALTATION
Larger sediment grains bounce along the bottom making frequent but only short term contact with the channel bottom.
TRACTION
The largest sediment in motion. These grains roll or slide along the bottom. (too heavy!)
