3 streams and floods Flashcards
Rivers
By far the most important agent of erosion and deposition on the surface of the Earth
Drainage networks
dominate earth’s surface
Drainage Basins and Divides
Drainage basin
The area drained by a certain river system
All streams flow to the ocean
Except not quite
we live in a basin that doesn’t reach the ocean because we have divides on all sides
Divides
Boundaries that separate one drainage basin from another
Mountains: always a higher point in
Drainage patterns: dendritic
Dendritic
Horizontal bed layers of soft rock
looks like a tree
drainage patterns: radial
radial
isolated peaks
star shaped around a peak
Drainage Patterns: rectangular
rectangular
jointed bed rock at 90 degree angles
keep eroding in the patterns of deformations
drainage patterns: trellis
trellis
deformed strata with alternating weak and resistant rock
see screen shot
like railroad track symbol
drainage patterns: parallel
parallel
steep sided hills with less vegetation
like lines on a paper
river vs. creek vs. stream vs. brook vs. etc.
Informal
River > Stream > Creek ~ Brook ~ Burn ~ Crick
Geology
River or Stream no matter the size
Any flowing channelized water
Permanent vs Ephemeral (temporary)
river systems
Any body of water that flows downhill in well defined channel
Streams erode the valleys through which they flow
valleys weren’t there until the stream cut down into it
River system parts
Collecting
Transporting
Dispersing
River Systems: collecting
Collecting
Occurs in tributaries
“Headwaters” top
Erosion= primary geologic characteristic
Transportation-some
Deposition-not much
Have the most energy
River Systems: Transporting
Transporting
Occurs in main “trunk”
Erosion-some
Transportation-main
Deposition-some
River Systems: dispersing
Dispersing
Occurs in distributaries
“Mouth” end
Erosion-not much
Transportation-some
Deposition-main
Stream Flow Variables
Discharge
Gradient
Velocity
Sediment load
Base level
Stream Flow Variables: Discharge
Volume of water flowing past a given point in the river in a certain amount of time
Varies with season and weather
Weir (like we saw on the bridge in rock canyon) Measured in cubic meters per second
Stream Flow Variables: Gradient
Slope of the river channel
Rise over the run
Changes over length of river
Steep at headwaters
- Sediments are coarse
Nearly flat at mouth
- Sediments are fine
Stream Flow Variables: Velocity
Directly related to gradient and volume
Higher gradient, higher velocity
Higher volume, higher velocity
Not uniform in stream channel
Highest in middle/center and top of channel
Stream Flow Variables: Sediment Load
Material carried by the stream
Three types
Suspended load
- Fine particles (makes it look dirty/murky)
- Usually largest fraction of mat’l carried
Bed load
- Coarse particles
- Rolling, sliding, or saltation (bouncing)
Dissolved load
- Ions in solution (not visible)
saltation see sediment load screenshot, little rocks jumps and land and bump another and that makes it jump and so on
Stream Flow Variables: Base Level
sediment load considerations
Capacity
Amount of sediment the stream can carry
Depends on velocity and volume
Varies seasonally, with weather, over time
Competence
Largest size of sediment the stream can carry
Depends on velocity and volume
Varies seasonally, with weather, over time
“dry” riverbeds
“Wadi”,“Wash”,“Arroyo”,“Slot Canyon”
Desert soils don’t soak up torrential rains
Hazardous flash floods after storms
Sometimes miles and miles away from the rain
deadly
base level
Level to which a stream will erode
Won’t erode lower than base level
Sea level is the ultimate base level
Local base level
Lakes/Reservoirs
Elevation of the stream’s mouth
Head: start of stream
Mouth: end of stream
great basin
Internal drainage
No outlet to the ocean
Topographic barriers
see screenshot for location west
What is the ultimate base level of the Provo River?
great salt lake
river erosion
Removal of regolith
Downcutting
The stream channel
Headward erosion
Removal of regolith (weathering products)
Washed downstream
About 6 cm/1000 yr removed from the continents
river erosion: v shape
Downcutting the stream channel
Abrasion by sand and gravel dragged across the sides and bottom
Most rapid during high flow
Slope retreat
V-shaped valley formation
canyons
Hard rock will not slump as easily as softer rock/regolith
valley and canyon
stair step canyon alternating soft and hard, see screenshot
headward river erosion
Headward erosion
Erosion rates are highest where gradients are highest
New channels develop at stream head
stream piracy
headward erosion can cause
Stream Piracy, me hearties
Headward erosion may allow one stream to capture the tributaries of a different stream
see screenshot
river erosion: braided streams
Braided Streams
High sediment load streams with relatively low water levels
Most common in tributary systems: near the head of the river
Distinctive stream pattern
Multiple channel system with lots of islands
Common in arid regions and in front of glaciers
river deposition
Primarily in the transporting and dispersing systems
Flow velocity reduction
Creates
Floodplains
Deltas
Alluvial valleys and fans
river deposition: floodplains
Floodplains
Flat area bordering a stream channel
Stream channel meanders through time across the floodplain
Natural state
Stream channel position changes
Artificial state
Stream channel remains same
river deposition: deltas
Deltas
Depositional place of most of river’s sediment
Where the river hits base level
Sudden drop in velocity
Stream drops everything off
May be modified by waves
deltas
Distributaries
As sediment is dropped off, a tributary channel becomes clogged
May split off into two smaller channels to seek easier route
Avulsion
Shift in main course
The Mississippi wants to switch to the Atchafalaya channel
Leaving New Orleans high and dry
river deposition: alluvial valleys and fans
Alluvial valleys and fans
Dry basin deposits
Rapid velocity drop as stream comes out of confined canyon
Spreads out from canyon mouth
High sediment load
Usually found in arid climates
bajada
Coalesced alluvial fans
Common in at the base of desert ranges
river deposition; meandering streams
Meandering Streams
Common in floodplains
Sinuous pattern
Erosion on one back, deposition on the other
changes course of river
Erosion – Cutbank: knocks into river bank
Deposition – Point bar: slower side of river more deposition
see screen shots for cutbank and evolution of meadnering streams
meandering streams
Meander: loop
- Point bar: deposition and cutbank: erosion
- Cutoff: bends back on itself creatingVVV
^^^^Oxbow lakes
Natural levees: sand ridges that parallel the channel
- Embankment created during flood stage: sediments dropped when the water enters a valley
- Sand ridges that parallel the channel
Yazoo streams: run down from high places and pareallel to river , but are blocked from entering the river by deposited material that creates river banks
Backswamps:?
