I
The geomorphic impact of the catastrophic October 1984 flood on the planform of
Squamish River, southwestern British Columbia
EDWARD
J. HICKIN
Department of Geography, Simon Fraser University, Burnaby, B.C., Canada V5A 1S6
AND
HENRYM. SICHINGABULA
Department of Geography, The University of Zambia, Lusaka Campus, Lusaka, Zambia
Received April 6 , 1987
Revision accepted August 14, 1987
Small-format aerial photography of Squamish River just prior to and immediately following the October 1984 flood of
record forms the basis of a quantitative evaluation of floodplain erosion and construction during this extreme event. Channel
changes in 1984 are compared with those determined from sequential aerial photography at various other times since 1947
and associated with less extreme flooding.
Depending on the type of river planform, the degree of channel change during the 1984 flood varied from a relatively minor
response to a major reorganization of the channel. Despite its large size, the 1984 flood accomplished little more floodplain
modification in the meandering and transitional semibraided reaches than had previous smaller floods of similar duration.
In general, greater erosion was accomplished here by relatively small but longer duration flood events. In contrast, in the
braided reach the 1984 flood caused floodplain erosion and major reorganization of the channel to an extent previously
unrecorded, apparently here exceeding a threshold for channel stability. For all reaches, variation in floodplain erosion among
sites was greater than at-a-site variation in erosion related to flood history.
Les photographies aCriennes de format rCduit de la rivikre Squamish prises juste avant et immkdiatement aprks la crue
d'octobre 1984 ont servi de base ?
une
i Cvaluation quantitative de 1'Crosion et de 1'Cdification de dCp6ts dans la plaine de
debordement durant cet CvCnement extreme. Les changements du chenal en 1984 sont comparCs avec ceux identifiCs au moyen
de photographies aCriennes ~Cquentiellesprises a diffkrentes pCriodes depuis 1947 et associCs h des CvCnements de crues moins
intenses.
DCpendant de la planarit6 des formes en bordure de la rivikre, le degrC de changement dans le chenal durant la crue de
1984 a variC entre un changement relativement mineur jusqu'h une rkorganisation majeure du chenal. En dCpit de son Cnorme
volume d'eau, la crue de 1984 n'a engendrC que ltgbrement plus de modifications dues au dCbordement dans les segments
mtandriques et les biefs semi-anastomosCs de transition que les modifications produites jadis par de plus petites inondations
de meme durCe. En gCnkral, une Crosion plus intense a eu lieu ici causCe par des CvCnements de crues relativement faibles
mais de plus longue durCe. Cependant, le long du bief anastornos6 la crue de 1984 a causC une Crosion sur la plaine de
debordement et une reorganisation du chenal d'une ampleur sans prCctdent et franchissant ici le seuil de la stabilitC du chenal.
Sur tous les biefs la variation de 1'Crosion due au debordement d'un site a l'autre dCpassait la variation d'trosion h n'importe
quel site pour toutes les crues de l'histoire connue de la rivikre.
[Traduit par la revue]
Can. J. Earth
Sci. 25,
1078- 1087 (1988)
Introduction
Although it is generally recognized that floods play an
important role in shaping the channels of rivers, there seems
to be little agreement among geomorphologists on the relative
importance of the related properties of flow frequency and
flow magnitude. A widely accepted view, closely linked to the
proposed equivalence of the concepts of "bankfull flow,"
"channel-forming discharge," and "the most probable annual
flood," sees river form and pattern largely as the consequence
of erosion and deposition accomplished by flows on the order
of the typical annual flood event. It was argued by Wolman
and Eiler (1958) and Wolman and Miller (1960) and affirmed
in the classic review by Leopold et al. (1964) that flows
smaller than the annual flood are not competent to achieve
more significant channel erosion despite their generally
greater frequency. It was further argued that flows larger than
the annual flood, although clearly more powerful, are less
effective in shaping the channel because of their lower frequency of occurrence and because overbank flows tend to
spread over the floodplain without notable geomorphic effect.
