Contemporary Squamish River sediment flux to Howe Sound, British Columbia
EDWARD
J. HICKIN
Department of Geography and Institute for Quaternary Research, Simon Fraser University, Bumaby, B. C . ,
Canada V5A IS6
Received October 1, 1988
Revision accepted February 13, 1989
Squamish River drains 3600 km2in the southern Coast Mountains of British Columbia to Howe Sound at Squamish, some
50 km north of Vancouver. This study uses differencing of digitized bathymetric surfaces based on the Canadian Hydrographic
Services surveys of 1930, 1973, and 1984 at the head of Howe Sound to yield a long-term sedimentation rate for Squamish
River delta. The sediment flux from Squamish River to Howe Sound is determined to be 1.29 x lo6 m3 a-' or 1.81 x
lo9 kg a-', rates consistent with loads calculated from flow and sediment-concentration regimes measured in the estuary in
1973- 1975 and 1987- 1988. The latter data indicate that the modal discharge-effectivenessclass is 600-700 m3 s-', moving
13% of the annual suspended-sediment load. Discharges up to 1400 m3 s-' constitute 99.8% of all flows and are responsible
for transporting 81.5 % of the annual load. A very significant 18.5% of the load is moved by large-magnitude floods
( > 1400 m3 s-'), which occur less than 0.2 % of the time.
Squamish Delta is prograding downfjord at an average rate of 3.86 m a-' although local extensions of the delta front in a
given year may approach 20 m. Some of the geomorphic implications of extrapolating these contemporary rates of fjord
infilling over the Holocene are discussed briefly.
Dans le sud de la chaine Coast de la Colombie-Britannique, la rivikre Squamish draine 3600 km2de terrains vers le dCtroit de
Howe Squamish, soit 9 environ 50 km au nord de Vancouver. Cette Btude est fondBe sur la diffCrenciation des aires
bathymktriques numkraliskes fournies par les moniteurs des Services hydrographiques pour les annBes 1930, 1973 et 1984, i la
t&tedu dCtroit de Howe, dans le but de dCterminer le taux de sedimentation sur une longue dude dans le delta de la riv5re
Squamish. Le flot de saiment de la rivikre Squamish dans le dCtroit de Howe est BvaluC a 1,29 x lo6 m3 a-' ou 1,81 x
lo9 kg a-', ces taux qui concordent avec les charges calculCes des rkgimes d'ecoulement et de concentration des sCdiments
mesuds dans l'estuaire en 1973- 1975 et 1987- 1988. Ces derniers ksultats indiquent que la classe de la capacitk modale de
dkbit est 600-700 m3 s-', dkpla~ant13%de la charge annuelle des saiments en suspension. Les dtbits, jusqu'i 1400 m3 s-',
constituent 99,8% de tous les Bcoulements et rkpondent du transport de 81,5 % de la charge annuelle. Une part importante de
18,5% de la charge est deplacke lors des grandes crues ( > 1400 m3 s-') qui se manifestent en moins de 0,2% du temps.
Le delta de la riv2re Squamish prograde en aval du fjord a un taux moyen de 3,86 m a-', quoique certaines extensions
locales du delta peuvent approcher 20 m en une annCe donnke. Une discussion portant sur quelques implications gkomorphologiques 9 l'Holoc5ne est fondCe sur une extrapolation des taux contemporains de remplissage du fjord.
[Traduit par la revue]
Can. J. Earth Sci. 26, 1953 - 1963 (1989)
Introduction
Squamish River drains a major alpine catchment in the Coast
Mountains of southwestern British Columbia and is the principal source of freshwater and fluvial sediment delivered to
Howe Sound, situated some 50 km north of Vancouver. This
fluvial system is a hydrologically representative component of
the coastal Cordillera of southwestern Canada and is reasonably accessible. The alluvial fill of Squamish Valley also represents a vast store of sedimentological information that may
provide insight into the environmental conditions of the last
10 000 years in this part of the Coast Mountains.
Squamish River supports a significant local fishery, and the
delta provides one of the very few wetland habitats in these
mountainous fjordlands. Furthermore, Squamish distributary
sedimentation represents a major engineering concern for
Squamish Harbour in general and for the deep-sea docking
facilities at Squamish Forest Products Terminal and F.M.C.
Chemicals in particular. These concerns have resulted in
expenditures in the millions of dollars on environmental engineering during the last two decades.
Despite the river's importance and a considerable amount of
environmental investigation, little is known about its performance as a sediment transport system. Even the average
annual sediment load remains unspecified by measurement.
The purpose of this paper is to present the results of analyses of
volumetric sediment yield based on comparative bathymetry of
Printed in Canada 1 Imprim.6 au Canada
Squamish Delta and of suspended-sediment-load measurements made in Squamish Estuary.
The environmental setting
Squamish River drains 3600 km2 of rugged Coast Mountains
terrain to the head of Howe Sound at the town of Squamish
(Fig. 1). It is one of many coastal fjords aligned with the
regional structural lineations that trend northeast-southwest
across the axis of the mountain range.
The general area has been the subject of considerable scientific scrutiny, which has been documented in a substantial
environmental literature. Much of this work to 1974 was catalogued in the major review undertaken by Environment
Canada (in particular, Hoos and Vold 1975). More recent studies of Squamish River geomorphology, hydraulics, sedimentology, and channel dynamics have been undertaken by Hickin
(1978, 1979, 1984), Brierley (1984), Brierley and Hickin
(1985), Sichingabula (1986), and Hickin and Sichingabula
(1988). Studies of Howe Sound oceanography and of sedimentation dynamics of Squamish Delta were reviewed by Bell
(1975) and more recently by Syvitski (1978), Syvitski et al.
