PART IV:
CHANNEL
PATTERNS
Channel Classification Systems
1) sequence with respect to structure
consequent
subsequent
obsequent
2) sequence with respect to time
antecedent
superimposed
3) plan-view shapes of drainage system
dendritic
complex
parallel
compound
4) Davis’ Cycle of Erosion
youthful
5) flow duration
mature
rectangular
old
ephemeral
perennial
intermittent
seasonal
bed-load
mixed-load
suspended-load
eroding
stable
depositing
6) Schumm’s % silt-clay (M) in channel boundary
7) regime
Knighton
8) boundary resistance
cohesive
9) planform regularity
braided
anastomosing
non-cohesive
meandering
10) number of flow paths in a reach
single channel
multi-channel
11) Montgomery & Buffington’s bedforms
cascade
pool-riffle
step-pool
dune-ripple (regime)
straight
compound
plane bed
12) Rosgen
multiple types
There is no generally accepted classification system, and
any individual channel can be described by several different
names, depending on the classification system used.
Montgomery &
Buffington,
1997
step-pool
cascade
pool-riffle
plane bed
Rosgen (Rosgen, 1994)
gradient
bed material
width/depth ratio
sinuosity
lateral constraint
The form of multiple channels is partly stage-dependent.
1) braided – multiple flow paths within a single channel
2) anastomosing/anabranching – multiple channels
Braided?
Anastomosing?
rivers on Alaskan coastal plain
The most commonly used classification is that of straight,
meandering, and braided, interpreted through a continuum –
channel pattern is controlled by interactions among a set
of continuous variables, so all channel patterns intergrade
The three basic types of patterns can be related to each
other by variations in stream power, sediment load,
boundary composition or stability, or slope-discharge
Progression from straight to meandering to braided
corresponds to
i) increasing width-depth ratio
decreasing bank stability
increasing bedload
ii) increasing stream power – which means increasing Q at
constant slope, or increasing slope at constant Q
iii) increasing sediment load, particularly bedload
Appears to be rapid change at thresholds
straight
meandering braided
anastomosing
*
0.05
BRAIDED
0.01
0.005
*
*
0.001
0.0005
0.0001
0.00005
s = 0.012 Q-0.44
**
*
MEANDERING
5
50
10
100
*
* *
500
1,000
+
*
5000
50,000
10,000
100,000
landform
instability
BANKFULL DISCHARGE (m3/s)
braided
island
braided
multichannel
straight
10-1
meandering
s = Fd/w
braided
-
10-2
single channel
(straight)
+ -
10-3
braided transition meandering
10-4
-
10-5
10-4
10-3
10-2
transition straight
10-1
(Width:Depth Ratio)-1 (d/w)
100
single channel
(meandering)
hydrogeomorphic
disturbance
catastrophic change
(bifurcation)
0.1
+
CONTROLS ON CHANNEL ADJUSTMENT
Dominant controls on channel form adjustment are
discharge of water & sediment
independent variables integrating effects of climate,
vegetation, soils, geology, & basin physiography
Because of the complexity & multiple feedbacks involved,
one of the first & most important steps is to choose a
suitable timescale on which to study the physical
relationship of interest
Important to define time span of interest because the
relationships between the variables change with time
(e.g., valley slope)
TIMESCALES
instantaneous or steady time
(< 10-1 years)
short time scale
(101 – 102 years)
graded time –
medium time scale
these are the most
3
4
(10 – 10 years)
relevant for channel
long or cyclic time
form adjustment
5
(> 10 years)
Schumm & Lichty (1965): time, space and causality
the distinction between cause & effect in landform
development is a function of time & space (area) because
the determining factor can be either a dependent or an
independent variable
during a long time period, a drainage basin may be an
open system progressively losing energy & mass (as