This view also has more recent supporters including Dury
Printed in Canada / Impr~mLau Canada
(1 973 and 1980), Gupta and Fox (1974), Gardner (1977), and
Andrews (1980) and-accords with the position taken in many
textbooks on fluvial geomorphology where the notion forms
the cornerstone of the theory of equilibrium river behaviour
(Hickin 1983). A variant on this theme was offered by Pickup
and Warner (1976), who suggested that for some stream channels in Australia, flows smaller than bankfull (defined as the
1.58 year flood) are more effective as agents of bedload transport than the bankfull flow itself. It is not entirely clear, however, how effective bedload-transporting discharges relate to
channel-forming discharges.
A contrasting view sees the effects of rare but major floods
as a very important part of contemporary channel morphology. It is argued that the most significant channel adjustments
are made in response to major floods and that although some
effects may be transient, others remain as a persistent part of
the geomorphic record because recovery times are much
longer than the recurrence interval of such events. This view
was taken, for example, by Anderson and Calver (1977), Wolman and Gerson (1978), Dury (1980), and Nanson (1986).
A rather more balanced perspective on this debate was pro-
HICKIN AND S
opportunity to assess the response of both braided and singlethread channels to the same extreme flood event.
The field area
-
Study reaches
B bra~ded
T translt~onal
FIG. 1. Location of the study reaches on Squamish River.
vided by Pickup (1976) and Pickup and Rieger (1979), who
attempted to move thinking away from the rather simplistic
notion of a channel-forming discharge to the more realistic
notion of channels as integrators of a range of processes
operating over a variety of time scales. It is this notion that in
recent years has focussed attention on the geomorphic effects
of large floods in rivers and on what is essentially a nonequilibrium property of channels (Hickin 1983).
The fact remains, however, that our understanding of the
relative importance of flood magnitude and frequency in shaping river channels is poorly developed. In part this lack of
understanding stems from the paucity of data on which our
ideas in this regard are based. There remains a need for a
much broader base of studies designed to document floodrelated channel changes in the context of the relevant flood
regime and in a variety of fluvial settings. It is our hope that
the present paper makes a useful contribution in this regard.
Some of the data presented here were obtained in fortuitous
circumstances. In late September 1984 an aerial photographic
survey was conducted along a series of reaches of Squamish
River in British Columbia in order to provide an end point to
a study of channel changes over the last four decades. Several
weeks after this survey was flown the river experienced the
flood of record. The reaches were rephotographed from the air
again in early November of that year, thus yielding a closely
bracketed photogrammetric record of the channel response to
a major flood event. Furthermore, the reaches photographed
have distinctly different planforms and provide an unusual
The Squamish River drains 3600 km2 of rugged Coast
Mountains terrain to the head of Howe Sound at the town of
Squamish some 80 km north of Vancouver in southwestern
British Columbia (Fig. 1). Like many other Coast Mountains
rivers, the Squamish displays a planform transition from
braiding in the steep sediment-supply-dominated headwaters
to meandering in the lower gradient reaches downvalley. It is
a gravel-bed river with banks of sandy silt; the geomorphic
setting, river hydraulics, bar sedimentology, and channel
dynamics were described elsewhere by Hickin (1978, 1979,
1984), Brierley (1984), Brierley and Hickin (1985), and
Sichingabula (1986). Here it will suffice to note that the mean
annual precipitation is 2100 mm in Squamish and that the river
has a mean annual discharge of 250 m3 s-' at Brackendale (see
Fig. 1); the flood record at Brackendale is summarized in Fig.
2a. There are two flood populations in the annual series: the
first relates to the April-June snowmelt freshet and the
second to intense fall and winter rainstorms during September
to December, when most of the steep headwater slopes are
frozen and relatively impermeable. Floods constituting this
second group generally are larger than those related to the
spring freshet and include the October 1984 flood of record.
