1. 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
single channel types
Straight (Sinuosity << 1.5)
Meandering (Sinuosity > 1.5)

2. 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 photo ixvt9i

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

4. Meander scaling laws

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

6. 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

7. 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

8. Perspective of Drainage Pattern
Reflect geology mostly

9. 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

10. Scaling Relations (Fig. 2.1 Knighton)

11. 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

12. (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)

13. 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)

14. 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?

15. 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

16. 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

17. 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

18. 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?

19. 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

20. 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 years
Range 1-10 years, depending on channel type
Applies best to gravel-cobble rivers with floodplains