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

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

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,

6. Channel Width
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
Kicking Horse River,British Columbia

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.
Tien Shan Mts, Kyrgyzstan

9. II. Braided vs Meandering
Yukon River, Alaska
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
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

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

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

16. At-Bashi River, Kyrgyzstan

17. Kicking Horse River,British Columbia

18. Animas River Silverton Colorado

20. Resurrection River, Alaska

21. Resurrection River, Alaska

22. Son-Kul River, Kyrgyzstan

23. Tien Shan Mts, Kyrgyzstan