The October 1984 flood
The flood hydrograph for Squamish River at Brackendale
for the period 84-10-4 to 84-10-13 is shown in Fig. 2b. Three
successive days of heavy rain from October 6 to October 8 culminated in a mean daily flow of 2150 m3 s-' on October 8 and
a peak instantaneous discharge of 2610 m3 s-' early in the
morning of the same day. No rainfall data are available for the
headwaters, but the Meteorological Branch station at Brackendale recorded 41.2 mm on the 6th, 12.6 mm on the 7th, and
10 mm on the 8th. Figure 2a suggests that this flood was no
less than a 30-year event. Field surveys indicate that for the
reaches considered here bankfull discharge is on the order of
1000 m3 s-' and banks are unequivocally overtopped by flows
exceeding 1600 m3 s-'. Thus, bankfull or greater flows were
sustained on Squarnish River for three consecutive days during
this flood.
Analytical procedures
Photographs for 1984 (35 mm coloured transparencies)
were obtained on 84-09-24 and 84-11-05 using the smallformat aerial survey system developed by Roberts and Griswold (1986). These film strips provided stereoscopic coverage
of the channel for interpretation and mapping. In order to
place the 1984 channel changes in a longer term context, timesequential aerial photography dating from 1947 was used to
construct earlier isochrone maps of Squarnish River channel.
British Columbia Government aerial photographs of the
study reaches are available for 47-07-15, 5 1-06-15, 60-07-22,
69-07-24, 76-09-27, 78-08-09, 80-09-14, and 82-07-24.
Channel boundaries established from the photographs were
inscribed on an emulsion-backed acetate paper with the aid of
a Bausch & Lomb mirror stereoscope. These maps were then
photographically adjusted to a uniform scale of 1 : 25 000 and
channel changes mapped using a Bausch & Lomb zoom transferscope. Ground truthing indicates that mapped distances are
CAN. J. EARTH SCI. VOL. 25, 1988
4
.
The 1984
f load
. .
I
1-01
I.'
I
1.5
2
I
5
Recurrence interval
# " I
10
(years)
1
1
:
50
4
5
:
:
6
:
7
:
8
:
:
9
10
:
m
11
1213
October 1984
FIG.2. (a) Magnitude and frequency of peak discharges on the annual series for Squamish River and (b) the flood hydrograph for the October
1984 flood on Squamish River (data from Water Survey of Canada 1985a, 1985b).
within 5 % of their directly measured values. It is estimated
that measurements of channel boundary displacements
obtained from these maps have an accuracy of f25 m (area
computations have a corresponding accuracy of k625 m2).
General measures of the degree of channel adjustment
adopted here are the floodplain area added by deposition and
that lost through erosion. The difference between floodplain
surface area added and that removed provides a first estimate
of relative sediment throughput by reach (given a sensibly
uniform bank height). These floodplain surface areas were
measured on superimposed 1 : 25 000 isochrone maps using
a compensating polar planimeter.
The effects of the flood on three reaches are considered: an
upstream braided reach, a downstream meandering reach, and
an intermediate reach of semibraided planform near the
Squamish -Ashlu junction referred to here as the transitional
reach (Fig. 1).
Observed channel changes
The upstream braided reach
The braided reach is formed by a very complex pattern of
anabranches, which weave their way around temporary bars
and more permanent vegetated islands where Squamish Valley
widens immediately downstream of a steep, canyonized reach.
These features may in part constitute sediment storage elements for bed-calibre material oversupplied locally by debris
flows from Mount Cayley and delivered to Squamish River
via eastern tributaries such as Turbid and Shovelnose creeks
(Clague and Souther 1982). Indeed the channel pattern
changes in this reach are far too complex to allow the abstraction of meaningful quantitative data on channel shifting. The
general qualitative pattern of change, however, is quite
evident.
