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

Channel Sinuosity Boggs, Principle of Sedimentology

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

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

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

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

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

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 http://en.wikipedia.org/wiki/Image:Braided_Drainage.jpg Animas River Silverton Colorado

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

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

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

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

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