envisioned in Davis’ erosion cycle),
whereas over shorter timespans, self-regulation is
important, & the basin may be graded, or in dynamic or
steady-state equilibrium
these concepts are not mutually exclusive
Adjustments to the internal geometry of the fluvial
system involve the variables of flow properties & sediment
properties, including channel form
Distinguish between
1) channel geometry: the 3d form of the channel over longer
time periods responding to the dominant discharges of
water and sediment
2) flow geometry: interactions among variables during
temporal changes in discharge, emphasizing crosssectional and reach scales
A stream must satisfy three physical relations in
adjusting flow geometry
1) continuity
2) flow resistance
Q=wdv
e.g., Manning: v = 1.49 R0.67 S0.5
n
3) a sediment transport equation
HYDRAULIC GEOMETRY
developed by Leopold and Maddock (1953)
assumes that water discharge is dominant independent
variable, & that dependent variables are related to Q in
the form of simple power functions
v = k Qm
S = g Qz
w = a Qb
d = c Qf
Q = w d v = a Qb c Qf k Qm
ack=1
b+f+m=1
technique is applicable to at-a-station & downstream
adjustment
neglects sediment transport
assumption of linearity is not quite adequate, but this
represents a way to break into a system with more
unknowns than independent equations
There have been attempts to quantify the equilibrium
cross section on the basis of cross-sectional parameters
(regime theory), & to explain downstream variations in
the channel
downstream hydraulic geometry: treats spatial variations in
channel properties at a reference discharge
at-a-station hydraulic geometry: deals with temporary
variations in flow variables as discharge fluctuates at a
cross section
Maddock (1970)
impossible to establish determinate relations for the
solution of the dual problems of resistance to flow &
sediment transport in alluvial channels
the best we can is to predict general patterns of stream
behavior
Graf (2001): the probabilistic river (probable patterns
of behavior)
Channel geometry: 3d entity; adjustment to external
controls can be considered in terms of four degrees
of freedom
cross-sectional form
bed configuration
planimetric geometry or channel pattern
channel bed slope
original Lane’s balance
expanded Lane’s balance
105
climate,
tectonics
104
profile
gradient
Increasing length scale, m
land use,
river engineering
profile
concavity
103
reach
gradient
102
meander
wavelength
bed configuration:
gravel-bed streams
101
channel
width
100
bed configuration:
sand-bed streams
10-1 10-1
channel
depth
profile
form
plan form
cross-sectional
form
100
101
102
Increasing time scale, years
103
104
Boulder knickpoint
Bedrock knickpoint
Sand bedforms
downstream variations f (Q, Qs)
e.g., flashy regime + high peak flows = wider channels
coarser load = wider, shallower channels
channel size may be adjusted to Qs, and channel shape to
the type of load (thus Schumm’s M classification)
in general, dominant controls on cross-sectional form are
discharge
absolute & relative amounts of bedload transport
composition of channel boundary/bank stability
multivariate character of these controls implies that the
approach of downstream hydraulic geometry can only
indicate trends – equilibrium
(underlying downstream trends are random & systematic
local fluctuations in xs form such as pools & riffles)
Equilibrium
Dictionary: Equilibrium exists in any system when the phases of the
system do not undergo any change of properties with the passage of
time, and provided that the phases have the same properties when the
same conditions, with respect to the variants, are again reached by a
different procedure.
The first part of this definition is most applicable to fluvial geomorphology.
