Sedimentary Geology, 75 (1991) 67-83
Elsevier Science Publishers B.V., Amsterdam
67
Channel planform as a non-controlling factor in fluvial
sedimentology: the case of the Squamish River floodplain,
British Columbia
G a r y J. B r i e r l e y * a n d E d w a r d J. H i c k i n
Department of Geography and The Institute for Quaternary Research, Simon Fraser University, Burnaby, BC V5A 1S6, Canada
(Received January 14, 1991; revised version accepted August 22, 1991)
ABSTRACT
Brierley, G.J. and Hickin, E.J., 1991. Channel planform as a non-controlling factor in fluvial sedimentology: the case of the
Squamish River floodplain, British Columbia. Sediment. Geol., 75: 67-83.
Channel planform styles demonstrate a continuum of variability. Whereas end member situations may exhibit diagnostic
sedimentologic characteristics, non-end member situations may be exceedingly difficult to reliably differentiate. The
relevance of channel planform-based sedimentologic differentiation of fluvial depositional units has been questioned for
some time (e.g. Jackson, 1978; Bridge, 1985; Brierley, 1989a). Despite reservations outlined in these papers, facies models
and interpretations of depositional suites in the rock record continue to be applied in terms of channel planform style. This
paper describes a test of this principle in one particular depositional setting.
The Squamish River, in southwestern British Columbia is a high-energy, gravel-based river in a fjord setting. In a 20-km
reach, this river displays a distinct downstream gradation in channel planform type, from braided through wandering
gravel-bed to meandering. Assemblages of bedform-scale facies units on bar surfaces in this river do not differ by planform
type (Brierley, 1989a); rather, facies associations relate directly to the pattern of morphostratigraphic units on bar surfaces
(Brierley, 1991a). When analysed in floodplain exposures, sediment sequences which are equivalent to morphostratigraphic
units are termed elements. These elements are defined in terms of their geometry, bounding surface and sediment
characteristics.
Detailed analysis of floodplain deposits in the braided, wandering gravel-bed and meandering reaches of the Squamish
River demonstrates that just as assemblages of bedform-scale facies cannot be differentiated by planform style, neither can
assemblages of floodplain elements. Indeed, for the Squamish River, the type, character, and spatial association of these
elements cannot be differentiated reliably at the channel planform scale. This indicates the limited relevance of planform
differentiation of sedimentary environments under transitional planform situations such as those of high-energy, gravel-bed
rivers in confined valleys. In such situations, sediment inventory at the element scale provides greater insight into
mechanisms of floodplain evolution than can be provided by interpretation at either the channel bedform or channel
planform scales.
Introduction
T h e p l a n f o r m of a river, or its c o n f i g u r a t i o n in
p l a n view, is generally described in t e r m s of the
come widely recognized, a l t h o u g h they are def i n e d u s i n g different p a r a m e t e r s : b r a i d e d , wand e r i n g gravel-bed, m e a n d e r i n g , a n a s t o m o s i n g a n d
straight. T h e s e styles are n o t m u t u a l l y exclusive,
n u m b e r , sinuosity, a n d lateral stability, of the
channel(s). Five c h a n n e l p l a n f o r m styles have be-
a n d overlap of p l a n f o r m styles can be significant.
F o r example, the a n a b r a n c h e s of a b r a i d e d river
* Present address: Department of Biogeography and Geomorphology, Research School of Pacific Studies, The Australian National University, GPO Box 4, Canberra, ACT
2601, Australia.
may m e a n d e r or b e straight. F u r t h e r m o r e , the
r a n g e of e n v i r o n m e n t a l settings for these p l a n forms are n o t well defined. F o r example, the two
p r i m a r y p l a n f o r m styles, b r a i d e d a n d m e a n d e r ing, can b e f o u n d along the e n t i r e s p e c t r u m of
l a t i t u d i n a l a n d a l t i t u d i n a l gradients, suggesting
0037-0738/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
68
that their occurrence is controlled more by local
slope, discharge and particle size characteristics
than by regional-scale geomorphic and environmental controls (see review by Ferguson, 1987).
Because individual planform types are found in a
range of geomorphic settings, and because sediment deposited by flow at bends in braids is not
fundamentally different from that deposited by
bend-flow in meanders, the supposed sedimentologic distinctiveness of these different planform
settings must be questioned. However, despite
reservations outlined by Jackson (1978), Bridge
(1985) and Brierley (1989a), the basic tenet that
the character of river deposition is a function of
channel planform type, and that one therefore
can be used to predict the other, continues to be
applied.
Because of the wide variation in the depositional settings of the various planform styles, it
has been necessary to develop suites of facies
models to account for sedimentologic variation in
rivers of a given planform. For braided rivers,
Miall (1977, 1978) and Rust (1978b) synthesized
sedimentary sequences into six braided planform
facies models based on observations of braids in
differing environmental settings and with different bed-material size. The deposits of wandering
gravel-bed rivers, with fewer channels and active
bar platform areas than braided rivers, have been
described by NeiU (1973), Church (1983), Desloges and Church (1987) and Morningstar (1988).
The classic meandering planform facies model
proposed by Allen (1965) has been shown only to
apply to particular meandering regimes, and
Jackson (1978) synthesized the fluvial sedimentology of meandering rivers into five facies models
based on hydrologic and sediment size controls.
