Geomorphology, 4 (1992) 381-391
381
Elsevier Science Publishers B.V., Amsterdam
Floodplain development based on selective preservation of
sediments, Squamish River, British Columbia
Gary J. Brierley I and Edward J. Hickin
Department of Geography and The Institute for Quaternary Research, Simon Fraser University, Burnaby, B.C., Canada, V5A 1S6
(Received July 27, 1990; accepted after revision May 16, 1991 )
ABSTRACT
Brierley, G.J. and Hickin, E.J., 1992. Floodplain development based on selective preservation of sediments, Squamish
River, British Columbia. In: G.R. Brakenridge and J. Hagedorn (Editors), Floodplain Evolution. Geomorphology, 4:
381-391.
In the conventional model of floodplain sediment accumulation, mechanisms of floodplain growth are differentiated
into lateral and vertical accretion processes, in which within-channel deposits are capped by overbank deposits. In the
high-energy, gravel-based Squamish River, sediments laid down on bar surfaces are composed of trough and planar crossbedded coarse sands. These sequences contrast incongruously with adjacent floodplain deposits which are composed in
large part of vertically accreted fine sands atop coarse alluvial gravels. Using element analysis it is inferred that bar platform sediments are stripped away by chute channels, which are subsequently infilled with lower-energy deposits. From
this, a model of floodplain growth based on selective preservation of bar platform sands and preferential preservation of
vertically accreted deposits is proposed. This mechanism of sediment replacement occurs independent of channel planform type.
Introduction
Floodplains are sediment sinks adjacent to
river channels in which eroded and sorted sediments accumulate and are reworked by various processes, producing an array of sedimentary forms. In the conventional model of
floodplain sediment accumulation, mechanisms of floodplain growth are differentiated
into lateral and vertical accretion processes, in
which within-channel deposits are capped by
overbank deposits. These are referred to as
bottom-stratum and top-stratum deposits, respectively. The relative abundance of these depositional units which make up floodplains is
conditioned largely by environmental setting
1 Present Address: Department of Biogeography and Geomorphology, Research School of Pacific Studies, The
Australian National University, GPO Box 4, Canberra,
ACT 2601, Australia.
and the history of geomorphic events.
Wolman and Leopold (1957) pointed out
the difficulty in reconciling substantial overbank deposits with regular recurrence intervals of bankfull discharge and considered topstratum deposits to make up only 20% of
floodplain sediments. These findings have been
confirmed in many studies (e.g. Lattman,
1960; Allen, 1965 ). In contrast, other authors
have noted the prominence of overbank deposits in migrating channel regimes (e.g.
Schumm and Lichty, 1963; Schmudde, 1963;
Blake and OUier, 1971; Chappell et al., 1992)
and the floodplains of streams which exhibit
little or no lateral migration may be composed
largely of vertical accretion deposits (e.g. Ritter et al., 1973; Nanson and Young, 1981 ). In
some settings, major floods may remove
coarse-grained floodplain sediments, with replacement by either bar and chute fill deposits
0169-555X/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
382
G.J. BRIERLEY AND E.J. HICKIN
son ( 1986, p. 1467) to comment that "... rather
than a single model of floodplain formation,
there is an array of processes leading to different floodplain types in different environments." In this paper an alternative mechanism of floodplain growth is demonstrated for
the high-energy, gravel-based Squamish River,
in southwestern British Columbia.
In order to understand mechanisms of
floodplain sediment accumulation it is necessary to link principles of floodplain sedimentology and fluvial geomorphology. This is
(e.g. Baker, 1977 ) or vertical accretion deposits (e.g. Schumm and Lichty, 1963; Burkham,
1972; Nanson, 1986; Ritter and Blakley, 1986 ).
Indeed, floodplain sediments initially deposited by one mechanism often are reworked and
redeposited by others (Schmudde, 1963;
Schumm and Lichty, 1963; Sigafoos, 1964;
Brakenridge, 1984; Hereford, 1984)o
As such, although floodplains are composed
solely of combinations of laterally and vertically accreted deposits, a wide range of floodplain types has been described, leading Nan-
MEANMONTHLYDISCHARGE
Study reach
SquamishRiverwatershed
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Fig. I. Regional setting of the Squamish River. Inset top fight: Mean monthly discharge of the Squamish River at Brackendale ( 1955-1987 ). Inset bottom left: Longitudinal profile of the Squamish River.