Idea of equilibrium first introduced to geomorphology by
Gilbert (1877), then modified by Mackin (1948), Leopold &
Maddock (1953), and Wolman (1955)
There is no exact equilibrium, but rivers tend to develop an
average behavior
Descriptions of fluvial geomorphic equilibrium concentrate on
adjustments to cross-sectional form & channel pattern,
rather than on channel profiles
Equilibrium also implies feedback mechanisms & thresholds
Types of Equilibrium
static
increasing
time
State of System
steady state
dynamic
Time
dynamic
metastable
Another way to express the ideas embodied in
equilibrium uses a method analogous to thermodynamics &
entropy – this defines a most probable state, which is a
balance between the uniform distribution of energy
expenditure & minimum total work: minimum variance
Both minimum variance & equilibrium are controversial --no universally-accepted set of criteria for determining
equilibrium
importance of transient fluctuations vs
long-term trends depends on timescale of interest
persistence & characteristic form times differ between
rivers & between components of a river
Minimum Variance Theory
1) minimize variance of selected variables at different
sections along a reach, and
2) minimize the sum of the squares of the exponents in the
hydraulic geometry relations
Theory proposed by Leopold & Langbein when they stated
that streams exhibit opposing tendencies which must be
balanced –
tendency toward equal energy expenditure
(constant stream power per unit area of bed)
and
tendency toward minimization of energy expenditure
over length of river
L
(total power, ∫ Ω dx
0
minimum)
Undulating wall example
Wohl et al., 1999
Extremal Hypotheses
Langbein & Leopold (1964): balance minimum total rate of work vs
uniform distribution of energy expenditure
Yang (1971): minimum energy dissipation rate
Davies & Sutherland (1980): maximum friction factor
Grant (1997): maintenance of critical flow in steep channels
Eaton et al. (2004): greatest relative stability achieved by
maximizing resistance to flow in the fluvial system
Nanson & Huang (2008): least action principle of how rivers adjust
toward conditions that minimize change & maximize operational
efficiency
Lag times – response of physical systems tends to lag behind
changes in process intensity
Different components have different lag times, depending on
1) resistance to change by various morphologic components
& the system as a whole
(e.g., bedrock is more resistant than a sand bed)
2) complexity of the system – the number of components
involved and the character of their interactions
3) magnitude & direction of the input change, which
reinforces or conflicts with existing tendencies
4) energy environment of the input
(e.g., headwaters are more responsive than the mouth)
Transient form ratio for persistence
TF = mean relaxation time
mean recurrence time
TF > 1, forms transient
TF < 1, forms persist
character
Threshold-equilibrium plot
T
agg.
E
altitude of floodplain, as
modeled by Bull
deg.
E
time
Conceptual models encourage studies of changes & of
self-enhancing feedback – easy to note passing of
threshold, but difficult to exactly predict threshold
because difficult to quantify driving & resisting forces
power of paradigms
Dominant discharge: can be defined as the flow which
determines channel parameters (geomorphic effectiveness),
or the flow which transports the most sediment
(geomorphic work)
geomorphic effectiveness
(persistence of features relative
to recurrence interval of flow)
geomorphic work
(sediment transport)
EW
EW
grad
student
What type of flow “controls” a river? Varies for different
climatic & topographic environments, & because of
uncertainty as to what constitutes control
Sediment transport rate
Frequency of occurrence
Product of magnitude + frequency
Dominant discharge defined in terms of magnitude and
frequency of sediment transport by a given range of
discharges (Wolman and Miller, 1960)
Discharge
Qd
Originally, dominant discharge was equated with a bankfull
discharge of 1-2 year recurrence interval on humid
temperate streams
This approach is limited because
bankfull channel can’t always be defined in field
Qb is not of constant frequency
channel form parameters don’t necessarily correlate
best with Qb
Qb may not be the most effective flow for sediment
transport
whole idea doesn’t work well for highly variable flow
regimes found outside humid temperate regions,
such as deserts or seasonal tropics
[problems with respect to channel restoration]