Depositional sequences subsequently have been
described for anastomosed rivers (for example,
Smith and Smith, 1980; Nanson et al., 1988), and
Bridge et al. (1986) described the sedimentology
of a gravel-based, low-sinuosity stream.
These and other descriptions of river sediments have become the basis for almost routine
identification of fluvial sediments by planform
type (for example, see Moody-Stuart, 1966, and
critical reviews by Jackson, 1978, and Bridge,
1985). Planform-sediment correlates are far from
G.J. BRIERLEY
A N D E.J. H I C K I N
unequivocal as channel planform facies models
have typically been derived for end-member situations, in which braided reaches are studied in
high-energy (i.e. typically steep mountain) environments, while meandering reaches are studied
in low-energy (i.e. typically low-slope "downstream") environments. While the environmental
domains of these planform styles may appear to
be well defined from discriminant analysis (sensu
Leopold and Wolman, 1957), causal grounds for
this differentiation in geomorphic process terms
remain unclear.
It is the purpose of this paper to report the
results of an intensive test of the supposed link
between river planform and fluvial sedimentology
in one particular field setting. The test involves a
detailed analysis of the contemporary bar and
floodplain sediments found along a reach of the
Squamish River, southwestern British Columbia,
which changes downstream from braided, through
wandering gravel-bed to meandering (Figs. 1 and
2). The rationale for this site selection is as follows. The place to test the validity of planformsediment correlation is not in end-member situations, but in field situations where differing planform styles are found in juxtaposition. The
Squamish River is one of many rivers in the Coast
Mountain Ranges of British Columbia which
demonstrates distinct downstream changes in
planform style within one environmental setting.
Characteristics of the Squamish River floodplain have been described in several previous
papers (Brierley, 1989a, 1991a, b; Brierley and
Hickin, 1991). In the first of these papers on this
study area, planform facies models were derived
using Markov analysis of facies assemblages examined at the bedform scale (Miall, 1977, 1978).
From analysis of sediment sequences observed in
pits dug on contemporary bars in each of the
three planform reaches it was shown that it was
not possible to differentiate between braided,
wandering gravel-bed and meandering reach deposits on the basis of the organization of smallscale sedimentary structures, (Brierley, 1989a).
Subsequent analysis showed that sediment trends
on bar surfaces relate directly to the spatial distribution of morphostratigraphic units (Brierley,
1991a). Four units, bar platform, chute channel,
69
CHANNEL PLANFORM AS A NON-CONTROLLING FACTOR IN FLUVIAL SEDIMENTOLOGY
ridge and remnant floodplain, demonstrated differing spatial organizations dependent on bar
type. When observed in cross-section in the
marginal floodplain, these units are referred to as
elements (Brierley, 1991b). A schematic sketch
illustrating the associations among these features
is presented in Fig. 3. Elements are geomorphic
floodplain features defined by their geometry,
scale, basal contact, and position within a sediment sequence, along with their associated sedi-
Mt.Garibaldi
VANCOUVER
l
i~
0
I
StudyReach
SquamishRiver
watershed
10 20 30 40 50km
I
I
I
I
I
Fig. 1. The Squamish River regional setting.
70
G.J. B R I E R L E Y
C. MEANDERING REACH: Note single channel, sinuous ouUine, and
restriction of bar platform areas to the insides of bends.
Fig. 2. Representative reaches of each channel planform type.
A N D E.J. H I C K I N
CHANNEL
PLANFORM
AS A NON-CONTROLLING
FACTOR
IN FLUVIAL
mentology. The spatial association of elements
helps elucidate the geomorphic history of the
floodplain (Brierley and Hickin, 1991).
In this paper the character and spatial association of elements are analysed in each planform
reach of the Squamish River. Using this sediment
inventory, mechanisms of floodplain evolution are
compared for each reach, and implications for
reliable sedimentologic differentiation of planform style are discussed.
71
SEDIMENTOLOGY
ronment with a catchment area of almost 3600
km 2. Discharge of the Squamish River is markedly
seasonal, ranging from a mean discharge of 90 m 3
s-t in March to over 500 m 3 s-1 in July. Mountain peaks in this glaciated environment are
greater than 2000 m in elevation on both sides of
the valley. On the eastern valley side there are
also two major volcanoes (Mount Cayley, 2393 m
and Mount Garibaldi, 2678 m). Holocene redistribution of glacially and landslide-derived materials by the Squamish River has resulted in a
distinct downstream gradation in channel planform styles. Just beyond the upstream end of the
study reach there have been enormous inputs of
coarse boulders and finer debris into the
Squamish River from massive landslides on
Regional setting of the Squamish River, British
Columbia
Located 60 km north of Vancouver (Fig. 1),
the Squamish Valley is a high-energy fjord envi(a) Schematic plan view of braided reach bars
I:.:~.:?..
