1. Module 5: Morphology, Facets, Ratios, and Stream Classification
Iowa’s River Restoration Toolbox Level 1 / Base Training

2. Fluvial geomorphology
Defines common forms of streams in plan, profile, and cross section
Describes processes (driven by flow regimes, lithology, sediment supply, land-use change, etc.) that influence channels and floodplains.
Fosters understanding that helps project work with, not against, the stream
Morphology, more simply, refers to the shape of a river from three views: Plan, profile, and dimension

3. Valley Types
Confined valleys result in lower sinuosity, step-pool channel formations, often with somewhat higher bed slopes.
Unconfined valleys result in higher sinuosity, alluvial channel formation lower slopes
Images from USDA Technical Supplement 3E, Rosgen Stream Classification Technique

4. Valley Length, Width, Slope
Length is “Big Picture” or “Crow Flies” alignment, not including meanders.
Width of the valley is where the stream’s boundaries occur, usually the edges of it’s meanders.
These dimensions derive from geological factors, such as faults, bedrock, glaciation, ancient lakebeds, etc.
They also derive from recent geomorphology history, such as abandonment of a terrace.

5. Stream Order Two of same order joined makes next highest order
Density affected by annual rainfall and geology
Affects applicability of reference reaches

6. Pattern measurements Point of Tangency (Pt) Belt Width (Wblt)
Point of Curvature
(Pc)
Arc Length
Channel
Distance
is on
Centerline
(Ch)
Radius of
Curvature (Rc)
Amplitude
(Am)
Linear
Wave
Length (λ)
Meander Length (Lm)

7. Sinuosity = Sinuosity Channel Distance Down Valley Length Channel
is on
Centerline
(Ch)
Down
Valley
Length
Ratio value always greater than 1
Channel Distance
= Sinuosity
Down Valley Length

8. Thalweg Thalweg is the energy gradient water of the stream.
Channel
Distance
is on
Centerline
(Ch)
Thalweg is the energy gradient water of the stream.
Survey the deepest point in the cross section.
Do not use Thalweg for stationing! It’s inconsistent.
Thalweg (Tw)

9. Facet Terminology Step-pool system Alluvial System Step (riffle)
flow
Step-pool system
Step (riffle)
Pool (scour pool)
flow
Alluvial System
Point bar
Riffle
Run
Glide
Pool

10. Facets in the Longitudinal Profile
Riffle
Pool
Glide
Run
Bankfull Slope
Low Bank Height
Degree of Incision Wedge

11. Slope… … is the ratio of fall over distance of stream
Calculations are most accurate using water surface(WS) slope from facet to facet
Bed slope (aka energy slope) goes from top of glide to top of glide
BKF Slope=.00233
Elevation (feet)
WS Slope=.00238
Bed slope =
Distance (feet)
Bankfull (BKF) slope is taken from field-observed calls
All three slopes should be roughly parallel in a stable stream.

12. Cross sectional area
Rivers and streams vary in width and depth. Pools tend to be deep and wide. The riffles tend to be the narrowest and shallowest.
It is important to run hydrologic calculations at the riffle(s), because it is the “pinch point” for flow at bankfull.
Pool (outside bend)
Riffle (straightaway)
Bankfull
Typical riffle XS
Typical pool XS

13. Cross sections Measured perpendicular to bankfull flow path / valley
Defines the flow path, and cross sectional area of water at various depths
Important for calculations, design, permitting
Important for categorizing streams into “like” groupings
Low terrace
Low terrace
Bankfull

14. The multi-stage channel
Critical stages to investigate are:
Base flow (depth maintained for habitat in draught)
Inner berm (important for base depth and sediment transport)
Bankfull floodplain (critical to dissipate shear stress through vegetation)
Broad floodplain (for the “really big” floods, 50-year+)

15. Why all this focus on bankfull stage?
Radically reduces shear stress per unit of discharge at a key moment for stability.
Defines difference between “channel” and “floodplain”
Consistent discharge at which to aggregate data (i.e. “normalizing factor”
Useful bodies of work (regional curves, classifications, etc.) are already built around bankfull stage

16. Hydrologic models
Can be useful to look in office prior to field work to calibrate.
Iowa Flood Center’s IFIS tool has 2-year versus 50-year inundations maps for larger streams
Upper Iowa River, Below Lower Dam

