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Intro

Intro. I) Rivers A. 4 types: 1. braided 2. anastomosing 3. straight to near straight 4. meandering. Braided Rivers. A. Rivers from low to high sinuousity 1. Braided = low sinuousity 2. Consist of channels & bars. Copper River, Alaska .

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Intro

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  1. Intro • I) Rivers • A. 4 types: • 1. braided • 2. anastomosing • 3. straight to near straight • 4. meandering

  2. Braided Rivers • A. Rivers from low to high sinuousity • 1. Braided = low sinuousity • 2. Consist of channels & bars Copper River, Alaska http://www.uwsp.edu/geO/faculty/ritter/images/lithosphere/fluvial/braided_stream_large.jpg

  3. Channel Sinuosity Boggs, Principle of Sedimentology

  4. Resurrection River, Alaska • B. Braided Rivers mostly in high relief areas • 1. Limited extent • 2. Grain size decrease to sandy braided • 3. Sediment abundant • 4. Water discharge high/sporadic http://www.uoregon.edu/~millerm/braided.html

  5. At-Bashi River, Tien Shan Mtns., Kyrgyzstan • C. Characterized by wide, shallow channels. • 1. Caused, primarily by weak banks, • 2. Channels can not incise very deeply, • 3. Increase in discharge channel banks erode • Channel becomes wider, http://www.uoregon.edu/~millerm/braided.html

  6. Channel Width Kicking Horse River,British Columbia • D. Wide channels = lower shear stress at base of bed • (flow can not get very deep). • 1. In modern braided streams shear stress is just above critical for the coarser grain sizes. • 2. Therefore, where have flow expansion a bit (eg bank collapse) • 3. Get deposition as shear stress drops (which takes place because flow widening also means flow shallowing). • 4. Have lots of local deposition and erosion taking place all the time = many, bars and channels http://www.uoregon.edu/~millerm/braided.html

  7. At-Bashi River, Kyrgyzstan • E. Why are banks weak? (i.e. what conditions lead to braiding): • 1. high sedimentation rates (especially coarser grained, noncohesive, material; • 2. reduced vegetation rates (vegetation can make banks cohesive); • 3. Flow variable ("flashy"), which is not necessary, but tough for plants depending on regular water table depth to take root

  8. Channel Steepness • F. Why are braided streams steeper than meandering streams in general? • 1. Assume two streams with identical water and sediment discharge. • 2. Both need similar shear stresses to transport the sediment it carries. • 3. If one stream deep, as meandering streams tend to be, it does not need to be steep to have high enough shear stress. • 4. If the other stream not deep (the braided case), only other way to get sufficient shear stress is to become streeper. http://www.uoregon.edu/~millerm/braided.html Tien Shan Mts, Kyrgyzstan

  9. Yukon River, Alaska II. Braided vs Meandering • A. General observations for braided streams: • 1. relative to meandering streams, in general braided streams have: • i. wider, shallower channels • get many channels separated by bars • ii. less fine grained material in channel and on flood plain • iii. steeper gradients • iv. weak, erodable banks • v. channels chock full of bars with lots of cutting and filling of channels http://en.wikipedia.org/wiki/Image:Braided_Drainage.jpg Animas River Silverton Colorado

  10. III. Longitudinal Bars • A, Gravelly braided streams tend to develop longitudinal bars. • 1. bars do not migrate much (in this way different than most bars) • 2. tend to elongate and become thinner more streamlined over time. • 3. Bar elongation is natural consequence of varying shear stresses • i. as bar deposited, channel becomes more constricted so flow tries to become a bit deeper, • ii. but material in stream is weak and easily eroded • iii. so as shear stress goes up get more local scouring due to increased shear stress. • iv. This process tends to elongate gravel bars. Son-Kul River, Kyrgyzstan http://www.uoregon.edu/~millerm/braided.html

  11. IV. Linguoid and Lateral Bars • A. Sandy braided streams tend to develop lingoid (tongue-shaped) bars (sometimes called sand waves ). • 1. Form under high flood conditions • B. Lateral Bars • Attached to bank • Develop in low energy Platte River, Nebraska http://faculty.gg.uwyo.edu/heller

  12. V. Why Braiding? • A. Braided channels not sinuous, • 1. Don't see much point bars. • 2. Why are they straight? • Because banks too weak to resist flow and don't get cut banks. • B. Rrivers can be both braided and meandering at different times (or even at the same

  13. VI. Depositional Sequence • A. Proximal= coarse bars overlain by less common x-bedded T bars • B. Ideal sequence is mixture of gravelly L & sandy T bars. L bars have erosional base • C .Distal sequence= less gravel, thinner L bars • D. Note transition to meandering river. More sand, flood plain deposits, abandoned channels

  14. Touttle River, Washington

  15. Bars in Jurassic Kayenta Fm, Colorado http://www.uoregon.edu/~millerm/braided.html

  16. At-Bashi River, Kyrgyzstan

  17. Kicking Horse River,British Columbia

  18. Animas River Silverton Colorado

  19. Resurrection River, Alaska

  20. Resurrection River, Alaska

  21. Son-Kul River, Kyrgyzstan

  22. Tien Shan Mts, Kyrgyzstan

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