(1987), and Syvitski and Murray (1981). Related research
includes the work of Syvitski and Macdonald (1982) and Syvitski et al. (1988).
Squamish drainage basin is cut into plutonic rocks of complex origin that formed over an extended period of time from
CAN. J. EARTH SCI. VOL. 26,1989
1954
- - --0
Contours in metres
Squomlsh Alvsr wolershed
10
20
30
40
50 krn
2
4
6
FIG. 1. Field area.
the Late Cretaceous and consist largely of quartz diorite,
granodiorite, and quartz monzonite. These rocks are associated
with older metasediments and with contemporaneous to
younger metavolcanics and Pleistocene volcanics. Mathews
(1958) mapped and described the geology of the Mount Garibaldi map area, which includes portions of Squamish, Cheakamus, and Mamquam basins. Mamquam River is unusual in
that its lower basin is dominated by extensive exposures of
metasediments and metavolcanics (massive and bedded greenstone chert, argillite, sandstone, conglomerate, and their schistose equivalents).
The highest peaks in the area are the Quaternary volcanic
piles of Mount Garibaldi (2678 m asl) on the CheakamusMamquam divide and Mount Caley (2393 m asl), forming the
northern divide between Cheakamus and Squamish basins
(Fig. 1). These peaks consist of highly unstable and rapidly
eroding andesitic lavas and tuffs, with some basalt and dacite
flows at lower elevations. Vesicular volcanics form a very
important component of Squamish River bed materials and
sediment load at the delta and are dominantly Mamquam,
Cheekye, and Cheakamus inputs.
Cheekye Creek, presently a tributary of Cheakamus River,
deserves special mention here because of its prolific sediment
production. Sediment supply from this steep stream coming off
Mount Garibaldi to Squamish River is so high that much of it is
stored in a 2.5 km3 fan, which has pushed Squamish River
against the western rock wall of its valley. The accumulated
sediment is such that it exercises upstream control on Squamish River.
About 6 km downstream from the Mamquam junction, Squamish River discharges into Howe Sound, forming Squamish
Delta. Here the fjord is about 2000 m wide and gradually
increases to about 3500 m, before it flares into Montagu Channel some 17 km downfjord.
Depth of water along the steep-walled fjord increases rapidly
to about 220 m near Woodfibre and then more gradually to
about 290 m in the basin at the foot of the inner sill about
17 km downfjord. The inner sill comes to within 30 m of the
'
I
I
surface and serves to divide the fjord into inner and outer
Howe Sound. This study is concerned only with the area of
inner Howe Sound. The sill is interpreted as a submarine
moraine marking the maximum advance of the ice during the
Sumas Stade (about 11 300 years ago) of the Fraser Glaciation
(Mathews et al. 1966). Inner Howe Sound was probably occupied by a valley glacier for some time after the end of the icesheet phase but was presumably ice free by 10 000 years ago
(Mathews et al. 1970).
Surficial sediments in inner Howe Sound range in size from
fine sand on the delta front to muddy sand and sandy silt in the
deeper water near Woodfibre. Farther downfjord there are
greenish grey clayey silts and pockets of clay and sandy silt
(Mathews et al. 1966; Syvitski and Macdonald 1982). In contrast, the inner sill consists of slightly granular to gravelly mud
with a high percentage of fresh vesicular lava from source
areas within Squarnish and Cheakamus basins (Mathews et al.
1966). On the delta front the western-sector sediments tend to
be coarser (fine to coarse grained and poorly sorted sand),
whereas those in the central to eastern sectors have a higher
proportion of silt and clay. This trend reflects the Squamish
sediment-plume path from west to east across the delta front to
the eastern shore, where it moves southward to Watts Point
and then reverses the earlier pattern by crossing the fjord
again, this time east to west towards Woodfibre.
The present plume path has changed significantly from that
of a century ago. Until early this century Mamquam River discharged directly into Howe Sound through the present Mamquam blind channel. A major flood caused the Mamquam to
avulse to its present course in 1921, and by 1947 all flow
through the Mamquam blind channel had ceased. In consequence, the eastern sector has been deprived of direct distributary sedimentation for half a century or so.
A further major change in distributary configuration occurred
in 1972, when all Squamish River flow was engineered into the
western distributary and held there to date by a training dyke in
order to control flooding and prevent siltation of Squarnish
Harbour.
This western distributary now carries the entire 300 m3 s-'
mean annual discharge from Squamish River. The period of
record (1956- 1988) indicates that summer snowmelt freshet
discharges typically are of the order of 600 m3 s-' and that
winter flows commonly drop to 100 m3 s-'. Large flows
(>2000 m3 s-'; return period > 10 years) are associated with
the freshet peak in July but more commonly occur in late
summer or autumn in response to heavy rains.
Daily mean discharges for Squarnish Estuary are taken as the
sum of the daily mean discharges on Squamish River at Brackendale (Water Survey of Canada (WSC) station 08GA022), on
Cheakamus River at Brackendale (WSC station 08GA043),
and on Mamquam River above Mashiter Creek (WSC station
08GA054). Missing data have been estimated by interstation
correlation. Discharge of Squamish River at Brackendale constitutes 83 % of the estuary discharge; these data are available
for all years from 1956 to the present (Inland Waters Directorate 1956- 1986).