17. Inner berms
In rain-driven systems (i.e. Iowa), sometimes confused with BKF because can form vegetated flats
Iowa Flood Center’s IFIS tool has 2-year versus 50-year inundations maps for larger streams
Bankfull
Inner berm

18. Inner berms
More prominent in lower drainage area streams (see Thomas Gardy paper)
Shape has implications for sediment transport and base flow depths
Note vegetated low flat well below Qbkf in this urban Des Moines stream

19. Why classify streams?
To keep fewer numbers and words in your head at once
To better understand stable ranges for a given type
To study processes that lead to form
To better apply design
Better communication

20. Entrenchment ratio Width of River’s Floodprone Valley
Width of Bankfull Channel at the Riffle
Floodprone Width is the width of the valley at an elevation at double the Max BKF Depth
Wfp is sometimes said to approximate the 50-yr flood stage, (i.e., A Really Big Flood)

21. Entrenchment ratio
Mathematical expression of how much floodplain a stream can access.
For example, the stream to the right does not access a floodplain at BKF discharge.

22. Width/Depth ratio Width of River’s Bankfull Channel at the Riffle
Mean Bankfull Depth at the Riffle

23. Width/Depth ratio
Expresses how shallow or deep a channel is at BKF discharge.
Low ratio indicates a deeper, narrower channel.
High ratio indicates a shallower, wider channel

24. BKF Width / Mean BKF Depth
Classifying streams
Classification A B C E DA D F G
Entrenchment ratio*
<1.4
>2.2
>4.0
n/a
FP width / BKF Width
BKF Width / Mean BKF Depth
W / D ratio*
<12
>12
<40
>40
Typical sinuosity
1 -1.2
>1.2
>1.5
Not predictive
Well-defined
point bars
Veg. islands
Heavy veg.
Both banks erode in wide dirt canyon
Confined, u-shaped valley
Often bedrock / log controlled
Visual Cues
Gully in soft materials
Sed. islands
Typical slope
<.02
<.005
<.04
Typical stability
Usually Stable
Likely Unstable
Classifications from Rosgen, 1996

25. “B” Channels “A” Channels
Often somewhat steep
U-shaped channel
Skinny floodplains
Tend to be riffle dominated
“A” Channels
Steep rocky channels
Most often controlled by bedrock or logs

26. “C” Channels Broad floodplains Point bars on inside bends
Moderately sinuous

27. “E” Channels Easy access to broad floodplains Tight, deep channel
point bars on inside bends
Extremely sinuous

28. “DA” Channels Steady high sediment supply
Vegetated islands promote braided channels with power to transport sediment, leading to a steady- state
Deltas into impoundments

29. “D” Channels High sediment supply
Aggrading channel due to overwide conditions
Braided channels through unvegetated islands

30. “F” Channels Widening channel Entrenched; no BKF floodplain connection
Channel bottoms often very uniform fine sediments in Iowa

31. Photo from John Thomas, Hungry Canyons Alliance
“G” Channels
Gully channel
Bed degrading, plainly seen by knick points / head cutting
Highly entrenched
Photo from John Thomas, Hungry Canyons Alliance

32. Categorize materials with pebble counts
Randomly select a particle from a stream through a reach
Measure medial diameter and record
Repeat 99 more times
Determine the size that 50 percent of the samples are small than. This is your “D50”
Medial diameter is along the intermediate access. It is the limiting dimension preventing it from fitting through a sieve.

33. Classification with bed types
Use pebble count to statistically determine D50 (meaning, 50 is percent finer than that width)
Bed number is assigned, and combined with stream type to quickly convey channel shape, floodplain connectivity, and dominant materials
Bed #
Materials Category
Diameter range (mm) 1
Bedrock
N/A 2
Boulder 3
Cobble
64-256 4
Gravel
2-64 5
Sand
.062 to 2 6
Silt/Clay
<.062

34. Examples of meanings inferred
B3: Moderately entrenched step pool stream with cobble bed
C4: High width/depth ratio stream with good access to floodplain, likely sinuous with point bars
E6: A deep, narrow stream with a bed, cohesive silt-clay bed, plenty of access to a floodplain, probably heavily vegetated