FLOODPLAIN DEVELOPMENT BASED ON SELECTIVE PRESERVATION OF SEDIMENTS
38 3
achieved using three-dimensional element
analysis (Miall, 1985, 1988a-c; Miall and
Turner-Peterson, 1989; Brierley, 1991 b). Elements are defined in geomorphic terms,
thereby providing insight into the associations
among sedimentary forms, processes responsible for them, and their environment of
deposition.
In this study an elemental approach is used
to characterize the fluvial sedimentology of a
20 km reach of the Squamish River in which
channel planform changes progressively downvalley from braided to meandering. By comparison of deposits laid down on contemporary bars with those sediments incorporated
and preserved in the adjacent floodplain, it is
shown that chute channels frequently rework
bar platform deposits and are subsequently
filled in by much lower-energy, finer sand deposits. This mechanism of sediment replacement results in floodplain sediments dominated by low-energy, vertically accreted
deposits atop coarse alluvial gravels. This
mechanism occurs independently of channel
planform.
Field setting
The Squamish River is a high-energy, gravelbased river, 60 km north of Vancouver in
southwestern British Columbia, Canada (Fig.
l ). Over 150 km long, the river has a drainage
basin area of almost 3600 km 2. During the
Holocene the Squamish River has moved,
sorted and redistributed glacially and slopederived sediments within its high-relief fiord
setting. Abundant sediments supplied from
slope failures on Mount Cayley (Fig. 1 ) have
imposed a braided channel planform downvalley. In contrast, base level control associated with Cheekye Fan adjacent to Mount
Garibaldi has imposed downstream channel
control on the meandering reach. The 20 km
study reach focusses on the planform transition reach, in which there is a change in bar
type from mid-channel and bank attached
Fig. 2. Aerialphotographs of the planform reachesof the
Squamish River. (a) The lower braided reach. Note the
confined nature of the valley, which broadens gradually
towards the lower end of the photograph. The active,
multi-channeledreach followsa broadly sinuous outline,
with substantial floodplainpreservedbetweenbends. This
photograph was taken immediatelyfollowingthe October
1984 flood event. Note the missing section of the bridge
in the foreground. (b, next page) The meanderingreach.
compound bars to lateral and point bars (Fig.
2).
In the study reach, channel sinuosity increases down-valley from 1.1 to 1.5, while the
braiding parameter (measured as number of
bars and islands per meander wavelength) decreases from 3.7 to 0.7. Dso values for the gravel
population on contemporary bars decrease
down-valley in a regular manner from in excess of 100 m m to less than 50 m m (Brierley
and Hickin, 1985). Valley slope diminishes
from about 0.006 to 0.00 l, while valley width
384
G.J. BRIERLEY AND E.J. HICKIN
Fig. 2. (Continued).
doubles from about 1 km to 2 km. In this reach,
channel shifting is prevalent, bend migration
is very rapid, and log jams are abundant.
Annual precipitation in Squamish exceeds
2000 mm. Annual runoff is highly seasonal,
and average monthly discharge of the
Squamish River at Brackendale ranges from 90
m 3 s-1 in March to over 500 m 3 S-1 in July
(Fig. 1 ). In an average year the river delivers
about 2 million metric tons of sand and silt to
the Squamish Delta in Howe Sound (Hickin,
1989 ). Occasional fall floods are especially severe; an event in October 1984 had a maximum instantaneous discharge of 2610 m 3 s - l ,
and brought about significant channel changes
in some reaches (Hickin and Sichingabula,
1988). Floodplain vegetation is mixed coniferous and deciduous woodland, with a welldeveloped lower canopy.
Floodplain growth of the Squamish River
To evaluate both contemporary bar sediments and preserved floodplain sediments of
the Squamish River, a three-fold sampling
procedure was employed. Contemporary bar
deposits were evaluated in a number of pit
transects, sediments were exposed in trenches
dug perpendicular to the main channel at the
bar/floodplain margin, and bank exposures
were examined at regular down-valley intervals. Sampling in the channel marginal zone
had several advantages in that the relationship
between known depositional environments and
their sediments could be evaluated by comparing contemporary bar sequences with adjacent
floodplain deposits.
Whenever possible, pits, trenches and bank
exposures were dug to gravel depth, and sediment sequences were analysed at both facies
and element scales. Particles sizes were recorded for each individual bedding unit. Specific sampling site locations and procedures for
sediment analysis are described elsewhere
(Brierley, 1991a, b).
