IntroIntro

I) RiversI) Rivers

A. 4 types:A. 4 types:

1. braided1. braided

2. anastomosing2. anastomosing

3. straight to near straight3. straight to near straight

4. meandering4. meandering

Braided RiversBraided Rivers

A. Rivers from A. Rivers from low to low to high high sinuousitysinuousity

1. Braided = 1. Braided = low low sinuousitysinuousity

2. Consist of 2. Consist of channels channels & bars& bars

http://www.uwsp.edu/geO/faculty/ritter/images/lithosphere/fluvial/braided_stream_large.jpg

Copper River, Alaska

Boggs, Principle of Sedimentology

Channel Sinuosity

Resurrection River, AlaskaResurrection River, Alaska

B. Braided Rivers B. Braided Rivers mostly in high mostly in high relief areasrelief areas1. Limited extent1. Limited extent

2. Grain size decrease 2. Grain size decrease to sandy braidedto sandy braided

3. Sediment abundant3. Sediment abundant

4. Water discharge 4. Water discharge high/sporadichigh/sporadic

http://www.uoregon.edu/~millerm/braided.html

At-Bashi River, Tien Shan Mtns., KyrgyzstanAt-Bashi River, Tien Shan Mtns., Kyrgyzstan

C. Characterized by C. Characterized by wide, shallow wide, shallow channels. channels.

1. Caused, primarily by 1. Caused, primarily by weak banks, weak banks,

2. Channels can not 2. Channels can not incise very deeply, incise very deeply,

3. Increase in 3. Increase in discharge channel discharge channel banks erodebanks erode Channel becomes Channel becomes

wider,wider,

http://www.uoregon.edu/~millerm/braided.html

Channel Channel WidthWidth

D. Wide channels = lower shear stress D. Wide channels = lower shear stress at base of bedat base of bed

(flow can not get very deep).(flow can not get very deep).

1. In modern braided streams 1. In modern braided streams shear stress is just above shear stress is just above critical for the coarser grain critical for the coarser grain sizes.sizes.

2. Therefore, where have flow 2. Therefore, where have flow expansion a bit (eg bank expansion a bit (eg bank collapse)collapse)

3. Get deposition as shear 3. Get deposition as shear stress drops (which takes stress drops (which takes place because flow place because flow widening also means flow widening also means flow shallowing).shallowing).

4. Have lots of local 4. Have lots of local deposition and erosion deposition and erosion taking place all the time = taking place all the time = many, bars and channelsmany, bars and channels

Kicking Horse River,British Columbia

http://www.uoregon.edu/~millerm/braided.html

E. Why are banks weak? (i.e. E. Why are banks weak? (i.e. what conditions lead to what conditions lead to braiding):braiding):

1. high sedimentation rates 1. high sedimentation rates (especially coarser (especially coarser grained, noncohesive, grained, noncohesive, material; material;

2. reduced vegetation rates 2. reduced vegetation rates (vegetation can make (vegetation can make banks cohesive); banks cohesive);

3. Flow variable ("flashy"), 3. Flow variable ("flashy"), which is not necessary, which is not necessary, but tough for plants but tough for plants depending on regular depending on regular water table depth to water table depth to take root take root

At-Bashi River, Kyrgyzstan

Channel SteepnessChannel Steepness

F. Why are braided streams F. Why are braided streams steeper than meandering steeper than meandering streams in general?streams in general?

1. Assume two streams with 1. Assume two streams with identical water and identical water and sediment discharge. sediment discharge.

2. Both need similar shear 2. Both need similar shear stresses to transport the stresses to transport the sediment it carries. sediment it carries.

3. If one stream deep, as 3. If one stream deep, as meandering streams tend meandering streams tend to be, it does not need to to be, it does not need to be steep to have high be steep to have high enough shear stress. enough shear stress.

4. If the other stream not deep 4. If the other stream not deep (the braided case), only (the braided case), only other way to get sufficient other way to get sufficient shear stress is to become shear stress is to become streeper.streeper.