t
(b) Channel cross-section
0
li0
Remnant
floodplain
Ber platform _ _;
Chute
channel
/
~
~
'
>
,0
1,00
Scale in metres
~
l
200
~io~:l:otstlreti°cl:ll~°;hlcunits
~
scale In metres
Bar platform
~
Chute channel
Ridge
~
R. . . . .
,
1(10
Y
\
(c) Pit data at the bedform-scale facies
Bedform-scale facies code
~
Wavy bedded sands r ~
Floodplain
Rippled sands
m
t
Trough
bedded croaosands
planar
bedded unde
oooooo Basal gravels
Scale in melres
(d) Trench data at the
element scale
N
-----"~ Send wedge top-stratum M _ _ . - " ~ " ~
(e) Bank exposure data at the element scale
B
Element code
~
0
s0
Proximal tup-stratum
~>--~---~Ridge
~
)
25
50
Scale in metres
~Bar
Chute channel
platform
ooo0 Basal channel gravels
Fig. 3. Schematic representation~of the sampling strategy, showing the relationship between the three scales of sediment analysis
employed. Part (a) shows a plan-view of a hypothetical reach of braided river. Sample locations are shown on, and adjacent to, a
mid-channel compound bar. The spatial distribution of morphostratigraphic units on this bar shows downstream alignment of units,
in which the chute channel cuts into the remnant floodplain unit. Part (b) shows five pit locations on the X - Y cross-section
indicated in part (a). Part (c) demonstrates the bedform-scale facies sedimentology of three of the five pits indicated in part (b).
Bedform-scale facies assemblages are shown in relation to morphostratigraphic units. Part (d) is an example of trench data
examined at the element scale for section M - N indicated in part (a). This is the equivalent location to the three pits examined in
part (c). Part (e) is an example of the third phase of the sampling strategy, in which bank exposures are examined at the element
scale (marked A - B in part (a)). Note that the scale of each sediment exposure differs.
72
G.J. B R I E R L E Y
Mount Cayley (Evans and Brooks, 1991; Fig. 1).
Coincident with the large fan from Mount Cayley,
the Squamish River is confined into a narrow
canyon. Immediately downstream of this canyon,
the large volumes of coarse sediment, along with
the steep valley flat slope and flashy discharge
regime have resulted in a braided planform style.
As slope decreases downstream, gravel-bed particle size diminishes dramatically (Brierley and
Hickin, 1985) and valley width roughly doubles
from 1 to 2 km. The dynamic nature of these
adjustments has resulted in a downstream transition in planform style from braided through wandering gravel-bed to meandering. Beyond the meandering reach, a low sinuosity (almost straight)
style has been imposed on the Squamish River, as
its single channel has been pinned against the
western valley wall by tributary fans from Mount
Cayley (Fig. 1).
This study focusses on the 20 km reach in
which channel planform changes down-valley
from braided through wandering gravel-bed to
A N D E.J. H 1 C K I N
meandering. In this reach there is an accordant
change in bar type from mid-channel compound
bars to bank-attached compound bars to point
bars (Brierley, 1991a). This reach is gravel-based
throughout, and D95 values of the > 8 mm gravel
fraction on bar surfaces decrease from about 200
mm to about 100 mm down-valley (Brierley and
Hickin, 1985). The general character of planform
variability in the study reach is described in Table
1 and representative air photographs of each
planform reach are shown in Fig. 2. Human modification of the river in the study reach has been
insignificant, enabling comprehensive description
and analysis of undisturbed floodplain sediments
in each planform reach.
Methods employed in the sedimentologic inventory
A three-phase sampling strategy was employed
to determine the floodplain sedimentology of the
TABLE 1
General character of planform variability in the study reach
Planform type
Braided
Wandering gravel-bed
Meandering
Channel multiplicity
Channel sinuosity ~
Braided parameter b
Slope
D95 channel bed particle
size (in mm) c
Mean valley width (in m) a
3-4
1.197
3.67
0.0058
2-3
1.317
2.50
0.0015
1-2
1.433
0.67
0.0013
150-260
800-1200
120-150
1100-1900
85-130
1700-2200
Predominant bar type d
Mid-channel or
bank-attached
compound bars
Bank-attached
compound or
lateral bars
Lateral or point
bars
Bar character
Chaotic pattern of
morphostratigraphic
units
Morphostratigraphic
units aligned downbar
Down-bar and
around-the-bend
pattern of morphostratigraphic units
3.5 m
4.5 m
6.0 m
Maximum observed floodplain
thickness (in m) e
a Ratio of thalweg length to valley length (Leopold and Wolman, 1957).
b Number of braids per mean meander wavelength (Rust, 1978a).
c Data from Brierley and Hickin (1985).
For further details, see Brierley (1991a).
e Measured as maximum thickness of sediments observed above basal channel gravels in bank exposures.
CHANNEL
PLANFORM
AS A NON-CONTROLLING
FACTOR
IN F L U V I A L
73
SEDIMENTOLOGY
l
i
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N
~
E
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~ ~
~~ ~
~ ~-
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~-~
~ o
~
"O
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8
'O
E
~
~
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~.~
0
..~
0
i
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0
~.~ ~ ~.~
~ ~.~
0
.~
8
~
.o
~,.. ~
..- -~ ~
0
.~.
"13 0J
"0
~'F,
e',
~
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t~
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R
74
braided, wandering gravel-bed and meandering
reaches of the Squamish River:
(1) Analysis of the sedimentologic composition
of contemporary bar surfaces. Over 290 pits were
dug in a grid-like pattern on 10 bar features and
sediments analysed as bedform-scale facies units
(Brierley, 1989a).