Bar sedimentology relates directly to the
spatial distribution of morphostratigraphic
units, with different within-bar trends by bar
type (Brierley, 1991a; Fig. 3). Fluvial morphostratigraphic units are defined as three-di-
FLOODPLAIN DEVELOPMENT BASED ON SELECTIVE PRESERVATION OF SEDIMENTS
(A) Floodplain sequences dominated by
p r o x i m atop-stretum
l
e i e r n
Mid-channel compound bar
o
385
~
,I
1oo
metres
Bank attachmd comnound bar
Bar platform
Chute channel
Ridge
R e m n a n t floodplain
,>>>>>:
>>>>>>
>>>>>;
>>>>>>
w
"~'
L.EMENTCODE
Point bar
E
~t
oximal
tnd Wedge
sial
dge
zu m
(B)
Floodplain sequences dominated by
distal top-stratum elements
_~ute Channel
Bar Platform
Gravels
Fig. 4. Three-dimensional elemental floodplain sedimentology models of the Squamish River. In model A, chute
Fig. 3. Characteristicspatial association of morphostratigraphic units on differing bar types in the study reach
(after Brierley, 1991a). In contrast to the randomlyorganized, remnantfeatureson midchannelcompoundbars,
morphostratigraphicunits are orienteddown-baron bankattachedcompoundbars and exhibitaroundthe bend and
arcuate patterns on point bars.
mensional morphologic features on bar surfaces which are associated directly with specific
sediment sequences. Four morphostratigraphic units are observed on Squamish River
bars, namely bar platform, chute channel, ridge
and remnant floodplain units. Unvegetated bar
platforms are the major features of contemporary river bars. Their facies composition differs markedly from the other morphostratigraphic units, with trough and planar crossbedded coarse sands dominant, frequently with
upward-fining particle size trends. In contrast,
other morphostratigraphic units are characterized by lower-energy facies composed primar-
channels have reworkedbar platform sands, and floodplain sequencesare dominatedby verticallyacreted fine
sands. In model B, bar platform sands are preservedbeneath distal top-stratum fine sands and silts, as the main
channel has avulsed acrossthe floodplain.
ily of medium-fine to fine sands. All four types
of morphostratigraphic unit are observed on
each bar type, although their scales and spatial
associations differ.
On compound bars of the braided reach,
morphostratigraphic units exhibit no characteristic spatial pattern, resulting in complex
within-bar trends in facies and particle size
characteristics. Sediment trends are much better defined on point bars in the meandering
reach, with both down-bar and lateral trends
in facies and particle size characteristics. The
primary control on observed within-bar sediment trends on all bar types is exerted by the
activity of chute channels. These actively rework bar platform sediments, scouring to the
depth of basal channel framework gravels. As
such, contemporary bars of the Squamish River
are dominated by discontinuous sheets of
386
G.J. BRIERLEYAND E.J. HICKIN
Fig. 5. Representative photographs of floodplain sedimentology of the Squamish River. (a) Vertically accreted fine sands
are observed directly atop basal channel framework gravels in this braided reach floodplain exposure. Sediments are 3.5
m deep. (b) Selectively preserved bar platform sands beneath overbank deposits in the meandering reach. Upward-fining
trough cross-bedded and rippled sands contrast markedly with the darker fine sand/silts above. These have lighter colouted proximal top-stratum (flood cycle) deposits atop. (c) 4 m pit dug in the ridge/swale sequence of a laterally confined point bar. Note the sharply tilted swale contacts at the base of the pit side wall. The floodplain is composed in large
part of vertically accreted fine sands.
FLOODPLAINDEVELOPMENTBASEDON SELECTIVEPRESERVATIONOF SEDIMENTS
387
Element code
Top stratum a)Sandwedge
1[
0
b)Proximal
metres
i
i
5
10
Chute
"-'-----~-'~
"J=~===
~
J
Bar platform
Gravels
)~___(~,~
Fig. 6. Chute channel reworking of bar platform sands in lateral (trench) sediment sequences.
coarse sands laid down on bar platforms, with
finer-grained deposits laid down in dissecting
chute channels and on marginal ridges and occasional remnant floodplain units.
To examine broader scale elemental sedimentology of the floodplain, sediment sequences were examined in 9 trenches, up to 26
m in length, dug perpendicular to the main
channel at bar/floodplain margins, and in regularly spaced longitudinal bank exposures (up
to 210 m in length). Elements are defined on
the basis of their geometry, position in the sediment sequences, their bounding surface and
their associated sedimentology. In this study,
elements simply represent morphostratigraphic units observed and analysed in vertical sediment exposures.