Aerial photographs between 1947 and 1984 reveal a consistent but complex pattern of channel shifting and accompanying
local shoaling, minor channel avulsion, back-channel infilling,
bar and island formation, and floodplain aggradation. But
despite the passage of 35 years and a major regional flood in
1948 and subsequent floods of note in 1958, 1968, 1975, and
1981, the character of the braided reach did not change significantly during this period (see Figs. 3a and 4). Of course, one
has to allow for the possibility that some channel recovery
from these events may have occurred between photography
dates, although these undocumented post-flood intervals
amount to less than 2 years in each case.
The channel changes caused by the October 1984 flood,
however, were quite dramatic in comparison with the geomorphic activity of the preceding four decades (Fig. 4). Not
only was the system of anabranches completely reorganized
within the active braid zone, but this zone was widened by
up to 0.5 km and extended upstream for 1.5 km. This expansion of braiding was made possible by the erosion of about
250 000 m2 of former floodplain (and 1.5 km of road),
reactivating areas of the valley floor for the first time in
decades. Prior to the 1984 flood this reach was showing signs
of conversion to a single-thread channel. Most of the flow
occupied the western side of the braid complex, and the
eastern zone largely consisted of abandoned back channels and
inactive braid bars and islands. Log jams had sealed off former
eastern anabranches (Hickin 1984), and vegetation was clearly
becoming established or was expanding on former bars and
islands. In short, the effect of the flood was to fully reinstate
the braided character of the channel. Major new depositional
zones were established, and the flow is now distributed more
evenly between the modified western anabranches and the
newly formed system on the eastern side of the valley.
The transitional reach
Channel changes in this reach are summarized in Fig. 5 and
Tables 1 and 2. Here the channel might be described as a
wandering gravel-bed river, although unlike with the braided
reach upstream, the major channel alignment throughout the
period of record persists, taking the form of a large bend. The
major change in this reach has been the downvalley migration
I
I
I
1
1
HICKIN AND SICHINGABULA
1081
FIG.3. (a) The braided reach in 1947, (b)the transitional reach in 1951, and (c)the meandering reach in 1982. (British Columbia Government
photographs).
of the upstream limb of the bend, resulting in the erosion of
the point-bar-like feature marked "island A" in Fig. 5. Here
downstream bank retreat of over 550 m (14.9 m year-') has
tightened the bend, although there has been no concomitant
deposition on the opposite bank. Consequently, channel width
has increased markedly during this period. Elsewhere in this
reach there have been only relativelx minor channel alignment
changes and bar migration.
A rather counterintuitive outcome of the analysis of bank
alignment changes on island A in particular and in the transitional reach in general is that the 1984 flood was not an
unusually significant geomorphic event despite its size.
Table 1, which lists total and mean bank retreat at measurement sites (a) through (f) in Fig. 5, together with the corresponding rates of floodplain-surface erosion and deposition,
reveals that the history of channel adjustment prior to the 1984
flood was far more impressive. Furthermore, two major
floods ( > 1600 m3 s-') in 1981 had a similarly minor effect on
the channel planform, and yet far smaller floods (in both
magnitude and duration) in the 1960's are associated with a
period of considerable bank erosion. This lack of correspondence between major flood and erosion -deposition events
CAN. J. EARTH SCI. VOL. 25, 1988
,/ Turbid C r e e k
,Turb~d C r e e k
//
m
1
Densely v e g e t a t e d b a r
1-
Thinly v e g e t a t e d b a r
- Main
-----
channel
Former channel location
Creek
-
G r a v e l road
FIG. 4. Planform of the braided reach (a) before and (b) after the October 1984 flood.
1. The history of flood duration and magnitude and channel boundary adjustments on the upstream side of "island A" in the transitional
TABLE
reach
Perioda
Bank
retreat
(m)
Mean annual
rate of bank
retreat
(m year-')
Floodplain
surface
erosion
(mZyear")
Floodplain
surface
accretion
(m2 year-')
700 5 Q 5 1000 m3 s-'
1947- 195 1
1951- 1960b
1960- 1964
1964- 1969b
1969- 1976
1976- 1980
1980- 1982
1982- 1984
1984 (Oct.)