(2) Analysis of floodplain sediments proximal
to contemporary bars. Nine trenches, each up to
26 m in length, were dug perpendicular to the
main channel at the bar/floodplain margin of
five bars. Analysis at facies and element scales
was carried out at between 6 and 15 sample
points in each trench, with 83 sample points in
total.
(3) Analysis of floodplain sediments in bank
exposures. Thirteen bank exposures, between 88
and 212 m in length, were selected for study.
These were divided equally among the three
planform reaches. Between 5 and 14 sample
points were described for each exposure, with 112
sample points in total. Facies and element composition of each exposure were analysed.
Relations among these three sampling styles
are shown in Fig. 3. Whenever possible, pits,
trenches and bank exposures were dug to the
depth of the basal channel gravels. Maximum
attained depth exceeded 6 m. Particle size and
basal contact character were recorded for each
individual bedded unit. Detailed outlines of the
specific sampling locations and sediment analysis
procedures are described elsewhere (Brierley,
1989b).
The rationale for the sampling procedure is as
follows. From analysis of contemporary depositional environments it is possible to evaluate and
interpret the character of sediment sequences
preserved in the floodplain. Observed facies associations on bar surfaces relate directly to the
spatial distribution of morphostratigraphic units
(Brierley, 1991a). Four morphostratigraphic units
are identified: bar platform, chute channel, ridge
and remnant floodplain. When analysed in
trenches and bank exposures, these preserved
floodplain depositional units are termed elements, and the remnant floodplain unit, composed of deposits laid down by unconfined flows
on bar/island surfaces, is differentiated into three
G.J. B R I E R L E Y
A N D E.J. H I C K I N
top-stratum elements, namely proximal, distal and
sand-wedge top-stratum elements (Table 2).
Elements are defined by their morphology,
their basal contact, their position within a sediment sequence and their associated sedimentologic composition (both texturally and structurally). As the Squamish River is gravel-based
throughout its course, the basal element is termed
the basal channel gravels element. As only the
upper surface of this element was analysed, few
comments can be made about the character of
these units. In general these are imbricated surfaces, which are characteristically inclined downvalley. Bar platform sands and gravels, deposited
as within-channel deposits on bar surfaces, typically overlie the basal gravel element. These are
composed primarily of trough and planar crossbedded coarse sands, which characteristically
grade upwards into interbedded rippled and
wavy-bedded facies, composed of darker-coloured, medium-fine sands.
Marginal to the bar platforms on contemporary bars are series of ridge features and chute
channels. Ridges are clinoforms upon which sediments accumulate as beds inclined towards the
adjacent channel. At their broadest scale they are
similar to levees. They are composed primarily of
medium-fine sands. Whereas ridges have distinct
morphologies and basal contacts, the character of
chute channels is highly variable, as their differing width/depth (W/D) ratios reflect different
erosive potentials. Accordingly, they have variable sediment infills, and generally chute channels with W/D ratios < 10 tend to be high-energy erosive channels with scoured basal outlines.
These are subsequently infilled with cross-bedded
coarse sands, which commonly grade upwards to
finer-sand units. Chute channels with W/D ratios
> 10 are characteristically shallow flood channels
which have fine-sand infills.
Basal channel gravels, bar platforms, chute
channels and ridge elements are within-channel
or channel marginal features, and represent bottom-stratum deposits. In contrast, top-stratum elements are overbank or vertical accretion deposits which tend to even out the floodplain
surface. In the Squamish River floodplain there
are three primary styles of top-stratum deposit.
CHANNEL PLANFORM AS A NON-CONTROLLINGFACTOR IN FLUVIALSEDIMENTOLOGY
Proximal top-stratum deposits are flood cycle sequences composed mainly of interbedded rippled
and wavy-bedded fine sands. In contrast, deposits
at the floodplain surface in distal floodplain areas
are composed of sheets of dark-coloured fine
sands and silts, with a massive appearance. Due
to channel avulsion, such deposits are found in
mid-bank exposure in the present channel banks.
Finally, sand wedges are wedge-shaped sand
sheets or splays of loose, fine to medium sands
which are launched onto the floodplain during
flood events.
From detailed analyses of both facies and element~cale features in lateral and longitudinal
75
exposures, similarities and differences in floodplain composition are evaluated for the three
planform reaches of the Squamish River.