Channel form is not the
product of a single formative
discharge, but of a
range of discharges & of the
temporal sequence of flows
instream flow
(e.g., Boulder Creek, 400 vs
50 m3/s; O’Connor et al., 1986)
channel maintenance flow
role of floods
(e.g., Grand Canyon rapids;
Kieffer, 1985, 1989)
Streams affected by floods have flashy hydrographs
produced by
high relief
high drainage density
equant basin shape
high frequency of first order streams
high basin magnitude
sparse vegetation
thin soils
Also characterized by
high channel gradients
abundant coarse bedload
relatively low bank cohesion
channel cross sections which enable flood discharges
to be accompanied by deep, high velocity flows
where macroturbulent flow phenomena exist
Kochel, 1988
Big Thompson River, Colorado
1976 flood
Patton (1988): Drainages in southern New England
for drainages in the highlands, sedimentation during floods
is an important process in constructing fluvial landforms
such as floodplains
erosion & deposition are localized, vegetation can mask the
flood effects, & moderate flows can re-work the floodcreated landforms, but periods of stasis & soil formation
are interrupted abruptly by major floods that add much
sediment to the floodplain surfaces (14C)
in lowland drainages, in contrast, flood deposits tend to be
completely reworked by moderate flows (original study in
1960)
Persistence vs recurrence
the persistence of landforms vs
the recurrence interval of a given discharge value
Regression models expressing some channel parameter
as a function of some discharge only apply to regime
or steady-state rivers – cannot describe a mean
behavior where large discharge fluctuations occur
Burdekin & Herbert rivers and Nepal GLOFs as examples
(Wohl, 1992a, b
Cenderelli & Wohl, 2001, 2003)
Herbert River, Australia
Bhote Kosi
1985 GLOF
Cenderelli, 1998
BED CONFIGURATION
Bedforms
means of adjustment in vertical dimension (related to
horizontal adjustments)
effect of instability at water-sediment interface
indicate systematic tendencies in the ability of natural
systems to sort & transport material over a wide
range of flow & bed material conditions
Bedforms
geometry depends on prevailing flow, sediment discharge,
& bed material properties
represent both response to changing discharge & load
conditions, & means of regulating, through resistance,
hydraulic variables
exert a drag on the flow additional to that of the grains –
bedform adjustment represents both a response to
changing discharge & load conditions, and a means of
regulating, through resistance, hydraulic variables
such as velocity & depth
Pool and riffle sequence
alternating deeps & shallows
occur in straight and meandering channels
successive pools spaced approx. 5-7 X channel width
riffles:
wider, shallower at all stages of flow
coarser materials
steeper water-surface at low flow
deposition at high flow
(velocity reversal?)
Thompson et al., 1996, 1998, 1999
North St. Vrain Creek,
CO
n CA
North Fork Poudre River, CO
Knighton
Explanations for the occurrence of pools & riffles include
kinematic waves
dispersion & sorting
convergence & divergence of flow
macroeddies in turbulent flow
essentially, a pool & riffle sequence is a means of selfadjustment in streams –
it is important in the attainment & maintenance of
quasi-equilibrium, and for the development of
meandering
pools & riffles and meandering are two sources of flow
resistance capable of modifying the rate & distribution
of energy loss at the reach scale
• velocity-reversal hypothesis(Keller, 1971) remains difficult to demonstrate,
but formation of a central jet in pools (Thompson et al., 1998, 1999) likely
to be important
Figure S4.12
eddy
jet
vortex
recirculating eddy
pool length
jet
shear
zone
main
flow
Keller & Melhorn (1978): Rhythmic spacing and origin of
pools and riffles
pools & riffles = meandering in the 3rd (vertical) dimension
fundamental characteristic of many streams, independent
of substrate type
70% of variability of spacing of pools can be explained by
variability of channel width
Roy & Abrahams (1980), commenting on Keller & Melhorn
separated bedrock & alluvial rivers from the K&M dataset,
found that mean pool spacing of bedrock streams is
greater
could be because bedrock channel forms are adjusted to
discharges of higher magnitude & lower frequency, or
because spacing is related to sediment size in transport
Lisle (1986)
large obstructions (bedrock outcrops, instream wood, rooted
bank projections) & bedrock bends stabilize