175
125
100
125
75
12.5
0
0
0
43
13
25
25
10.7
3.1
0
0
1.9
15.6
6.8
11.2
13.5
3.7
3.7
0
1.3
1.9
2.4
0.9
2.8
2.3
1.1
1.6
0
0
0
28
37
42
41
4
11
16
1
Number of days when
1000 5 Q 5 1600 m3 s-'
No data recorded for this period
6
5
5
5
2
3
1
1
Q r 1600 m3 s-'
5
0
1
1
0
2
0
2
"Years of photography, not complete calendar years.
bIncomplete discharge record.
also applies to this reach in general (Table 2).
The meandering reach
Of the several meandering reaches examined, the upper
meandering reach (Fig. 1) is typical; its planform history is
shown in Fig. 6, with derived erosion and deposition rates
listed in Table 3. Because the planform in this reach is relatively simple and the flow is well organised, floodplain ero-
sion-deposition is a rather predictable response to lateral
migration of the individual meander bends. The westerly
migrating meander bends in this reach have impinged on the
bedrock valley wall and are no longer fully alluvial in character. Although there has been some downvalley channel-bend
displacement along these bedrock "hard points" over the last
4 decades, migration normal to the valley axis has been
negligible. The easterly migrating channel bends, on the other
1083
HICKIN AND SICHINGABULA
----
-----
1951
----
Floodplain l t m i t
84-09-24
[m
Gravel b a r
----
F l o o d p l a ~ nl ~ m ~ t
Floodplain limit.
FIG. 5. Channel changes in the Squamish-Ashlu bend in the transitional reach between 1947 and 1984.
hand, are fully alluvial and have eroded both laterally and
downvalley over the period of record. It is clear from Fig. 6c,
however, that the 1984 flood had little geomorphic impact on
this reach compared with the effects of earlier floods in the
period prior to the flood of record.
Discussion and conclusions
In general it would appear that the response of the braided
reach to the 1984 flood has been quite catastrophic with
respect to the change exhibited by the channel farther downstream. We suggest that there are several reasons for this
difference in channel behaviour.
First, the 1984 flood was the only event in the last 40 years
to flow over the surface of the major bars and islands in this
reach. The depth of inundation was greater in this reach than
in others because bank heights in this multiple-channeled system are lower and because here the floodplain is confined
between the resistant walls of the relatively narrow valley. In
consequence, the flood was accompanied by surface scour of
bars and islands, as well as by lateral erosion and deposition.
Furthermore, log jams that had gradually built up during the
decades-long succession of more moderate floods and completely sealed off some of the eastern anabranches were floated
off during the high water of the 1984 flood. Thus, the flow
regained unrestricted access to older parts of the floodplain at
a level below the protective cover of vegetation and where
lateral erosion could be accomplished with relative ease.
Second, bars and islands in this reach are less well colonized
by vegetation than point bars and floodplain surfaces elsewhere on the river. In consequence, they may be more vulnerable to surface scour by overbank flows.
Third, although the braided reach is steeper than those
downstream, it also is composed of far coarser bed material
in some localities than are the other reaches. It is certain that
threshold flow conditions for sediment transport (erosion) are
much greater for many of these braid bars than for point bars
farther downstream. In consequence, there are many more
anchor points to secure the general planform alignment during
moderate floods (which of course are also divided flows) while
at the same time allowing for some reorganization of gravel
bars composed of finer material.
In short, the geomorphic response of the braided reach to
floods is characterized by threshold behaviour at a critical discharge equal to or less than the 1984 flood (approximately
2600 m3 s-') but apparently greater than the next highest flood
(approximately 2300 m3 s-') in the record (or some similar
hydraulic bracket based on flood magnitude and duration).