Because the river is both steep and energetic the estuary is
only 6 krn in length even at low flow. Tidal influences are felt
upstream to the Mamquam junction at low flow but are less
evident during the freshet. The tidal regime is of the mixed
type typical of the Pacific coast, with two highs and two lows
in a tidal day. Mean tidal range is 3.2 m, and the maximum is
5 m. Lower low water for a large tide is -0.33 m in elevation,
sufficiently low to expose the baymouth bar forming the top of
the delta front.
Changes in Squamish Delta bathymetry
The technique of differencing reservoir bottom surveys to
determine rates of sediment infilling behind dams has been
routine engineering practice for many years. Considerable use
has also been made of resurveys by Earth scientists interested
in sediment erosion -accumulation rates in a variety of settings
from gravel-bed mountain channels (e.g., Nanson 1974) to
lacustrine environments (e.g., Gilbert 1973, 1975) and coastal
deltas (e.g., Mathews and Shepard 1962). Nevertheless, such
survey differencing is not widely utilized in geomorphology
because, even if sequential surveys are available, most geomorphic surfaces of interest are usually too extensive and the
survey interval so short relative to the typically small erosion accumulation rates that the correspondingly small form differences are exceeded by the measurement errors involved.
Fjord-head delta progradation provides an interesting exception. It is ideally suited to measurement by bathymetric survey
differencing because the accumulating sediment pile is highly
confined and sedimentation rates are relatively great. Furthermore, the environment of deposition, with its single sediment
source and largely unidirectional sediment pathways, is relatively simple and quite orderly.
The approach demonstrated here was recommended to the
WSC in a review of the sediment survey programme (Church
et al. 1985) as an alternative to conventional sedimenttransport monitoring. Church et al. noted that although measurement programmes rarely are maintained for more than a
few years, survey differencing can provide well-averaged
results at relatively low cost.
The data and methods of analysis
The data
Howe Sound in the vicinity of the town of Squamish was
first surveyed by the Canadian Hydrographic Service (CHS) in
1930, and the most recent resurveys were completed in 1973
and again in 1984. The interval between 1930 and 1973 is a 43
year period during which Squamish sedimentation was largely
natural. Construction of the river training dyke in 1972 and
subsequent dredging of the channel and foreshore have meant
that the following decade and beyond to the present represents
a post-construction period of distinctly disturbed sedimentation.
Comparison of the hydrographic surveys of 1930 and 1973
was first made by Bell (1975) in an unpublished thesis in
which he presented two charts of the bathymetry at a scale of
1:8000. The bathymetry for 1930 is based on CHS field sheet
2313-2 (photographically enlarged from an original scale of
1 : 24 300). The bathymetry for 1973 is based on CHS field
sheets 2235-S (Squamish Harbour) and 2278-L (northern
Howe Sound) and involved photographic enlargement from an
original scale of 1 : 30 000. Bottom contours were plotted from
soundings shown on the field sheets in fathoms (reduced to
mean sea level). Bell's original charts, now held in special collections at the Main Library of The University of British
Columbia, constitute the main data base for the present study.
The bathymetry for 1984 was obtained by Prior and Bornhold
1956
CAN. J. EARTH SCI. VOL. 26, 1989
(1984) and was compiled in a chart at a scale of 1 : 10 000,
with a contour interval of 5 m.
Bell (1975) employed the 1930 and 1973 bathymetric charts
to characterize general geomorphic change in the delta over the
period, but quantitative comparison was limited to the measurement along six longitudinal profiles of vertical and lateral
accretion rates at depths of 60, 120, 180,240, 300, and 360 ft
(1 ft = 0.3 m) bsl. Apparently, no attempt was made to compute the accumulated sediment volume.
In the present study the receiving basin for Squarnish River
sediments is taken as the submarine valley floor of Howe
Sound from the mouth of Squamish River to the fjord sill, a
surface bounded laterally by the abrupt break in slope at the
foot of the steep valley walls (the 200 m contour downfjord
from Woodfibre). Thus, the surface of accumulation is about
16 krn in length, almost 2 km wide, and about 2.9 X lo7 mZ
in area.
Bottom elevations over the surface of accumulation to a
depth of 200 m below sea level were determined from the contours on each chart for all intersections of a superimposed
100 m x 100 m regular grid. Intermediate elevations were
computed by linear interpolation between contours. Graticule
alignment was identical on each chart, with a principal ny
reference at 49"40t0"N, 123" 10'O"W. This surface digitization exercise yielded about a thousand coordinates for each
chart, of which 642 subsequently were utilized in the final
analysis. This elevation array is sufficiently dense to integrate
the rather regular surface without imposing an excessive datacollection burden; all subsequent data processing was done by
computer.
Measurement error
The accuracy of the survey data in a study of this type obviously must be a prime concern. But it also is very difficult to
establish. Errors in chart soundings relate to echo-sounder calibration, ship positioning, bottom-slope characteristics, and
mapping factors. The target accuracy of the CHS, incorporating all of these error types, is f3 % of the operating depth of
water. It is considered (CHS, Victoria, personal cornmunication, 1988) that this level of accuracy is generally not achieved
in distinctly less than ideal conditions (poor weather in deep
open waters) but is significantly exceeded (*1%) in modem
surveys of relatively quiet and shallow conditions in enclosed
waters and harbours.
The problem of independently assessing errors in the case of
the present analysis is that little is known of the operating conditions during the 1930 survey. Operating in favour of good
resolution are the facts that the depositional surface is regular
and not highly crenulated and that bottom slopes almost everywhere are less than 3". Furthermore, where slopes are steepest, water depths are at their shallowest. Working against good
resolution is the generally considerable depth of water, almost
everywhere in excess of 50 m (2000 m from the river mouth,
the delta surface is 100 m bsl).