Tien Shan Mts, Kyrgyzstanhttp://www.uoregon.edu/~millerm/braided.html

II. Braided vs MeanderingII. Braided vs Meandering

A. General observations A. General observations for braided streams:for braided streams:

1. relative to meandering 1. relative to meandering streams, in general streams, in general braided streams have: braided streams have:

i. wider, shallower i. wider, shallower channels channels

get many channels get many channels separated by bars separated by bars

ii. less fine grained ii. less fine grained material in channel material in channel and on flood plain and on flood plain

iii. steeper gradients iii. steeper gradients iv. weak, erodable banks iv. weak, erodable banks v. channels chock full of v. channels chock full of

bars with lots of bars with lots of cutting and filling of cutting and filling of channels channels

Animas River Silverton Colorado

Yukon River, Alaska

http://en.wikipedia.org/wiki/Image:Braided_Drainage.jpg

III. Longitudinal BarsIII. Longitudinal BarsA, Gravelly braided streams A, Gravelly braided streams

tend to develop tend to develop longitudinal bars. longitudinal bars.

1. bars do not migrate much 1. bars do not migrate much (in this way different (in this way different than most bars)than most bars)

2. tend to elongate and 2. tend to elongate and become thinner more become thinner more streamlined over time. streamlined over time.

3. Bar elongation is natural 3. Bar elongation is natural consequence of varying consequence of varying shear stressesshear stressesi. as bar deposited, i. as bar deposited,

channel becomes channel becomes more constricted so more constricted so flow tries to become flow tries to become a bit deeper, a bit deeper,

ii. but material in stream ii. but material in stream is weak and easily is weak and easily erodederoded

iii. so as shear stress iii. so as shear stress goes up get more goes up get more local scouring due to local scouring due to increased shear increased shear stress. stress.

iv. This process tends to iv. This process tends to elongate gravel bars.elongate gravel bars.

Son-Kul River, Kyrgyzstan

http://www.uoregon.edu/~millerm/braided.html

IV. Linguoid and Lateral IV. Linguoid and Lateral BarsBars

A. Sandy braided streams A. Sandy braided streams tend to develop lingoid tend to develop lingoid (tongue-shaped) bars (tongue-shaped) bars (sometimes called sand (sometimes called sand waves ).waves ).

1. Form under high 1. Form under high flood conditionsflood conditions

B. Lateral BarsB. Lateral BarsAttached to bankAttached to bankDevelop in low Develop in low

energyenergy

http://faculty.gg.uwyo.edu/heller

Platte River, Nebraska

V. Why Braiding?V. Why Braiding?

A. Braided channels not A. Braided channels not sinuous,sinuous,

1. Don't see much point 1. Don't see much point bars. bars.

2. Why are they straight? 2. Why are they straight? Because banks too Because banks too

weak to resist flow weak to resist flow and don't get cut and don't get cut banks.banks.

B. Rrivers can be both B. Rrivers can be both braided and braided and meandering at meandering at different times (or different times (or even at the same even at the same

VI. VI. DepositionDepositional al SequenceSequence

A. Proximal= coarse bars overlain by less common x-bedded T barsA. Proximal= coarse bars overlain by less common x-bedded T barsB. Ideal sequence is mixture of gravelly L & sandy T bars. L bars B. Ideal sequence is mixture of gravelly L & sandy T bars. L bars

have erosional basehave erosional baseC .Distal sequence= less gravel, thinner L barsC .Distal sequence= less gravel, thinner L barsD. Note transition to meandering river. More sand, flood plain D. Note transition to meandering river. More sand, flood plain

deposits, abandoned channelsdeposits, abandoned channels

Touttle River, Washington

Bars in Jurassic Kayenta Fm, Colorado

http://www.uoregon.edu/~millerm/braided.html

At-Bashi River, Kyrgyzstan

Kicking Horse River,British Columbia

Animas River Silverton Colorado

Resurrection River, Alaska

Resurrection River, Alaska

Son-Kul River, Kyrgyzstan

Tien Shan Mts, Kyrgyzstan