The geomorphic response of the channel to the 1984 flood
through the better organized meandering and transitional
reaches appears to be quite minor. The relatively small
amounts of erosion and deposition caused by the 1984 flood
1084
CAN. J. EARTH SCI. VOL. 25, 1988
76-09-27
-
-
---
84-09-24
H a r d poirrt
Gravel
road
,
Floodplain limit
FIG. 6. Channel changes in the meandering reach between 1947 and 1984.
TABLE2. The history of flood duration and magnitude and floodplain surface-area adjustments in the transitional reach
Perioda
Floodplain
surface
erosion
(m2)
Floodplain
surface
accretion
(m2)
Floodplain
surface
change AA
(mZ)
1947- 1951
1951- 1960b
1960- 1964
1964- 1969b
1969- 1976
1976- 1980
1980- 1982
1982- 1984
1984 (Oct.)
99 510
150 590
110 430
150 150
111 860
24 650
36 430
77 620
31 880
13 800
71 290
60 680
22 660
70 200
49 000
7 870
47 100
0
113 310
221 880
171 110
172 810
182 060
73 650
44 300
124 720
31 800
Number of days when
7 0 0 ~ Q ~ 1 0 0 0 m ~ s -1' 0 0 0 ~ Q ~ 1 6 0 0 m ~ sQ- r' 1 6 0 0 m 3 s - I
No data recorded during this period
28
37
42
41
4
11
16
1
6
5
5
5
2
3
1
1
5
0
1
1
0
2
0
2
"Years of photography, not complete calendar years.
bIncomplete discharge record.
in the single-channeled reaches suggest that it is not so much
the magnitude of extreme flows that is important here as it is
the duration of flows above the threshold for sediment transport. In other words, most of the channel formation in the
meandering reaches of Squamish River occurs at flows more
moderate but more frequent than the 1984 flood. Figure 7
illustrates this point for the total area of floodplain erosion and
accretion recorded for the transitional reach between 1947 and
1984. Figure 7a shows the relation between the change in
floodplain surface area (AA) and the number of days river
discharge was between 700 and 1000 m3 s-', i.e., No(700 I
Q % low), a range bracketing bankfull flow at many sites;
Fig. 7b, 7c, and 7d show the relation AA versus, respectively,
No(1000 I Q I 1600), No(Q 2 1600), and No(Q r 700).
1085
HICKIN AND SICHINGABULA
TABLE3. The history of flood duration and magnitude and floodplain surface-area adjustments in the meandering reach
-
Perioda
Floodplain
surface
erosion
(m2)
Floodplain
surface
accretion
(m2)
Floodplain
surface
change AA
(m2)
Number of days when
700sQs1000m3s-'
1947-1951
1951 - 195gb
1958- 1960
1960- 1964
1964 - 1969b
1969- 1976b
1976- 1977
1977- 1980
1980- 1982
1982- 1984
1984 (Oct.)
21
1 0 0 0 ~ Q ~ 1 6 0 0 m ~ sQ>1600m3s-'
-'
No data recorded during this period
4
'Years of photography, not complete calendar years.
bIncomplete discharge record.
TABLE4. Table of correlation coefficients for relations among floodplain surface-area adjustments in the transitional
reach and the duration of various flood-magnitude classes
Number of days when
Floodplain surface
erosion E
Floodplain surface
accretion, D
Change in floodplain
surface area, AA
+0.89
+0.53
+0.86
TABLE5. Table of correlation coefficients for relations among floodplain surface-area adjustments in the meandering reach and the duration of various flood-magnitude classes
Number of days when
700 Q I.1000 m3 s-'
Floodplain surface
erosion, E
Floodplain surface
accretion, D
Change in floodplain
surface area, AA
1000 s Q 5 1600 m3 s-'
Q 2 1600 m%-'
Q r 700 m3 s-'
+0.61
+0.66
+0.74
Clearly, about three quarters of the variance in AA is statistically explained by either the variation in No(700 I Q I
1000) or No(1000 5 Q I1600), but little further explanation
is offered by considering the number of days that the flow
exceeded 1600 m3 s-'. A stepwise multiple-linear-regression
analysis confirms this conclusion: in addition to the 73.96%
of the variance in AA explained by No(700 5 Q 5 1000)
alone, No(1000 I Q I 1600) and No(Q 2 1600) further
explain just 1.15 and 4.60% of the variance, respectively. Of
course, the colinearity between No(700 I Q I 1000) and
No(1000 5 Q 5 1600) means that it is not possible to make
a distinction between the causal significance of these low and
intermediate flood groups; reversing their order in a
multiple-regression analysis merely reallocates most (68.9%)
of the variance to No(1000 I Q I 1600). We can conclude
from these data, however, that most of the adjustments in
floodplain surface area are related to the low and intermediate
floods caused by the spring freshet (Fig. 2a) and that the geomorphic effect of the major floods in excess of 1000 m3 s-' is
no greater than one would expect from their frequency in the
record (about 5 %).