In any case, even if single point measurements of depth have
a wide margin of error, in a large enough sample of measurements errors may be largely self-cancelling and therefore depth
measurement may be distinctly more accurate in the mean.
Some sense of the accuracv of the differenced 1930 and 1973
bathymetric surveys can be obtained by direct measurements
over parts of the surface where the elevation differences are
known to be near zero. Here apparent differences should
largely reflect measurement error.
A candidate area for such a test is the fjord bottom near
Woodfibre. Here in over 200 m of water 5 km from the mouth
of Squamish River, where sedimentation rates can be taken as
negligible, an array of 77 grid points (data for all depths >
200 m) yields E,, = 212.805 m, E7j = 208.052 m, and E,, =
210.548 m. The distribution of elevation differences ((E30 E7,)lE7, and (E,, - E,,)IE,,) have respective means of 0.023
and 0.012. In other words, for the 1930- 1973 data, measurement error in the mean appears to be 2.3 % of the mean depth
(4.79 m in 208.05 m) and for the 1973- 1984 data the associated error in the mean is f 1.2 % of the depth; the upper 99 %
confidence limits are, respectively, 2.9 and 1.7% . Again,
these errors are very conservative in the sense that if sedimentation has occurred, assumed zero differences actually will be
finite and the errors consequently will be smaller still. For
example, limited sediment-trap data (Syvitski and Murray
1981) indicate an annual accumulation rate here of at least
0.01 m a-l. Thus, there may actually have been about 0.43 m
of sediment accumulation in the test area between 1930 and
1973, in which case the associated measurement error is 2.1 %
rather than 2.3%.
The average elevation of the fjord surface over which sediment volumes have been computed by bathymetry differencing
is about 110 m. In light of the foregoing observations on measurement errors it is suggested that measurement accuracy for
the mean elevation differences between surveys can be conservatively estimated at +2% of the mean depth or k2.2 m for
the 1930- 1973 surveys and f1% or f1.1 m for the 19731984 survey.
*
Volumetric-analysis procedure
The total volume of accumulated sediment for the
1930-1973 period was calculated in two components.
Component I is the sediment accumulated on the fjord floor to
a depth of 175 m and includes the active delta front and prodelta area. Although this component represents only 30% of
the receiving basin, it accounts for almost all of the total
volume change. Volume computations here are based on direct
survey differencing utilizing 642 grid points.
Component I1 is the sediment that has settled on the fjord
floor in water depths greater than 175 m. Here water depths are
too great and sediment accumulation rates too slow for bathymetric differencing to be reliably employed. Instead, a diffusion model based on component I data has been used to
generate volume estimates for component 11. The component I
data set of elevation differences was divided into 12 subsets of
about 50 grid reference points each at increasing distance
downfjord from the mouth of Squamish River. As expected,
the depth of accumulated sediment declines exponentially with
distance (Fig. 2), and the relationship can be used to extrapolate sedimentation rates into the lower fjord.
Although such extrapolation needs to be done cautiously, the
main point of the exercise in the present context is to confirm
that component I1 sedimentation is a negligible proportion of
the total accumulated-sediment volume. Several factors suggest that eq. [I] in Fig. 2 provides an appropriate picture of the
lower fjord sedimentation. First, physical reasoning suggests
that the sediment-accumulation-distance relationship should
display an exponential form. Although sediment transport
other than diffusion and grain settling (slumping and turbidity
currents, for example) has produced departures from the ideal
trend (Fig. 2), 85 % of the variance in the depth of accumulated
sediment is statistically explained by a distance decay func-
-1.31
500
1000
1500
2000
2500
3000
0.05
3500
Distance. X (metres)
FIG. 2. Relation of average annual depth of sedimentation (D) to downfjord distance (X) from the mouth of Squamish River.
TABLE1. Volume rate of sediment accumulation in inner Howe Sound (1930- 1973)
Component I
Component 11
Total
54 114 180
855 608
54 969 788
600 000
55 569 788
Area (mz)
Mean accumulation (m)
Volume (m3)
Net dredging volume (m3)
Total volume of accumulation (m3)
Mean volume rate of accumulation (m3 a-')
Upper limit (+25 %)
Lower limit (-25 %)
tion. Second, the few sediment-trap data available for the
lower fjord (Syvitski and Murray 1981) indicate an average
rate of the same order of magnitude as those predicted by eq.
[I]. Third, there is some seismic sparker evidence (Syvitski
and Macdonald 1982) suggesting that the Holocene sediment
pile in Howe Sound may be about 50 - 100 m thick in places.
Given that paraglacial sedimentation rates were almost certainly higher than the contemporary rate (Church and Ryder
1972) and that some of the sediments of Howe Sound are
derived from non-Squamish sources (for example, there is an
annual incursion of Fraser River freshet water into lower Howe
Sound), 50 m 1 10 000 a (0.005 mla) provides an upper limit
on the fluvial sedimentation rate in the lower fjord.
Equation [I] predicts a rate of 0.004 m a-I at Squamish Harbour limit near Britannia Beach, some 10 km distant from
Squamish, and 0.0002 m a-' in the 285 m deep basin at the foot
of the sill, 16 krn from Squamish.