Two element-scale floodplain sedimentology models are proposed for the Squamish
River (Brierley, 1991b; Fig. 4). In the first
model, floodplain sequences marginal to contemporary bars are dominated by chute, ridge
and top-stratum deposits (up to 3 m thick),
with minimal preservation of bar platform
sands (Fig. 5a). The latter occur as remnant
units of planar or trough cross-bedded coarse
sands at the base of sediment sequences. As
observed on contemporary bars, bar platform
sands are removed during flood stages and replaced by lower-energy depositional sequences, especially in chute channels. In many
instances these chute channels have scoured to
basal channel gravel depth. Sediments infilling
chute channels and their adjoining ridges are
composed of vertically acrreted fine sands (up
to 2 m deep), with up to 1.5 m of top-stratum
(flood-cycle and sand wedge) deposits atop
(Fig. 6). As such, floodplain sediments in
trenches and bank exposures immediately adjacent to contemporary river bars contrast
markedly with deposits observed on the bars
themselves. While the former are dominated
by fine sand, lower-energy facies, the latter are
composed primarily of coarse sand, high-energy facies.
In the second model of the floodplain sedimentology of the Squamish River, channel
avulsion has resulted in preservation of up to
388
2 m thick sequences of bar platform units composed of coarse sand. These are preserved beneath uniform sequences of fine sands, up to
2.5 m thick, which were deposited in a distal
floodplain environment. Subsequent avulsion
h as resulted in placement of proximal top-stratum deposits above these distal top-stratum
u nits. The former are composed of flood cycle
sequences up to 3 m thick (Fig. 5b).
Bar platform sands also are preferentially
preserved beneath vertically accreted top-strat u m deposits adjacent to a point bar in a laterally confined bend. As this bend impinges
against a bedrock bluff, inhibiting bend migration (Hickin and Sichingabula, 1988), no
chute channels are observed, and bar platform
deposits are laterally continuous, with overbank deposits of fine sands atop, away from the
main channel (Fig. 5c). These sediment sequences are typical of those ascribed to a
meandering planform style (e.g. Allen, 1965 ),
but are atypical for floodplain sequences of the
Squamish River in which distal top-stratum
elements are not observed in mid sediment sequence. This implies that the mechanism of bar
platform preservation is tied to chute channel
processes.
Discussion
Vertical stacking of elements is consistent
throughout the study reach, and the two floodplain sedimentology models apply in both the
braided and meandering reaches. A mechanism of selective preservation of bar platform
deposits is proposed in which these units are
preferentially preserved beneath distal overbank elements. The latter accumulate above
bar platform sands following channel avulsion. Under conditions of more gradual channel migration, however, bar platform sands are
stripped away by chute channels, and replaced
by fine sands. The coarse sand which makes up
comtemporary bar platforms is flushed through
the system and accumulates on the delta. Various sources of evidence support this assertion:
G.J. BRIERLEY AND E.J. HICKIN
Bar
0
Head
3750
lm
Bearing 52°N
3250
a
I
b
I
I
Bearing 348°N
Mid-bar
3 2• 5 e
~
<4e
)
e
Bar Tail
444e
<
<4o
<4o ~ o
4
Bearing 334°N
~4o
3
,
I
2
a
~
I b I
Fig. 7. Ridge accretion during a 1986 flood event. Sediments both thin and becomefiner u~ridge (i.e. awayfrom
the main channel). The medium-fine sands adjacent to
the main channel were washed away during the next rising-stageflowevent (labelleda on the figure), leavingthe
very fine sands and silts as the only record of the initial
flood event (labelled b).
(1) During a June 1986 flood event, the
dune field making up the bar platform of a
point bar was mobilized in its entirety, and the
configuration of dunes and avalanche faces on
the bar were completely altered. Following
passage of this event, the only sediments preserved on the floodplain adjacent to a point bar
were fine sands accreted to ridges during the
waning flood stage (Fig. 7). Any m e d i u m or
coarse sands which accumulated during this
event were subsequently removed. The fine
sands and silts deposited on the ridge are the
only sediments deposited during this event
FLOODPLAIN DEVELOPMENT BASED ON SELECTIVE PRESERVATION OF SEDIMENTS
which have any longer term preservation
potential.
(2) At the river mouth, boils on the water
surface caused by dune fields. Materials convected to the surface are coarse sands (sampled by Rood and Hickin, 1989). When the
main channel was dredged prior to dyking, a
large volume of coarse sand was removed from
the channel.