Planform sedimentology of the Squamish River
While elements in themselves are distinct from
each other in terms of their summary properties
(Table 2), they exhibit no regular variability in
either their sedimentologic character or their vertical stacking arrangement by channel planform
type. Although elemental composition of each
lateral and longitudinal exposure varies markedly
(Table 3), facies composition of each exposure is
TABLE 3
Summary sedimentologic composition of each trench and bank exposure
Planform
type
Element abundance (%)
Facies abundance > 20%
Proximal and Distal Chute
Bar
sand-wedge topchannels platform
top-stratum stratum and ridges sands
a
Mean particle
size (~ units)
Braided
Trenches
Basbar
Bank exposures Dbas
Brabend
Upstat
Statbar
Dstat
94
98
73
90
29
58
6
21
Sr 32%; Sw 31%; Sh 21%
2.81
3
10
31
Sr 53%;
Sr 54%;
Sr 71%;
Sr 61%;
Sr 46%;
2.98
3.22
2.80
2.34
2.52
41
84
83
24
99
39
7
3
19
1
St 35%; Sr 23%
1.65
Sw 54%
2.15
St 30%; Sp 24%; Sr 20%
1.70
Sw 27%; Sr 24%; Fm 20%; St 20% 2.37
Sr 39%; Fm 27%
2.80
21
4
26
32
31
55
Fm 40%; Sr 27%
Sw 52%
Sw 66%
Sw 28%; St 25%; Sr 24%
3.35
2.68
2.76
2.39
92
61
44
8
27
28
Sw 92%
Sw 52%; Sp 27%
Sw 45%
2.77
2.92
2.08
3
4
2
17
18
14
8
Fm 40%; Sw 23%
Sw 37%; Sr 35%
Fm 42%
Sw 60%; Sr 25%
2.35
2.40
3.13
2.97
6
7
61
11
2
Sw 27%
Sw 22%
Sw 22%
Sw 23%
Sw 27%
Wandering graL'el-bed
Trenches
Upash
Tflent Head
Tflent Mid-I
Tflent Mid-II
Tflent Tail
Bank exposures Widewand
Upash
Tflent
Fallop
Meandering
Trenches
20
9
14
57
30
47
65
27
44
18
Beach Bar Head
Beach Bar Tail 12
Pillbend
28
Bank exposures Campup
Dcamp
Sumart
Pillbend
48
79
48
90
35
34
Sr = rippled sands; Sw = wavy-bedded sands; Sh = horizontally bedded sands; St = trough cross-bedded sands; Fm = massive fine
sands and silts; Sp = planar bedded sands. For further details on these facies types see Brierley (1989).
76
G.J. BRIERLEY AND E.J. HICKIN
to the main channel. From this figure, element
associations can be differentiated into two primary styles which occur independently of planform type. In those instances in which chute
channels are prevalent (e.g. Upash, Tflent
trenches, Pillbend), bar platform units are observed as small features (typically < 10 m wide,
0.5 m thick) which have been scoured by chute
channels. These chute channels, which are ob-
remarkably consistent, with rippled and wavybedded medium-fine sands and massive fine
sands/silts dominant throughout. Particle size
trends are either upwardly uniform or upwardfining, with medium-fine sands dominant. These
facies and particle size trends are independent of
channel planform type.
Figure 4 summarizes sediment sequences analysed in trench exposures oriented perpendicular
BASBAR
Braided
reach
UPASH
00000000(
TFLENT HEAD
Wandering
gravel.bed
river
reach
TFLENT MID-I
/
TFLENT TAIL
BAR
~,
BEACH
\
END
\
~
oooooooooooooooooooooo~
ooooooooo~oooOOOOO
BEACHBAR TAIL
oooooooooo°°
~
Meandering
re.o,
PILLBEND
Top-Stratum
/
0i
ooooooooooo
BEACH BAR HEAD
~
~Band
Proximal
~r--~ Ridge
Chute Channel
Wedge
Scale0f exposures
2rn]
J
0
i
I
3km
,
~Bar
o o o o
Platform
0
2
~im
Basal Gravels
Fig. 4. Trench sediment exposures. For presentation purposes, element associations are simplified for each trench.
CHANNEL PLANFORM AS A NON-CONTROLLING FACTOR IN FLUVIAL SEDIMENTOLOGY
served up to 16 m wide and 2 m deep, may also
scour basal gravels. Ridge deposits at chute channel margins accumulate to a roughly equivalent
depth. These sequences are subsequently capped
by proximal and sand-wedge top-stratum units
which frequently extend laterally across trenches,
up to 1.5 m deep. Although chute channel reworking of bar platform deposits is evidenced
primarily in trenches dug in the wandering
gravel-bed river and meandering reaches, bar
platform deposits are similarly poorly preserved
in the one trench examined in the braided reach
(Basbar; Fig. 4). An alternative to this depositional sequence is evidenced by Beach Bar
trenches. Ridges observed in these trenches have
developed marginal to the main channel and are
77
much larger than ridges observed elsewhere, as
they extend across-exposure. Rather than relating
to either down-valley position or channel planform type, these two styles of element associations in lateral exposures are determined by local
characteristics, especially the manner of chute
channel reworking of sediments.
The operation of particular depositional mechanisms independent of planform type is also apparent in longitudinal bank exposures (Fig. 5).
Braided reach bank stratigraphy
Given their differing local environmental settings, these five bank exposures vary remarkably
little (Fig. 5). Sediment sequences are dominated
DBAS
~°°°b°°°°°°°°°
BRABEND
•
Top.Stratum
/
0,, + y +
ooo ~
m
ooo o
Proximal
~
Ridge
Distal
~
Chute Channel
Sand Wedge
~
Bar Platform
"~ UPSTAT
o o o o Basal Gravels
~m]
0
0
.~ STATBAR
DSTAT
Scaleof
exposures
20
40
60m
CAMPUP
WIDEWAND
1
DCAMP
(o
~o
~--:]
~:::.:.:.. oo~O~o
~" UPASH
....:::.. . . . . .