Results of the volumetric analysis
Comparison of the 1930 and 1973 surveys
Sediment-volume calculations for components I and I1 are
summarized in Table 1. Component I sedimentation constitutes 98.4% of the total volume of accumulated sediment; both
component I1 volume and the net dredging volume are relatively small in comparison. The spatially averaged accumulation rate for component I is 8.429 2.20 m, equivalent to a
+
1 292 321
1 615 401
969 241
+
mean annual rate of vertical sediment accumulation of 0.196
0.05 m a-' (assuming that any depth error associated with
spatial averaging is small and subsumed in the specified measurement error). The 25% error margin and the associated
volume-rate limits simply recognize that the sounding-error
differencing of 2.2 m in a total accumulated depth of 8.429 m
rounds off to 25 % . Since determining the receiving basin area
presents no special measurement problem, it seems reasonable
to assume that the error in the volume rate is sensibly equivalent to the estimated error in the rate of vertical accumulation.
But of course these average figures mask considerable spatial
variability. For example, it is not uncommon (9 % of cases) for
scouring and (or) slumping to have resulted in localized sediment losses of up to 5 m in places (-0.12 m a-I); extreme
point degradation associated with submarine gully development peaks at 18 m for the survey period (-0.42 m a-'). In
most places, however, the depth of sediment accumulation is
positive and generally increases towards the river mouth
(Fig. 2). Maximum rates of sediment accumulation of 3040 m (0.7 - 1.0 m a-') occur on the upper part of the steep
(20") delta front as a result of grain avalanching rather than
settling.
The pattern of delta progradation between 40 and 175 m bsl
is shown in Fig. 3. The average delta profile is semilogarithmic in form and appears to represent an equilibrium morphology, since it has remained virtually unchanged during 166 m of
downvalley translation during the survey period. This transla-
CAN. J. EARTH SCI. VOL. 26,1989
+ log
2.3r
Olog E, 1973
E,1930
t 0.1
R' = 0.996
[3]
1.44
500
log E (1930) = -0.0002890X
+ 2.374
1
1000
1500
2000
Distance. X (metres)
2500
3000
0.05
3500
FIG.3. Relation of average elevation ( E ) of the delta surface to the downfjord distance (X) from the mouth of Squamish River (1930, 1973).
tion implies that the alluvial valley fill presently being deposited by-Squamish River is advancing into Howe Sound at an
average annual rate of 3.86 m a-' .
In water depths of <40 m, sedimentation and progradation
rates are less well ordered with respect to depth or distance
downfjord because proximity to particular distributaries
becomes a controlling factor. Furthermore, in the areas of high
sedimentation rates, much of the accumulating sediment is
transient. Bell's (1975) profiles show as much as 300 m of progradation at the present active delta front (7 m a-'), steepening
the delta-front slope at 20-50 m bsl from about 4" to 15"
during the 43 year survey period. If the profiles in Fig. 3 represent long-term equilibrium, then these rapid morphological
changes at the present delta front (also see Wiebe 1976;
Zrymiak and Durette 1979) must be regarded as nonsustainable short-term effects. Clearly, the sediment wedge accumulating near the surface of the delta front must be redistributed
fromtime to time to deeper regions of the delta by mass movement and turbidity currents. Certainly it is a matter of observation by dredge operators that the delta front near sea level can
suddenly retreat by as much as 20 m (Mr. Gabor Veres, Sandwell Swan Wooster Engineering, Vancouver, personal communication, 1988), presumably as the result of slope failure.
Such short-term variability is a cautionary note to those who
might be inclined to generalize to annual progradation rates
from a limited series of annual delta bathymetry.
Comparison of the 1973 and 1984 surveys
Unfortunately, the data generated by differencing the 19731984 bathymetries are not particularly useful, although the
mean accumulation rate derived from them is not inconsistent
with that based on the 1930- 1973 data set. The mean difference in the 642 grid reference elevations for component I
between 1973 and 1984 is 0.925 m. Allowance for a 5 X
lo6 m3 dredging loss yields a component I sediment accumulation of 1.0 m. In other words, given the error estimate of
1 m, the actual accumulation could be as much as 2.0 m,
corresponding to an annual rate of 0.182 m a-', one not inconsistent with the 0.196
0.051 m a-' based on the 19301973 data.
+
Suspended-sediment loads in Squamish River
Independent confirmation of the magnitude of the annual
volumetric sediment flux calculated from bathymetry differencing can be sought in the suspended-sediment-load data
available for Squamish River below Mamquam River.
Suspended-sediment concentrations are not routinely measured
on this river, but WSC did operate an automatic pump sampler
at its water-level recorder (WSC station 08GA053) during late
1973 and the hydrologically active months of 1974 and 1975.
The pump-sampler intake was located about 1 m above the
boundary on the eastern thalweg side of the channel. The daily
mean concentrations reported here are WSC estimates based
on integration of their sample record. Although the data have
been published by the WSC as daily "mean" concentrations
(Inland Waters Directorate 1973- 1975), the pump readings
have never been calibrated with measured mean concentrations
for the section, although others have assumed a direct equivalence (Syvitski and Farrow 1983; Syvitski et al. 1988).
Another complication with the WSC data is that WSC
station 08GA053 is located at the upstream end of Squamish
Estuary, and here the river is distinctly tidal at low discharges
( < 300 m3 s-') and not completely free of tidal influences even
at moderate to high discharges.
Despite these limitations, there are several reasons to believe
that the data are useful in the present context. The data seem to
be internally consistent in the sense that much of their variability is explained by variations in discharge. Figure 4a shows
daily mean suspended-sediment concentration versus daily
mean discharge, a relationship for which the second-order
polynomial least-squares rating curve statistically explains
60 % of the variance in concentration. Much of the 40 % unexplained variance is associated with discharges less than about
200 m3 s-', a domain in which suspended-sedimentconcentration becomes independent of discharge but very dependent on
the occurrence of sediment-supplying events such as storms.
