Why study streams Flashcards
Why study streams?
- They provide water and deposit fertile soil for agriculture
- They’re pathways for commerce and trade
- They erode, flood, and sculpt the landscape
Evapotranspiration:
Moves water into the atmosphere and precipitation brings it back to earth
Drainage basin:
Is the area that drains into a stream.
Divides
Are ridges that serrate basins
What is the largest river basin?
The Mississippi basin
Discharge
Is the amount of water that flows through a channel
Where does sediment come from?
- Loosened particles from weathering are picked up in surface runoff and transported to the stream channel
- Mass movement events may move loose material downslope into the stream channel
- The stream itself may also erode the banks of its channel
Sediment moves as…
Bedload and suspended load
Bedload:
Is large grains that cannot be picked up, but still are able to be moved. They roll, bounce, and slide along the bottom
- When a stream is at bankfull, all sizes of particles move due to increased stress
Suspended Load:
Small sediment grains that mix with the flowing water are transported above the bed, rarely touch the bottom, and came make the water cloudy/muddy
Dissolved load:
The hidden load. Ions in solution form chemical weathering are also carried by water in streams
Alluvial Stream
Is where the channel is in water transported sediment. It has a different energy structure than a bedrock stream which is cut into solid rock
How do streams pick up sediment?
Force, not velocity is exerted on a particle to move it.
When does shear stress increase?
Shear stress increases with increasing water depth and increased slope. Both cases require shear force to overcome the resisting forces of friction and cohesion
Do small particles take as much shear force as large particles?
Yes, small particles of silt and clay actually have large cohesive forces for their size that keeps them in place
Stream power:
Is the result of shear stress and velocity .
A stream must..
Lose power to move sediment when it switches form erosion to deposition
Loss of shear stress will cause..
Deposition
Loss of velocity will cause…
Deposition
Loss of discharge will cause…
Deposition because without water, there is no shear stress or any velocity
Increase of water depth…
Increases shear stress, which increases stream power which results in Erosion
Decrease in water depth…
Decreases shear stress, which decreases stream power which results in deposition
Increase in channel slope…
Increases shear stress, which increases stream power which results in erosion
Decrease in channel slope…
Decreases shear stress, which decreases stream power which results in deposition
Increase in flow of velocity…
Increases velocity, which increases stream power, which results in erosion
Decrease in flow of velocity…
Decreases velocity, which decreases stream power which results in deposition
When does deposition happen?
Deposition happens where water depth decreases and stream power drops off
An alluvial fan forms where…
The stream goes from confined to unconfined
Where do deltas form?
They form where streams enter the ocean or lake
What factors determine channel pattern?
- Meandering Patterns
- Braided patterns
Meandering Patterns:
Is a single main channel that has various loops and/or curves down its length. It is longer and has a lower average slope.
Braided Patterns:
Are a series of small channels that are interwoven into a larger whole
Channel Patterns:
Are most often a combination of meandering and braided patterns
How do banks determine channel pattern?
- Bank erodibility influences channel pattern
- Stable banks are due to clay, vegetation or a narrow, deep, and sinuous channel
Why do streams flood?
- Discharge exceeds a stream channels capacity
- The drainage basin cannot absorb the water from precipitation or snowmelt and then must runoff onto the surface
Causes of floods:
- Extreme rainfall
- Rapid snowmelt
- Storm surge
- Breaching of dams/embankments
Flash floods
Associated with canyons and are generally the deepest and most damaging. They are rapid and difficult to predict
Prolonged floods:
Last more than two days and inundate large areas
Storm surge
Strong onshore winds cause ocean water to ‘pile up’ along the shore which raises the level of all rivers flowing into that coastal area
