Rivers: Fluvial Processes Yukon and Charley Rivers.
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Transcript of Rivers: Fluvial Processes Yukon and Charley Rivers.
Rivers: Fluvial Processes
Yukon and Charley Rivers
Topics
1. Introduction to fluvial processes
2. Types of channels
3. Processes associated with large meandering rivers
1. Introduction to fluvial processes
Streams “work” by eroding, carrying and depositing sediment
Solid load: suspended load & bedload (alluvium)
Suspended LoadLittle Colorado River
Large bedload, carried during monsoon rains in Nepal
Constant suspended load of glacial flour (ground up rock)
Also dissolved load
Suspended Load
Suspended Load
Dissolved Load
Khartoum
Manaus
Suspended Load
Dissolve
Load
Organic
Acids!
Manaus
Erode? Carry? Deposit?
200 cm/s ~ 4.5 mi/hr
Discharge & Velocity:key
Elements Of Channel Formation
Erosion in headwaters Transport in
middle
Deposition closer to ocean
High gradient (mountain) streams – erode and carry large bedload (high competence)
High competence load creates potholes & other evidence of abrasion
Middle – mixture of erode, transport, deposit, and for much of the year – like this
Depends on flood or not
Lower end - deposit
Mississippi
River delta
has very high
capacity
(lots of sediment,
Just small competence)
These are generalizations that work for larger system and not smaller
pieces
Flow Velocity Relationships
Note: always concave upward profile – characteristic of streams
Even these small washes
Base Level & Grade
The ability of a stream to
erode is based on velocity
of water. Velocity is
proportional to slope. At a sufficiently low slope, streams will run without
eroding: this is called the base level. The ultimate base
level is sea level, although local base levels exist, since
flow paths are seldom one consistent slope.
Grade – concave upward
profile
Base Level
Base level is a short-term
concept. Change in sea
level, for instance, can
either increase or
decrease slope of a
stream profile.
Dams create artificial
local base levels, and in
so doing force steeper
slopes (and hence
erosion) downstream.
Profile –
concave
upward
Effects Of Dams
From FEMA
When base level drops, every tributary incises because streams are steeper and have more energy
When Grand Canyon formed, it “entrenched” meanders
Base level dropped ~ 5 million years ago, freezing in place the old stream patterns
2. Types channels
3 basic types of channel patterns:• Straight Channels • Meandering channels • Braided Channels
Straight – in nature, requires a weakness (fault,
jointing) for stream
Straight – when you see, usually from human activity (channelized, Nasca, Peru)
Meandering rivers typically contain one channel that winds its way across the floodplain. As it flows, it deposits sediment on banks that lie on the insides of curves (point bar deposits), and erode the banks on the outside of curves.
Meandering
Point Bar Cut Bank
Asymmetrical
Point Bar Cut Bank
Braided Channel
Braided Rivers exhibit numerous channels that split off and rejoin each other to give a braided appearance. They typically carry coarse-grained sediment down a steep gradient
Braided – carries mostly bedload, so
stream spreads out to bring high velocity
close to bed
Meandering – carries much suspended load, so stream
narrows to carry load more efficiently
Can get very complicated
3. Large Meandering Rivers
Meandering stream, Phnom Penh, Cambodia
Oxbow lake
Point barCut bank
Zooming into a Reach
Pool
Riffle
Bank Erosion at Cut Bank
Bank ErosionNewaukum River (WA)
Point Bar Deposit
Point bars
Floodplains
These are satellite images before and during Summer, 1993 floods of the Mississippi river north of St. Louis.
A Floodplain
FloodsIn natural streams,
floods occur
periodically when
seasonal discharges
grow extremely large
and waters exceed
channel banks.
The first thing that
happens after water
goes over bank is the
velocity drops, and
sediment is deposited.
This makes natural
levees along channels.
Yazoo Stream
Societal Problem
Living below the river!
Cities grow & obligation to protect
Certainty: levee will always break. Only question: when
Tendency to increase meander amplitude over time
Tendency to increase meander amplitude over time
Tendency towards avulsion (sudden change in channel)
Sacramento R.
Oxbow lakes