4 channel types defined at reach scale, based on 3 features Number of channels Channel stability Sinuosity photo: S. Hillebrand. U.S. Fish & Wildlife Service Digital Library System http://images.fws.gov/ single channel types Straight (Sinuosity << 1.5) Meandering (Sinuosity > 1.5)

multiple channel types Braided Qs >> Q … channels unstable, move during floods Anastomosing / Anabranching Cohesive banks. … channels stable, don’t move photo: © Michael Collier. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images, photo ixvt9i

How do meanders form? Differential erosion Cut off At a point in time Movement through time Cut off

Meander scaling laws

Sine curves (meanders) represent the most uniform distribution of change of energy dissipation along a curve. From Leopold (1994, A View of the River)

C) View#3: Whole Basin scale Perspective of Downstream Changes in … Discharge Stream gradient Sediment grain-size Influence of riparian Distance Elevation outlet max source High gradient Width Depth Velocity (why?) Low gradient

More scaling relations (Fig.3.3 text) Low order High order W What changes fastest downstream? D Is average velocity greater in large river than in small stream? U

Perspective of Drainage Pattern Reflect geology mostly

Channel Classification at whole basin scale 2 Classifications (Fig. 2.1 Knighton): Horton/Strahler (1945, 1952) - Stream Order Shreve (1967) - Link Order For point X, what is: stream order? link order? X

Scaling Relations (Fig. 2.1 Knighton)

Fluvial Geomorphology Ecological Perspective (1) Sediment size distribution (2) Channel Form X-sec Planform Basin Downstream changes Drainage pattern Channel size (3) Sediment movement Distance Elevation

(3) Sediment Movement Classes of sediment transported Dissolved load Suspended (and “wash load”) Bedload (5- 10% total load) (moves during floods) Big Thompson Flood (1978)

critical erosion velocity: "Graded stream" -- dimensions in "balance" (adjusted to) current discharge (Q) and sediment (Qs) regimes. Aggradation (Qs > Q) Degradation (Qs < Q) critical erosion velocity: velocity that is sufficient to move the particles on the bed (varies with particle size) stream competence: the largest particle that may be moved as bedload (for a given Q)

Hjulstrom curve (Fig.3.8, Allan text) For individual particle ... Erosion When critical erosion velocity exceeded Transport Continued movement once eroded Sedimentation Velocity drops below transport threshold Bed load suspended Why is sand the most erodible? Low cohesion, small grain size Why is clay least erosible? High cohesion resists erosion Why is clay most transportable? Small grain size Once suspended … ? Thought question: How can particles continue to be transported when average velocity < fall velocity?

Streambeds are well sorted Distribution of particle sizes reflects hydraulic conditions coarse grains in faster flowing areas (erosional) finer grains in slow-flowing areas (depositional) Sorting occurs longitudinally (e.g., pool-riffle) and laterally (thalweg to shore) Sorting reflects a range of flows (Fig. 3.8 Allan) Coarse sediments sorted by flood flows Finer sediments sorted by less than flood flows

Movement of streambed during high flows maintains channel form What determines mobility? Sediment size (Hjulstrom curve) Shear stress acting on streambed (particle) Recall for a particle on the streambed: -- Shear stress, t =  * DU/Dy For streambed mobility, we generally focus on the population of particles, and estimate the average shear stress acting on the bed (Allan Eq’n 3.5): t = rgRS r is water density, g is gravitional force R is hydraulic radius, which is ~ mean Depth S is gradient R IS HYDRAULIC RADIUS WHICH IS X-SECTIONAL AREA DIVIDED BY WETTED PERIMETER. FOR CHANNELS MUCH WIDER THAN MEAN FLOW DEPTH, DEPTH IS GOOD ESTIMATE

So, shear stress acting on streambed is Directly proportional to Mean Depth Channel Slope (or gradient) As shear stress increases, what happens to stream “competence”? Factors that dictate changes in depth and slope with increasing Q? DDepth: depends on channel “constraint” DSlope: depends on channel bedform

DDepth: depends on channel “constraint” (Fig. 4.15 Knighton) Constrained channel Depth increases rapidly with Q energy doesn’t dissipate laterally Unconstrained channel (e.g., with floodplain) Depth increases to fill channel Higher flows spill onto floodplain (energy dissipated laterally) For same Q and slope, which channel type has greatest competence?

DSlope: depends on channel bedform (riffle v. pool) Which has greater shear stress at LOW FLOW? (Fig. 1.12) At low flow, velocity, slope, and shear greater in Riffle Pool has greater depth What about during flood flows? Depth? Velocity? Slope? Shear Stress? Sediment transport in riffles vs. pools At low flow, pools are depositional At flood flow, pools are erosional coarse grains move through pools, deposit at head of riffles as flood recedes

Bankfull discharge (Qbkf), an important concept Fills the channel and does work on boundaries "Dominant discharge” Greatest total volume of sediment moved Frequency-Magnitude Concept (Fig. 3.10, text) Magnitude sediment load Freq. of Q The highest flows are not the “dominant discharge”! Suspended Bedload For suspended: Qd is ~Qbkf a “channel forming flow” Peak discharge that occurs on average once in 1.5 - 2 years Range 1-10 years, depending on channel type Applies best to gravel-cobble rivers with floodplains