But these sediment-supplying events are also important at
higher discharges, and inspection of the temporal dimension of
the data reveals a distinct seasonal hysteresis in the concentration-discharge relation (Fig. 4b) (also see Syvitski et al.
1987).
It is convenient to partition the year into two periods, each
with its own rating curve. The first, from April to July, encompasses the rising limb and crest of the annual hydrograph,
hydrologically a relatively homogeneous period in which
-
3.5~
.
[4]
3.
log Co = 3.393 - 2.mlog Q
+ 0.793(log Q)'
a
O0
o
0
'
0 log 6 (August-March)
log Co (April-July)
3.5
( 0 )
.
3
-
2.5.
2.5
2
u0
-
F
2
1.5
1
0.5
1-6
?.
1.8
2.2
2.4
2.6
2.8
3
32
1.6
1.8
2
22
2.4
log Q
2.8
3
3.2
log Co field (1987)
3.5~
,(
2.6
log Q
c
l
[ 5 ] 1cqC.a
= 2.W4.3.143IogQt
l.019(log0)
log Co = 1 .47.0.91 leg C1
a
0
+ 0.459 (log 0 )'
---, 2.5.
-7
3
-2
2.
,
0
"m
-
1.5-
.
1.
0.5,
1.6
1
.
j
1.8
.
,
2
,
,
2.2
.
,
2.4
log Q
.
2.6
2.8
3
1
3.2
-
0.5,
1.6
0
.
,
1 .8
1
.
0
0
,
2
,
,
,
2.2
,
2.4
.
.
2.6
,
a
2.8
,
3
log Q
FIG. 4. Suspended-sediment rating curves for Squamish River at Squamish. ( a ) All WSC daily mean suspended-sediment concentrations
(1973 - 1975). (b) Seasonal hysteresis in concentrations. (c) April -July rating curve. (d) August -March rating curve.
suspended-sediment concentration is strongly linked to rising
spring temperatures and the associated melting of winter snow
and glacier ice (Fig. 4c). The rating curve here is such that
90% of the variance in sediment concentration can be explained
statistically by variation in discharge.
The second period, from August to March (Fig. 4d), encompasses the falling limb of the annual hydrograph as declining
temperatures reduce the melt rate of snow and ice. But this also
is a period of less order in the concentration-discharge relation, as only 64% of the variance in concentration is explained
statistically by variation in the daily mean discharge (Fig. 4d).
The reason for this increase in unexplained variance is that
there are in fact two hydrologic populations involved. Superimposed on the declining snow- and ice-melt-driven discharge
and sediment flux are numerous upward excursions signalling
the return to increased winter storminess. These storm events,
particularly prevalent in October to December, produce intense
pulses of sediment-charged flow. Indeed, although the duration of these high storm discharges is short at this time of the
year, they often are of far greater magnitude than the peak flow
of the earlier freshet. In consequence, the suspended-sediment
rating curve for August to March reflects both more variable
and higher sediment concentrations per unit discharge than
those characterizing April to July (Figs. 4c, 4d).
The question of how the pump-sample concentrations relate
to the cross-sectional mean concentration has been answered in
part for the April -July rating curve (Fig. 4c). The mean concentrations were measured in 1987-1988 at WSC station
08GA053. Depth-integrated samples were obtained from RV
Endeavour at five equally spaced verticals with a cablesuspended DH-48 sampler. Sets of samples were obtained
across the tidal cycle on 7 days in June and July, representing a
wide range of discharges. It turns out that, whether by seren-
dipity or design, the daily mean suspended-sediment cotlcentrations for the section and those based on the automatic pump
samples appear to be sensibly identical for the measured discharge range.
No similar data are available for the winter months, but there
is no apparent reason why the relationship of the pump sample
to the cross-sectional mean should change across the seasons.
What remains quite uncertain is the reliability of eqs. [5] and
[6] (Fig. 4) to predict the concentrations at the very important
high discharges for which no data of any sort are available.
One might expect a fixed sampler to overestimate the heavy
concentrations at high discharges, but then eqs. [5] and [6] do
tend to account for such an upward drift by predicting low in
the high discharge range. Predicted concentrations of 40005000 mg L-' at the highest discharges (2000-2500 m3 s-')
may still seem heavy, but measurements elsewhere in the
channel do suggest that they are of the right order. About
1000 m downstream from WSC station 08GA053 the channel
steepens and near-surface flow velocities increase by about one
third to 2.0 m s-'. Here the channel presents something of an
analogue of sediment-transport conditions at higher discharges
at WSC station O8GA053. In July 1987, at a discharge of about
1100 m3 s-', 50 samples of surface water (flow depth = 4 m)
collected from RV Endeavour for another study commonly
yielded concentrations in the 3000 - 5000 mg L-' range. Sedimentological evidence of these high sediment concentrations is
most recently apparent for the 3132 m3 s-' flood of record in
October 1984. At this time a 30 cm thick blanket of fine sand
was deposited along the top of the western bank of the estuary
floodplain (measured by I. Hutchinson, personal communication, 1988).