the location & form of gravel bars
large obstructions and bends cause intense secondary
circulation in scour holes that terminate upstream
bars at fixed locations
bars are deposited upstream from large obstructions due
to backwater reductions in stream power; they are
deposited downstream because flow energy is
expended around obstructions, & flow expands
upstream
85% of pools are next to large obstructions or bends
Lisle (1982)
studied effects of Dec. 1964 flood on 12 rivers in n CA
hydraulic changes with aggradation indicate an increase
in the effectiveness of moderate discharges (1-2 year
recurrence interval) to transport bed load & shape the bed
bars become smaller, pools preferentially fill, & riffles
armored with relatively small gravel erode headward
during the falling stages & form a gentler gradient
excess sediment can thus be more readily transported out
of channels
degraded reaches show higher percentage length of channel
as pools, & lower percentage as runs than do aggraded
reaches
step-pool channels
alternating vertical steps (clasts, wood, bedrock) and
plunge pools (Chin & Wohl, 2005; Church & Zimmermann, 2007)
S > 0.02
1 < H/L/S < 2 (Abrahams et al., 1995)
anti-dune hypothesis
(Whittaker & Jaeggi, 1982)
jammed state hypothesis
(Zimmermann & Church, 2001)
Estimated recurrence interval for step formation varies from
annual in Japan (Sawada et al., 1983) to 30-50 years in Italy
(Lenzi, 2001) to > 50 years in US Cascades (Grant et al., 1990)
nappe vs skimming flow
dramatic decrease in
flow resistance &
increase in velocity
(Comiti et al., 2009)
• bedload transport is extremely spatially & temporally variable
• Yager et al. (2007) proposed a modified version of the
Parker (1990) bedload equation to include the resistance
associated with steps & selective transport of relatively
mobile sediment using a range of hiding functions
qsm* = 5.7 (τm* - τcm*)1.5 (Am/At)
qsm*
τm*
τcm*
Am
At
dimensionless transport rate of mobile sediment
dimensionless stress borne by the mobile sediment
dimensionless critical shear stress of the mobile sediment
bed-parallel area of mobile sediment
total bed area
• particles in pools are preferentially entrained & transported
longer distances
• step-pool channel segments are transport reaches that are less
sensitive to changes in water & sediment discharge
(Montgomery & Buffington, 1997; Ryan, 1997; Wohl & Dust, 2012)
pool-riffle sites with
augmented flow
1.80
1.60
Corral 1
Corral 2
Corral 3
Bankfull depth (m)
1.40
Hague 1
Hague 2
1.20
Hague 3
1.00
La Poudre Pass 1
La Poudre Pass 2
0.80
La Poudre Pass 3
0.60
La Poudre Pass 4
0.40
La Poudre Pass 5
step-pool sites with
augmented flow
0.20
0.00
0
10
20
30
40
50
A (km 2)
60
70
80
Poudre 1
Poudre 2
Poudre 3
90
100
“THE GREAT GRAND CANYON DEBATE”
Leopold (1969): occurrence of rapids & pools represents a
state of quasi-equilibrium, independent of bedrock type
& valley characteristics
Dolan et al. (1978): pools & rapids in the Canyon are located
where the river crosses regional & local fracture zones
Graf (1979): Colorado Plateau rivers – spacing of rapids in
canyon rivers is random, & local-site conditions are more
significant than canyon-long operations of the main river
system
Webb et al. (1988): large boulders transported into the
Colorado River by debris flows along tributary canyons
create or change hydraulic controls (rapids) – controls are
governed by the magnitude-frequency of tributary flows,
& by history of discharges on Colorado River
Channel Form Adjustment
occurs in the horizontal plane
influences resistance to flow, & is an alternative to slope
adjustment when slope is constant on short & medium
time scales
compound
meandering
bank
resistance
braided
sinuosity
or d/w
stream power
A catastrophe theory representation of the interaction of
the controlling variables stream power & channel slope with
a responding variable, channel configuration as measured
by sinuosity
Compound channels occur in the folded portion of the
diagram where two possible states exist for given
combinations of power & slope (Graf, 1979a, 1988b)
Compound Channels
e.g., series of low gradient, anastomosing, savanna streams
in Australia – two theories
1) Rust (1981): the sand underlying the clays indicates a
former braided system
2) Nanson et al. (1986): the clay-bed anastomosing streams are
a modern phenomenon in which the clays dry out,
aggregate, & are transported as bedload by high flow,
braided rivers until they dissolve & are transported