SUMART
....+... r
-oo~ooooo
"~ TFLENT
FALLOP
PILLBEND
Fig. 5. B a n k e x p o s u r e s e d i m e n t s e q u e n c e s . F o r p r e s e n t a t i o n p u r p o s e s , e l e m e n t associations a r e simplified for e a c h exposure. N o t e
that t h e h o r i z o n t a l scale is an o r d e r of m a g n i t u d e l a r g e r t h a n in Fig. 4.
78
by proximal top-stratum deposits, which rest atop
basal gravels and extend virtually to the floodplain surface. These are over 3 m thick in Brabend
exposure, but are between 1 and 2 m thick in
other braided reach exposures. Statbar exposure
is made up primarily of chute channel infill deposits and only in Dstat exposure are bar platform deposits preserved to a significant extent
and, even in this case, 67% of the element is
composed of rippled and wavy-bedded, mediumfine sands. Elements in the braided reach are not
horizontally aligned and are longitudinally discontinuous.
Wandering gravel-bed river reach bank stratigraphy
These four bank exposures have highly variable element compositions (Table 3; Fig. 5). In
Widewand section, bar platform deposits up to
2.5 m thick adopt an irregular arch-like form atop
distal and sand-wedge top-stratum elements. The
vertical elemental arrangement from bar platform
to proximal top-stratum to sand-wedge elements
is evidenced in both Upash and Tflent exposures,
but is disrupted in the former instance by three
chute channels. In Fallop exposure, bar platform,
distal and proximal top-stratum elements are
stacked as horizontally aligned units down the
entire section. Of the four bank exposures studied in the wandering gravel-bed river reach, only
in Upash section are elements longitudinally discontinuous. The primary difference between these
exposures and those described for the braided
reach is the significantly greater proportion of bar
platform deposits in the former. These extend
across the base of most exposures, their thickness
generally increasing down-valley. Indeed, in Fallop section these are the dominant element, and
are up to 3 m in thickness.
Meandering reach bank stratigraphy
The roughly horizontal stacking of elements
evidenced in the wandering gravel-bed river reach
exposures is echoed by bank exposures studied in
the meandering reach, although elements are longitudinally more extensive in this instance (Fig.
G.J. B R I E R L E Y A N D E.J. H I C K I N
5). Floodplain sequences are also thicker than
elsewhere, with depths up to 6 m to basal gravels.
Top-stratum deposits are the major element of
each exposure. However, while proximal topstratum elements dominate Dcamp exposure,
Pillbend exposure is composed primarily of sandwedge deposits, and both Campup and Sumart
exposures have thick (> 2 m) distal top-stratum
deposits in mid-sequence. Bar platform sands are
preserved beneath these cohesive dark brown/
grey deposits of finely laminated, or massive, very
fine sands. Atop distal top-stratum elements are
extensive proximal top-stratum and sand-wedge
deposits. Indeed, the latter deposits extend atop
the surface of Campup, Dcamp and Sumart exposures, with depths < 0.5 m. In Sumart exposure
there are small ridge and chute channel deposits
adjacent to the distal top-stratum unit. Hence,
while Campup and Sumart exposures exhibit thick
floodplain sequences with longitudinally continuous, horizontally aligned elements, Dcamp exposure is more reminiscent of wandering gravel-bed
river reach sections, and Pillbend exposure closely
resembles sediment sequences observed in the
braided reach (Dbas and Upstat).
Element-scale sediment composition
Squamish River floodplain
of the
Element composition of the Squamish River
floodplain varies from site to site, related specifically to the character and extent of sediment
reworking and not necessarily to channel planform type. Two element-scale models of floodplain sedimentologic sequence can be discerned
(Fig. 6; Brierley, 1991b). In the first instance,
basal channel gravels are transitional upwards to
thin, discontinuous bar platform deposits and,
more frequently, to thick proximal top-stratum
deposits. These deposits have relatively thin
chute, ridge and sand-wedge deposits above. In
general, such sequences are associated with relatively "recent" floodplain deposits, marginal to
contemporary bars, in which sediment reworking
is prevalent. For example, Dbas, Upstat, Statbar,
Upash, Tflent, Dcamp and Pillbend exposures
are all located immediately upstream of contemporary bars, and are composed of sediments which
79
CHANNEL PLANFORM AS A NON-CONTROLLING FACTOR IN FLUVIAL SEDIMENTOLOGY
(A) Floodplain sequences dominated by
proximal t o p - s t r n ~ ,
greater, this element is also observed in the
braided and wandering gravel-bed river reaches
(Brabend, Widewand and Fallop exposures). Certainly, the five exposures in which these deposits
are observed are much more similar in type, and
more different from any other exposure, than any
tendencies that are observed at the channel planform scale.
In summary, the applicability of the two models of elemental floodplain composition is better
explained in relation to local field setting rather
¢~/~
Braided Reach
E
~"
zorn
(B) Floodplain sequences dominated by
distal top-stratum elements
Sande~.x X
LEMENTCODE
roximal
andWedge
istal
idge
huteChannel
BarPlafform
Gravels
Fig. 6. Elemental sedimentology models of the Squamish
River floodplain (Brierley, 1991b). (A) Basal channel gravels
have thin, remnant bar platform sands atop. The remainder of
the exposure is composed primarily of proximal top-stratum
deposits with occasional chute channels and ridges. Discontinuous sand wedges are observed at the floodplain surface. (B)
Bar platform sands atop basal channel gravels are much
thicker than in model (A), as they are preserved beneath
distal top-stratum elements.
