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Grant Howard
Geomorphology 42:278
Williams River in Alaska The Upper Waimakariri River in NewZealand
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Table of ContentsyWhat is Channel Pattern
y Meandering Channels
y Braided ChannelsyWhy Meandering? or Why Braided?
y Head-to-Head Comparison of Meandering & BraidedChannels
y Conclusion
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Wha
t is Cha
nnelPa
ttern?y Channel pattern refers to the overall appearance of thestream or river
y 3 main types of channel patterns: meandering, braided andstraight
y It is possible to have a river in which has more than onetype of channel type
Tigre River, Argentina Braided river in Iceland
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What is Channel Pattern?
Meandering /Braidedcombination
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Mea
ndering Cha
nnels
y Meanders in meandering channels aremostly initiated by the turbulence and
velocity of the flow
y
The deflection of flow in meanderingrivers is due to centrifugal forces on theouter edge of the meander
y The migration of meanders is a positivefeedback loop because the system ismoving away from the original statethat it was in
Oxbowlake
MeanderMigration
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Mea
ndering Cha
nnels - continued
yAnd because of all thesediment deposited onthe inside of the
meander a point bar maybe formed
And because of the highervelocity on the outer edgeof meander a helical flowis created
Meandering rivers aremeasured by sinuosity
index
Sinuosity =
stream length/valleylength
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Mea
ndering Cha
nnels - continued
Point bars
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Braided Channelsy Measured in terms of the degree of braiding and expressed
as a ratio of bar length to channel length
y Primarily bedload deposition; resulting in braid bars
y In braided rivers of dominantly coarse material the bars arelongitudinal and diamond-shaped
y Whereas in dominantly sandy material braided rivers thebars are frequently covered in ripples and dunes
y2 variations of braided channels: confined braided systemsand free-braided systems
y And during a flood event in a braided river finer sedimentcan end up on top of the coarser sediment on the bars
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Braided Channels - continued
Braidbars
Multiplechannels
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Braided Channels - continuedy Braided rivers are consistently changing there plan
form just like meandering channels do
y
The bars/islands are constantly shifting and increasingor decreasing in size
y In Canada braided rivers are especially common onglacial outwash deposits in the Cordillera and in the
Arctic
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Bra
ided Cha
nnel
s - continued
Deposited
sediments creating abraid bar
Fossil Creek - Coral Harbour, Nunavut
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WhyMeandering? or WhyBraided?
(channel formation)
yWhy a channel results in one or the other is adebateable topic
y But the consensus answer is that is the main influence isthe erodibility of the bank material
y And erodibilitys biggest factor sediment type andvegetation
y Of course there are other factors influencing thechannel type, such as: channel width, depth ratio, water
velocity, sediment transport/deposition and the make-up of the sediment
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Riverba
nk Erosion
Bank erosion on the Upper Iowa River
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Meandering vs. BraidedMeandering Braided
y Series of curves of
approximately same shapey Deep, narrow channels and
banks
y Banks and channels are fairlyresistant to erosion
y Meandering rivers tend toonly have one primarychannel
y Less steep gradient
y Numerous dividing andreuniting channels withintervening bars or islands
y Braided river channels tendto be wide and shallow
y Braided river banks are easilyeroded
y One or more channels areusually dominant in largebraided rivers
y Steeper gradient
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Meandering vs. BraidedMeandering Braided
y Sediment load is primarilycarried in suspended load
y The flow tends to be moreturbulent
y Well developed f loodplains
y Associated with more finer
materials, like clay and silty Meandering channels are the
dominant pattern today
y Meandering rivers tend tohave stable discharge levels
y Sediment load is primarilycarried in bedload
y Slower, less turbulent flow
y Floodplains are not as welldeveloped
y Associated with coarser
materials , like gravely Far fewer braided channels in
todays world
y Braided rivers discharge levelscan vary a lot
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Meandering vs. Braided
Braided river - East Fork Toklat River -Denali National Park, Alaska
Meandering river on theNorth Slope, Yukon
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Conclusiony 3 main types of channel pattern: meandering, braided, and
straighty The consensus answer for what channel pattern a river takes on is
primarily because of the erodibility of the channel banky Point bars and oxbow lakes are associated with meandering
rivers....braid bars are associated with braided riversy Meandering rivers are measured by sinuosity....braided rivers aremeasured by the degree of braiding in the river
y Meandering rivers are primarily finer sediment and carried insuspension....braided rivers are primarily coarser sediment andmostly carried by bedload
y Meandering rivers are much more common....there are only 2braided rivers in all of Europe!y Meandering rivers have deep, narrow channels, whereas braided
rivers tend to have wide, shallow channelsy Meandering rivers typically have just one dominant channel and
braided rivers will have multiple channels
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Questions?
Point bar
Polar bear