by low flow, anastomosing rivers (deeper, with
lower w/d ratios)
rip-up clast, Buckskin Gulch, AZ
e.g., Graf (1988a) noted that arid region rivers frequently
accomplish radical adjustments to extreme events by
complete changes of channel configuration – paths are
1) gradual infilling during sustained periods of low flows,
causing a change from braided to meandering
configurations, &
2) rapid erosion during rare floods causing a change from
meandering to braided
these changes present problems in defining the floodplain
for land-use zoning
e.g., Fahnestock (1963)
White River of Mt. Rainier, WA – glacial stream
marked change from meandering to braided occurred with
onset of high summer flows, & pattern returned to
meanders with the low flows of autumn
Fahnestock concluded that both braided & meandering
stretches can occur along the same stream, which
may be aggrading, stable, or degrading – pattern
alone does not conclusively define the regime of the
stream
no slope change between meandering & braided – braiding
occurred because of coarse load, which when
deposited caused diversion to other channels
rate of pattern change is related to amount of bedload
e.g., Gupta and Dutt (1989), Auranga River, India
seasonal tropics
during low flows of dry season, active channel is a
relatively narrow braided reach flanked by exposed
point bars &, above and beyond, flood bars
point bars are covered by bankfull wet season flows,
when the river assumes a meandering form
flood bars are covered by episodic large floods
MEANDERS
river on Alaskan coastal plain
Yukon River in Alaska
Wind River Range, WY
Chena, Alaska – meandering & permafrost
North Park,
CO
Wood River, Alaska
the interrelation of channel width, radius, & wavelength of
meanders is only one of several shape characteristics
that show a tendency toward energy conservation
meanders tend to approximate a sine-generated curve
θ = ω sin S/M 2 ∏
θ = angle between the direction measured at a given point along the
curve & the mean downstream direction
ω = maximum value of θ
S = distance along the path
M = total path distance in a unit wavelength
sine-generated curve represents the most uniform
distribution of change along the curve – minimization of
the sum of the turning angles will tend to minimize the
total work of erosion on the bank
this minimization is in opposition to the uniformity
represented by equal angles of deflection, provided the
curve were in the form of a circle – minimized variance
(least work) is approached at the cost of less uniformity
Leopold, 1994, A View of the River
Leopold, 1994, A View of the River
Meandering rivers in Alaska
Yukon River
Leopold, 1994, A View of the River
San Juan Goosenecks, UT
Paria River, UT
Natural Bridges
National Monument, UT
meanders incised
into bedrock
Why meander?
1) flow properties
flumes show that helicoidal flow & secondary currents
create meanders
flow pattern through meander bends involves superelevation
of water against the outer bank, with a secondary
transverse cell toward the outer bank at the surface &
the inner bank at the bed
OUTER
high
high
*
*
low
*
*
low
low
*
*
high
INNER
BANK
REGION
MID-CHANNEL
BANK
REGION
REGION
Super-elevated
water surface
Outward shoaling flow
across point bar
path lines of secondary flow
* relative water surface elevation
break in bed slope
maximum velocity is toward the inner bank in the upstream
limb of the bend, & then below the surface around the
curve – so there is erosion at the outside of the bend &
deposition on the inside
explanations of meanders focusing on flow properties regard
helicoidal flow & secondary currents as inherent properties of
turbulent flow, which are manifested in meanders as
deformable boundaries permit observation of the underlying
wave-like structure in flows
near-surface velocity
near-bed velocity
bar exposed
at low flow
thalweg
break in
bed slope
2) mechanics of sediment transport
secondary flow is the result, rather than the cause, of
meandering
mobile bed of a channel with stable banks is unstable –
instability occurs as alternating bars which grow in
amplitude & form meanders – in this views, sediment
transport is the essential factor
other explanations include Chang’s minimization of stream
power, but meanders are not necessarily the outcome of
a single cause
meandering is mainly a means of slope reduction for given
external constraints
Mattole River, n CA
meander belt, Missouri River near
St. Louis
meandering channel scar,
Rocky Mountain N.P.