/
/
/
Proximal~
Top-Stratum ~
/~ ÷ ~
/
Bar
Platform~l
l
Distal
/ Top-Stratum
Gravels
Wandering Gravel-Bed
River Reach
Sand 4 . _
//~Wedge~
/j~.
have accumulated as bars which have migrated
down- or across-valley.
Just as this model can be applied to each of
the three channel planform styles, so can the
floodplain model which characterizes those exposures dominated by distal top-stratum deposits.
These occur either in concave banks of bends, or
in areas which have recently experienced channel
shifting into older areas of the floodplain. In this
model, bar platform deposits are notably thicker,
since channel shifting probably occurred relatively quickly, thereby minimizing chute channel
reworking of bar platform deposits. Sediment accumulations above the distal top-stratum element
are composed primarily of proximal top-stratum
units, with occasional chute channels, ridges, and
discontinuous sand-wedge deposits. Although distal top-stratum deposits are more commonly observed down-valley, where floodplain width is
\\
\\
\.I'%
/
Bar/
Proximal~
Top.stratu
~
Distal
Top-Stratum
Platform ~ . . . . . .
Gravels
Meandering Reach
Sande,~....
/
Proximal~
\\
Top-Stratum ~ \ \
/ /__...--~
Distal
B a r ~
Top-Stratum
Platform~..,,,,,~
Gravels
Fig. 7. Upward element transitions which occur with a frequency of ~>20%. Transitions are counted upward from each
element at each analysis point in the trench and bank exposures.
80
than to channel planform type. Floodplain sedimentology of each exposure relates primarily to
the character of chute channel reworking of deposits at that site. This process occurs throughout
the study reach. Just as bar platform deposits are
poorly preserved in exposures at the head of the
braided reach, they also occupy < 20% of each
of the four meandering reach exposures. These
findings are confirmed by examination of the
vertical stacking arrangement of elements in each
planform reach (Fig. 7). Upward-element transitions observed with a frequency >/20% demonstrate remarkably similar patterns in each planform reach, related to the operation of similar
geomorphic processes throughout.
In the Squamish River, the only sedimentologic differences which can be described by planform are purely qualitative, reflecting the relative
abundance of certain elements or greater continuity of particular elemental associations. Moving
downstream from the braided to the meandering
reach, floodplain depositional sequences become
thicker, and elements are more longitudinally extensive and are more horizontally aligned. In
environmental settings analogous to the Squamish
River, recognition of these trends in the ancient
rock record would require extensive, undisturbed
exposures, in which equally preserved deposits
from each planform reach can be viewed directly
in juxtaposition. As such, it is unlikely that sediments reflecting the down-valley progression of
channel planform styles, so evident in many field
situations, can always be identified in stratigraphic section.
Discussion
Unequivocal planform differences in the
Squamish River study reach have failed to produce differences in floodplain sedimentology.
When analysed at either bedform-scale facies or
element scales, there are no predictable variations in floodplain sedimentology by planform. As
such, greater differences in environment than
demonstrated in the Squamish Valley are necessary to produce the sedimentological differences
on which floodplain facies models are built.
Observed sediment patterns both on contem-
G.J. B R I E R L E Y AND E.J. HICKIN
porary bars and on the floodplain of the braided,
wandering gravel-bed and meandering reaches of
the Squamish River are best explained in terms
of local-scale depositional histories. For example,
analysis of sediment trends on contemporary bars
revealed that sequences relate more closely to
their local environment of deposition than they
do to position on bar or bar type (Brierley, 1991a).
These occasional spatial and scalar trends provide the only basis upon which sediment sequences can be differentiated by planform. In
high-energy fluvial environments such as the
Squamish River, however, these within-bar sediment trends are unlikely to be preserved in the
floodplain because chute channels rework bar
sediments.
The dynamic geomorphic setting of the
Squamish River presents an environment prone
to rapid adjustment, and the modern river clearly
is adapting to recent sediment supply events from
major landslides, and is also recovering from very
recent glaciation in the Coast Mountain Ranges.
While the study reach is presently transitional in
planform style, such a downstream pattern may
not reflect long-term adjustment of this system.
Furthermore, given recurrent glacial reworking of
surficial deposits in such settings, the thin veneer
of deposits recorded in this work seems relatively
insignificant in the context of the thick accumulations of coarse gravels that lie beneath the contemporary floodplain. Nevertheless, modern-day
processes have resulted in visually distinct planform styles for the Squamish River which are
difficult to differentiate on the basis of their
sedimentologic properties when measured as either bedform-scale facies assemblages or associations of element units. It is difficult to imagine
why such process-sedimentologic associations
should not be relevant in other field settings.
Because the three contiguous channel planform reaches examined in this study do not have
clearly distinguishing sedimentologic characteristics, the question has to be asked "How large
does a sedimentologic exposure have to be to
make a reliable interpretation of channel planform style?" It seems that the answer depends on
the type of floodplain being analysed. While it
may be relatively easy to differentiate the end
C H A N N E L P L A N F O R M AS A N O N - C O N T R O L L I N G F A C T O R IN F L U V I A L S E D I M E N T O L O G Y
members of the broad spectrum of planform styles
in lateral, across-floodplain exposures, it may be
virtually impossible to do so in other situations.