The annual sediment-load regime for the 2 complete years of
measurement is shown in Fig. 5; the estimated total annual
1960
CAN. J. EARTH SCI. VOL. 26,1989
FIG. 5. Mean daily discharge and relative suspended-sediment load of Squamish River at Squamish (1974, 1975).
suspended-sediment loads for 1974 and 1975 are, respectively,
2.334 and 2.653 x lo9 kg. The load data include an upward
correction (averaging 16.87%) to account for the antilogtransform bias (Church et al. 1985; Ferguson 1986) implicit in
eqs. [5] and [6]. These two years illustrate what appear to be
the two most common scenarios. In 1974 the annual peak discharge was associated with the June-July freshet, and
although a major high flow was recorded in mid-January, no
severe floods were recorded for the fall. Seventy-sevenpercent
of the total annual sediment load was delivered between June 1
and August 31, although no one day contributed more than
5 %. In contrast, only 63 % of the total suspended-sediment
load was moved during the June-August period of 1975, but
21 % of the annual load was moved during 5 days of a stormrelated flood in early November, including 1 day (November
4) on which almost 10% of the annual load was discharged into
Howe Sound.
In order to set these measurements in the longer term context,
eqs. [5] and [6] were applied to the daily mean discharges for
the years of record (1956- 1986) to generate the sequence of
total annual suspended-sedimentloads shown in Fig. 6. A considerable amount of year to year variability is apparent
between the minimum of 0.91 x lo9 kg a-' (1985) and the
maximum of 4.23 x lo9 kg a-' (1968). The mean annual
suspended-sediment load for the period is 2.30 X lo9 kg a-',
and the 95 % confidence limits are 2 .OO-2.60 x lo9 kg a-' .
Since the record for the Squarnish does not extend back
beyond 1956, it is not possible to determine directly how representative the data in Fig. 6 are of the periods 1930- 1973 and
1930-1984 used in the volumetric analysis. Nevertheless,
some indication is given by the much longer record for the
adjacent Lillooet River near Pemberton (WSC station
08MG005). Maximum daily discharges here correlate well
with those on Squamish River at Brackendale (RZ= 0.63, with
P < 0.0001) and indicate that the means for 1930- 1973,
1930-1984, and 1956-1986 (respectively, 515, 536, and
557 m3 s-') are close enough (within 8%) to be considered
identical for the present purpose. The same conclusion is
reached by comparing the mean discharges for July.
In order to compare the mean suspended-sediment load of
2.30 x lo9kg a-' with the volumetric sediment flux, one must
make allowance for bedload transport and mass-to-volume
conversion via the bulk density of the sediment.
The principal source of bedload at WSC station 08GA053 is
likely the Mamquam River, immediately upstream. Survey
differencing of the river bed indicates, however, that bedload
volumes delivered to Squarnish River (K. M. Rood, personal
communication, 1988) are relatively low (25 000 m3 a-' for the
period 1971- 1981). In any case, because Squamish channel at
WSC station 08GA053 is relatively confined and steep, the
sand fraction of the sediment load appears to be fully suspended at most times. Even at relatively low flow the boundary
consists of coarse gravel. In distinct contrast with the estuary
farther downstream, repeated echo-sounder profiling here over
a wide range of discharges failed to detect any bedforms consistent with active bedload transport of the sand fraction. For
these reasons, the bedload fraction at this station is considered
quite minor (probably less than 5% of total load); it will be
ignored in the sediment-budget calculation to follow.
The average bulk density of the 8 m deep increment of'
sediment deposited over component I of the delta between
1930 and 1973 has not been measured directly. Although measurements of sands on the exposed baymouth bar and beaches
of Squamish River (D,, = 0.19 mm) yielded a mean bulk den-
.
1955
3
1960
.
,
1965
.
.
1970
. - 1-9 7 5
T
.
,
1980
.
1985
1990
Year
FIG.6. Estimated annual suspended-sediment load of Squamish River at Squamish (1956- 1986).
sity of 1620 kg m-', much of the sediment transported to the
delta is finer and therefore likely lower in bulk density than
these foreshore sediments. Water Survey of Canada analyses
of 13 Squamish River sediment samples collected in 1973
(unpublished data) yield an average grain-size composition of
58.2% sand, 37.4% silt, and 4.4% clay. United States reservoir data (Lane and Koelzer 1953) suggest a mean bulk density
of about 1400 kg m-3 (1260 - 1540 kg m-3) for such a sediment
mix. This value is consistent with the 1440 kg m-3 (13651520 kg m-3) obtained by Gilbert (1973) for the finer 1.5 m
deep lacustrine sediments in nearby Lilloet Lake and with the
1355 kg m3 obtained for Fraser River delta sands by Mathews
and Shepherd (1962).
If bulk density is 1400-1500 kg m-3, it follows that the
equivalent volumetric sediment flux based on the mean suspended-sediment load is (2.00 -2.60 x lo9 kg)/(1.3 - 1.50 x
lo3 kg m-3) = 1.33 -2.00 x lo6m3, comfortably overlapping
the upper half of the likely range of sediment flux based on
bathymetry differencing (0.96- 1.62 X lo6 m3). Given the
measurement error estimates involved, it appears that the suspended-sediment data certainly are not inconsistent with the
sediment flux based on bathymetric differencing.
Discussion and conclusions
Based on bathymetric differencing, the mean annual sediment flux to Howe Sound from Squamish River is 1.29 x
lo6m3 a-' or 1.81 x lo9 kg a-', rates consistent with the flux
based on suspended-sediment concentrations in Squamish
Estuary. The average rate of vertical sedimentation in the delta
and prodelta region is about 0.2 m a-', and the Squamish
Valley alluvial fill is prograding into Howe Sound at an
average rate of 3.86 m a-' .