meanders
North Park, Colorado
rivers on Alaskan coastal plain
Braided rivers
high width-depth ratios
steep slopes
large bedloads
rapid shifts in channel position at high discharge
geometrical properties of braided rivers not as well studied
as those of meandering rivers, partly because degree of
braiding changes with discharge (bars covered at high
stages)
similarly, a characteristic sedimentary sequence equivalent
to the fining-upward sequence of meander point bars has
not been defined for braided rivers, partly because of the
high sedimentary variability
braided rivers occur in a range of climates, from proglacial
to arid, & at a range of scales
central AZ
Arid-region braided rivers
1969 flood boulder train, Jordan River
Death Valley, CA
coastal Peru
Glacial braided rivers
Annapurna region, Nepal
Jasper National Park,
British Columbia
Matanuska River, Alaska
northeastern Brooks Range,
Alaska
initiation of braiding involves longitudinal bars, followed by
transverse & point bars, & islands
braiding, like meandering, is interpreted as a mode of
adjustment to modify the stream’s energy use
braided rivers are the default river morphology
Yukon River, central Alaska
anabranching
meandering
Platte River, Nebraska
Probabilistic River
Summary of methods used to convert
air photos to a locational probability map
(Graf, 2000, Figure 5)
Examples of locational probability maps for
Salt River near Phoenix based on 1935-1996
data
(Graf, 2000, Figure 9)
anastomosing – low sinuosity channels that split & rejoin
anabranching – high sinuosity channels that split & rejoin
reticulate – finer scale channels that split & rejoin
Glacial braided rivers
Annapurna region, Nepal
Jasper National Park,
British Columbia
Matanuska River, Alaska
Aggradation above tributary
junction, Khumbu, Nepal
upper Amazon basin, Ecuador
Mt. St. Helens
area, WA, 1997
braided rivers on coastal plain near
Kotzebue, Alaska
Biotic Influences on Channel Form
Riparian vegetation
roots increase soil cohesion by increasing shear strength
of soil via root reinforcement
decreases bank stability when weight of vegetation
increases driving forces acting in downslope direction
above-ground portion of vegetation intercepts
precipitation & decreases infiltration that can decrease
bank strength, as well as removing water from root zone
above-ground portion increases hydraulic roughness on
banks & floodplains & facilitates sediment deposition
Rio Puerco, NM: moderate to densely vegetated banks =
40% reduction in perimeter-averaged shear stress and
20% reduction in shear stress in channel center
(Griffin et al., 2005)
riparian vegetation can cause a braided channel to
self-organize to, & maintain, a single-thread channel
(Tal & Paola, 2010)
Riparian vegetation
bank erosion can isolate rootmass of tree, creating scalloped
bank that alters near-bank hydraulics & habitat
(Rutherfurd and Grove, 2004)
Riparian vegetation
spatial changes in vegetation can alter downstream hydraulic
geometry relationships (Huang & Nanson, 1997) and
channel type
different types can influence stream response to augmented
peaks flows associated with snow-making (David et al., 2009)
Aquatic & riparian animals
microbial biofilms & fine sediment
macroinvertebrates & silk
fish & nests or redds
crayfish & clast displacement
Beaver dams
pond water
accumulate sediment & organic matter
enhance extent, frequency & duration of overbank flooding
reduce bed & bank erosion (Pollock et al., 2007)
increase habitat diversity & stability (Naiman et al., 1988)
facilitate formation of multi-thread channels & floodplain
wetlands (Westbrook et al., 2006)
Channel gradient & longitudinal profile
H = f (L)
longitudinal profiles tend to be concave upward, although
they are rarely smooth, & may contain evidence of past
events in their irregularities
at a constant cross-sectional shape, the controls on channel