(3) Drilling by Swan Wooster Engineering
and Simon Fraser University has yielded sediment cores from the delta to depths of 50 m,
and all show substantial amounts of coarse
sand.
Although similar mechanisms for removal of
coarse sand and gravel facies have been described elsewhere (e.g. McGowen and Garner,
1970; Alexander and Nunnally, 1972; Bluck,
1976; Baker, 1977; Brakenridge, 1984), their
replacement by lower energy deposits which
dominate the floodplain has not previously
been described. In the Squamish River system,
coarse sands on bar platforms seemingly are
transitory evidence of sediment pulses, mobilized during flood events and replaced at bar
margins by lower-energy deposits during waning flood stages. The pronounced seasonal variability in discharge of the Squamish River
probably promotes this process, as sediments
accumulated at lower discharge stages are regularly mobilized during freshet (snow melt)
flood events.
Bar platforms are the least vegetated and
most accessible depositional unit in a river system. Given their abundance on bar surface,
coarse sands appear to be the dominant contemporary sediment unit of the Squamish
River. This impression is highly misleading as
an analogy in floodplain modelling, as fine
sands dominate the floodplain. Although considerable amounts of coarse sand have been
deposited in both distributary channels and estuarine environments of the delta, coarse sands
make up only a small proportion of the overall
depositional suite of the Squamish River
system.
389
Summary and implications
Given the high energy, dynamic geomorphic
setting of the Squamish Valley, floodplain sediments do not meet intuitive expectations.
Bottom-stratum deposits, which have been
shown to be preferentially preserved under
certain environmental conditions (e.g. Cant
and Walker, 1978; Mader, 1985 ) are only partially preserved in the Squamish River floodplain, and channel framework gravels predominantly are transitional upwards to fine sand,
top-stratum deposits. From analysis of trench
and bank exposures it is evident that chute
channels have partially removed bar platform
sands and replaced them with lower-energy,
chute channel infills. The abundant coarse
sands which make up active bar surfaces are
largely flushed through the system and stored
in the delta. The following implications can be
made:
( 1) There is a need for greater input from
geomorphology into floodplain sedimentology
if mechanisms of floodplain growth and preservation potential of sediment units are to be
understood. This is particularly relevant with
increased interest in Holocene floodplain research and studies of environmental change.
Since elements are defined specifically in geomorphic terms, they provide an ideal framework for sediment analysis.
(2) The fact that elements stack in identical
vertical sequence in both the braided and
meandering planform reaches indicates that
processes are operating in a similar fashion independent of planform style. Chute channel
reworking of bar platform deposits is associated with lateral and down-valley migration of
the main channel (s). Bar platform sands also
are preserved beneath distal overbank sediments, associated with channel avulsion, in
each of the planform reaches. As such, the
preservation potential of individual sediment
units relates directly to the local scale geomorphic history of the floodplain.
(3) Given this mechanism of sediment re-
390
working, the d i s t i n c t i o n b e t w e e n w i t h i n - c h a n nel a n d o v e r b a n k m e c h a n i s m s o f f l o o d p l a i n
g r o w t h has e v e n less r e l e v a n c e t h a n p r e v i o u s l y
has b e e n t h o u g h t . B e t t e r t e r m s m a y be " c o n s t r u c t i v e " (flowing w a t e r ) a n d " p a s s i v e "
(slackwater) sediments, reflecting tractive a n d
s u s p e n d e d deposits respectively. Passive deposits d o m i n a t e large parts o f the S q u a m i s h
R i v e r floodplain. T h e p r o m i n e n c e o f these fine
sands c e r t a i n l y is c o n t r a r y to i n t u i t i v e expect a t i o n s in the high-energy b r a i d e d reach.
Acknowledgements
T h i s s t u d y f o r m s p a r t o f a d o c t o r a l thesis
c o m p l e t e d b y G. Brierley u n d e r the supervision o f E.J. H i c k i n . Digging a n d field c o m p a n i o n s h i p were p r o v i d e d b y J o h n Olyslager, R o d
M a l c o l m , Kelly T h o m p s o n , G r a h a m Schnare,
a n d m a n y o t h e r g e o g r a p h y s t u d e n t s a n d colleagues f r o m S i m o n F r a s e r U n i v e r s i t y . T h e editorial c o m m e n t s o f Ian P r o s s e r are greatly app r e c i a t e d . T h e p r o j e c t was f u n d e d b y N S E R C
G r a n t ( A 8 3 7 6 ) a w a r d e d to Dr. H i c k i n .
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