Geo Morph Meandering Braiding

8/8/2019 Geo Morph Meandering Braiding

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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

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Geo Morph Meandering Braiding