The corresponding mean annual specific-sediment yield for
Squamish River is 5.0 x lo5 kg krn-2 a-'. This yield is comparable to the only other long-term sediment-transport rate
available for a British Columbian basin, that obtained by
Gilbert (1973) for the adjacent Lillooet River. He used a 57
year sequence of aerial photography of the Lillooet delta front
to obtain a mean annual specific-sedimentyield of about 3.5 x
lo5 kg
a-'. Both of these yields appear to fall squarely
within the specific-yield domain of somewhat disturbed
(logged) glacierized basins in the region (Church et al. 1989).
The pattern of Squarnish River sediment discharge
throughout an average year based on eqs. [5] and [6] and the
discharge record for the period 1956- 1986 in Squamish Estuary is shown in Figs. 7a and 7b. The importance of both
freshet-related loads and those produced by individual stormrelated major floods is quite apparent. The product of the suspended-sediment rating curves and the flow-duration curve for
1956- 1986 (Fig. 7c) yields the discharge-effectivenessgraph
for sediment load shown in Fig. 7d. The modal dischargeeffectiveness class is 600-700 m3 s-' and is responsible for
moving almost 13% of the annual sediment load. Discharges
up to 1400 m3 s-' constitute 99.8 % of all flows and are responsible for transporting 8 1.5% of the annual load. A very significant 18.5% of the load is moved by large-magnitude floods
( > 1400 m3 s-') occurring less than 0.2 % of the time; 7.6 % is
transported by floods in the range 1400-2000 m3 s-' (0.14%
duration); and 10.9% is transported by the largest floods in the
range 2000 -2700 m3 s-' .
Changes in Squamish Delta during the next century are likely
to have minimal impact on Squarnish Harbour if the present
hydrologic regime continues and the river is kept confined to
the western distributary. Local delta advance here could be as
high as 20 m a-' for the next several decades but will decline as
progradation leaves the dyke behind and sedimentation
becomes less confined. It seems likely, however, that the
3.86 m a-' average annual progradation rate will be a minimum
if logging in the valley continues and predictions of the global
trend to warmer and locally moister conditions are realized.
The sediment-flux data presented here provide a basis for
retrodiction of the Squamish delta-front position during the
Holocene. Such retrodiction obviously is quite speculative and
could only apply to the last 6000 years, if at all. This is a
period during which sea level likely has been stable at the present level (Clague et al. 1982) and one well removed in time
from other immediate effects of deglaciation, paraglacial sedimentation, and the hypsithermal.
If the long-term volume rate of sediment flux is taken as
1.2 x lo6 m3 a-' (allowing for a 10% compaction of the contemporary surface volume of 1.3 x lo6 m3 a-') and the rock-
CAN. J. EARTH SCI. VOL. 26, 1989
1962
Q (m3 11)
Percent tlme flow equalled or exceeded
FIG.7. Long-term (1956 - 1986) flow and sediment-loadcharacteristics of Squamish River at Squamish . (a)Annual hydrograph of mean daily
discharge. (b) Annual sedigraph of mean daily sediment load. (c) Flow-duration curve. (d) Discharge effectiveness for sediment transport.
cut valley the alluvium has filled is assumed to have a
rectangular cross section 250 m deep, the delta front would
have stood about 8 km upvalley, at the Cheakamus-Squamish
confluence, about 3000 & 875 years ago and a farther 10 krn
upstream, at the Ashlu - Squamish confluence, about 6000
1500 years ago; the error estimates are simply direct translations of the 25% uncertainty in the volume flux.
The dates and places are intriguing in both of these cases. In
the case of the ~heakamus-squarnish confluence it is known
that most of the enormous Cheakamus-Cheekye fan was in
place to the level of the present sea as early as 6000 years ago
(Eisbacher 1983). It seems quite likely, therefore, that lower
Squamish Valley upstream of Cheakamus junction may have
been filled instead with a lake or lagoon dammed behind this
localized sediment pile in the early to middle Holocene. Certainly the very low "backwater" slope to the valley fill here is
consistent with a lacustrine or lagoonal environment.
In the case of the Ashlu iunction. there is a distinctive break
in the valley slope here where low-slope meandering changes
abruptly to high-slope braiding upstream. Although the junction is 18 km upstream from the delta, it is only 30 m above sea
level, but within the next 1000 m upstream, the elevation
increases by as much again and the fill continues to steepen
upvalley. It is tempting to interpret the steep surface over
which the braided Squamish River flows as an essentially
moribund early Holocene paraglacial sandur of sandy gravel
and the downstream meandering reach as the low-slope fluvial
cap of a fill deposited by later delta progradation.
These propositions are speculative but they are testable.
They raise questions that currently are being addressed in
reflection seismic and coring surveys of the lower Squamish
Valley fill. This work forms part of a larger ongoing project
+
concerned with the nature and impact of Holocene sediment
supply to British coastal rivers.
Acknowledgments
I would like to express my gratitude to Oliver Nagy and
Bruno Tassone and Guy Vallieres of the Inland Waters Directorate of the Water Survey of Canada in Vancouver for their
assistance in data searches, particularly in locating the original
field-data sheets for the Squamish River sediment survey. I am
also grateful for the assistance of the staff of the Canadian
Hydrographic Service in Victoria and of Gabor Veres of Sandwell Swan Wooster operations in Squamish.
This paper incorporates a number improvements suggested
by M. Church and J. Syvitski in reviews of an earlier version
of the manuscript.
The study is part of a project funded by Simon Fraser
University and the Natural Sciences and Engineering Research
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