gradient are Qs, φs, and Q (inversely)
generally, steep slopes reflect coarse sediment, a small
drainage area, & a wide, shallow channel
concavity is also affected; profiles are more concave where
bed-material size decreases rapidly, & they don’t have
much concavity if particle size is constant or increasing
downstream
increasing discharge also produces concavity, because an
increase in discharge means that the same load can be
transported over lower slopes
The adjustment of slope to sediment load is expressed in
the concept of the graded stream, as proposed by
Mackin (1948):
graded alluvial streams with a stable flow regime have
slopes just sufficient to transport the load supplied, &
have a smoothly concave profile
increasing discharge & decreasing bed material size provide
a general explanation of profile concavity, but the profile
reflects adjustments to many variables whose interaction
in different combinations leads to a wide variety of profile
forms: another example of a system with more dependent
than independent factors
knickpoints, for example, may indicate lithologic controls,
or they may show progressive upstream erosion due to
baselevel drop or other tectonic changes
Hack
equilibrium landscape development: all aspects of the
landscape are mutually adjusted so that they downwaste
at the same rate
stream gradient index allows comparison of slope between
streams
SL = ∆ H L/ ∆ L
∆ H = reach fall
∆ L = reach length
L = distance from drainage divide
influenced by climate
tectonics
relief
stream regimen
geomorphic history
Example of application of stream gradient indices
Merritts & Vincent (1989) examined a series of rivers along
coastal northern California, in region of Mendocino Triple
Junction, where 3 tectonic plates meet
25 basins spread over areas of
low (< 1 m/ky)
intermediate (1-3 m/ky)
high (> 3 m/ky)
rates of uplift
using stream gradient index & a variety of other geomorphic
indices, they found channel gradients to be the best
indicator of tectonism in the landscape
lower order tributaries best reflect tectonically-controlled
differences because large streams are able to adjust to
base level changes & maintain their profile form, whereas
lower-order streams farther upstream accumulate the
effects of net base-level fall & have the steepest profiles
in the areas of highest uplift rates
semi-log plots
arithmetic plots
low uplift
intermediate uplift
high uplift
Other work
Goldrick & Bishop (2007)
main channel flow
tributary knickpoint, Wulik River, sw Brooks Range, Alaska
Knickpoints & hanging valleys
Prediction of Shang, the channel gradient at which incision rate falls below
the rate of base level fall & permanent hanging valleys form
(Crosby et al., 2007):



1



1bc1 / 4
1


1
1 2 S t K '  A
S hang  2
 cos cos 
3S t
1
3


3



27
S
t







 4 
   
 3 




2
(depends on steady-state transport-limited gradient St,
channel erodibility K, percent of eroded material transported as
bedload ß, and drainage area A)
Prediction of maximum drainage area at which a temporary hanging
1
valley can form:
Atemp
 k w k q b I max  1bc


 K GA  U initial 


(Uinitial is background rate of base level fall)
Effect of Uplift & Subsidence on Channel Morphology
subsidence uplift
uplift
subsidence
Meandering channel
Braided channel
Zone
A
Aggradation
Thalweg shift
Submerged bars
Degradation
Single thalweg
Aggradation
Thalweg shift
Multiple channels
axis
B
C
D
Degradation
Terrace fm.
Single bars
Aggradation
Braided
Aggradation
Braided
Degradation
Single thalweg
flooding
Degradation
Sinuosity increase
Bank erosion
Degradation
Sinuosity increase
Bank erosion
After Schumm, Mosley & Weaver (1987)
Aggradation
Aggradation
Local scour
Flooding, cutoffs,
multiple channels
(Schumm et al., 2000)
Channel evolution models
Simon & Hupp
(1986)
Long debate on relative importance
of external (land use, climate)
versus internal triggers for channel
incision