For example, exposures across the floodplain of a
single-channeled, gravel-based meandering river
with well-defined scroll bars and distal overbank
units will contrast markedly with a gravel-based
braid-plain in which there are numerous shallow
channel infills and little overbank sedimentation.
Distinguishing these features, however, may not
be so straightforward in longitudinal exposure
and, without doubt, reliable differentiation of
planform style from the depositional record may
be questionable in the more common non endmember planform situations. In this study, sediment characteristics identified at scales ranging
from tens to hundreds of metres can not be used
to reliably differentiate planform styles. At the
scale of valley cross-sections, however, the sedimentologic characteristics of differing planform
styles may be quite distinct, and greater complexity may be expected in the spatial association of
elements in braided reaches, and greater differentiation between proximal and distal floodplain
sequences may be predicted in meandering
reaches.
Given this situation, it may be more appropriate to change the question posed above. Rather
than focussing attention on planform type, it may
be more appropriate to focus interpretation on
mechanisms of floodplain development, examining exposures at the scale of those processes by
which sediments become preserved in the floodplain. Prior to the introduction of fluvial architecture and element analysis (e.g. Allen and
Williams, 1982; Allen, 1983; Miall, 1985), the role
played by fluvial geomorphology largely has been
neglected within sedimentology. Until what we
know about processes in fluvial geomorphology
are fully incorporated into fluvial sedimentology
models, past-environmental interpretations of
sediment sequences will remain less reliable than
they might otherwise be.
While moves towards three-dimensional approaches have increased geomorphic inputs into
fluvial sedimentology, the question about which
scale provides the most reliable framework for
sediment analysis remains. Our conclusion is that
81
such analysis is best applied at the scale of floodplain geomorphic units. Several interrelated components make up the architectural framework of
a river floodplain, such as the main channel, bar
units, ridges and swales, chute channels, levees,
crevasse splays, proximal and distal floodplain
units, backswamps, oxbows and abandoned channel infills (Happ et al., 1940; Lewin, 1978). The
relative abundance and spatial association of
these geomorphic elements depend upon local
field conditions; each river system has its own
local suite. Although each of these elements may
be observed in a range of environmental settings,
with similar character both morphologically and
sedimentologically (i.e. these are morphostratigraphic units), their summary presence/absence,
scale and spatial associations within any sedimentary sequence are the primary indicators of process and environment of deposition. From analysis of the spatial association of these elements, at
the scale of valley cross-sections, floodplain style
may be reliably interpreted. To date, however,
knowledge of geomorphic controls on the presence/absence of elements, their spatial associations, environmental domains and response to
changing environmental conditions is sorely lacking for differing floodplain styles.
A range of floodplain types exists which are
not necessarily a direct function of channel planform type. The challenge now is to characterize
the sedimentology of these differing floodplain
types in such a manner that floodplain evolution
can be reliably interpreted. Element analysis provides an appropriate framework for such research
as elements are related directly to known field
correlatives. As elements represent local-scale
depositional environments, floodplain evolution
can be evaluated by analysis of their spatial distribution. This "constructivist" framework views
floodplains as particular associations of elemental
units, which may or may not relate to pre-existing
models. This does not necessarily equate, or lead
towards, sedimentological anarchy, as suggested
by Walker (1990, p. 779), but provides a rigorous
framework with which to analyse the complexity
of modern-day floodplains, thereby permitting
more reliable interpretation of mechanisms of
floodplain evolution.
82
Summary
Findings of this paper can be summarized as
follows:
(1) Just as planform depositional sequences
cannot always be differentiated in terms of facies
associations of bedform-scale features, neither
can they always be differentiated reliably in terms
of the abundance and vertical stacking arrangement of element-scale features. Processes by
which sediments become incorporated into the
Squamish River floodplain, recorded as geomorphic elements, do not differ by, and hence are not
controlled by, planform type. Lack of sedimentologic differentiation in the three contiguous planform reaches of the Squamish River is explained
by the fact that primary mechanisms of floodplain
evolution, especially sediment incorporation, reworking and preservation of deposits in the floodplain, are consistent throughout and independent
of channel planform style (Brierley and Hickin,
1991).
(2) The only aspect of the elemental floodplain
inventory of the Squamish River which varies by
planform type is the scale or the lateral/longitudinal continuity of elements. This aspect is purely
relative in character and could not be evaluated
from individual sedimentary exposures examined
in isolation.
(3) Using element analysis it is possible to
determine mechanisms of floodplain evolution
which may occur independently of planform type.
Accordingly, in situations in which it can be applied, element analysis will usually provide a more
meaningful and reliable framework for interpretation of floodplain depositional environments
than will by sediment inventories described at
either channel bedform or channel planform
scales.
Acknowledgements
Funding for the work was provided from
NSERC Grant A8376 to examine the morphodynamics of high energy gravel-based rivers. The
efforts of many individuals who put in many
hours of digging involved in compiling this floodplain sediment inventory are gratefully acknowl-
G.J. B R I E R L E Y
A N D E.J. H I C K I N
edged. Useful and constructive referees comments by Gerald Nanson and an unknown referee are gratefully acknowledged.
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