If AA is partitioned into floodplain erosion and accretion, it
is clear (Table 4) that the former is much more closely linked
with the flood regime than is the latter. The fact remains, however, that even for floodplain erosion most of its variance
(79.2%) is statistically explained by floods near bankfull, and
1086
CAN. J. EARTH SCI. VOL. 25, 1988
0
1
2
No(Q
3
3
4
1600)
5
6
0
10
20
No(Q
30
2
40
50
700)
FIG.7. Relations among adjustments in floodplain surface area in
the transitional reach and the duration of four flood-magnitude
classes.
play a rate reduction over time to zero during the last few
years because the bank erosion has widened the channel to the
extent that even the 1984 flood appears to have been accommodated without undue stress on the banks. The notion of
bank erosion outstripping concomitant deposition on the opposite side of the channel, thus enlarging the channel cross section and thereby reducing the subsequent rate of lateral
erosion, has been identified elsewhere (Nanson and Hickin
1984) as an important control on short-term migration rates in
river bends. Clearly, the immediately preceding flood history
is very important in conditioning a channel for subsequent
erosion. It seems likely that the 1984 flood on Squamish River
would have had greater impact locally if it were not for the fact
that the river experienced a conditioning 15-year flood in
1981.
In the meandering reach the geomorphic effects of the 1984
flood also appear to be negligible. Again, it seems to be the
case that the energy of this large flow was broadly dissipated
over the heavily vegetated floodplain without causing any significant scour. The effects on lateral migration in the bends
apparently was little different from that achieved by much
smaller floods of the same duration. Table 5 lists the simple
correlation coefficients for relations among AA, floodplain
erosion (E), and floodplain accretion (D), and the durations of
three classes of floods; the relations involving AA graphed in
Fig. 8. Once again we see the geomorphic effects of extreme
floods (Q 2 1600 m3 s-') as being quite insignificant in the
overall pattern of channel adjustments.
Summary
FIG.8. Relations among adjustments in floodplain surface area in
the meandering reach and the duration of four flood-magnitude
classes.
only about 7 % can be accounted for in terms of major floods.
We should not conclude, of course, that the geomorphic
effects of the large floods in general and of the October 1984
event in particular are insignificant in an absolute sense at particular localities. It is true to say, however, that in the general
scheme of things in this reach, variation in site-specific factors
are overwhelmingly more important than is variation in AA
caused by high discharges. This result should be no surprise,
since it has been demonstrated elsewhere that the morphology
of channel bends or the site location in a given bend can profoundly influence bank erosion rates, other things being equal
(Hickin and Nanson 1984).
This problem of site-specific variability obscuring the
effects of flood magnitude and duration is not necessarily
overcome by monitoring the channel-boundary displacement
at one site. For example, the bank displacements (a) through
(e) on the upstream side of island A (Fig. 5 and Table 1) dis-
In summary, observations on the braided reach of Squamish
River support the notion that extreme floods such as the
October 1984 event can produce catastrophic channel change
of far greater significance than its low frequency would suggest. In contrast, observations on the effects of the 1984 flood
in the meandering and transitional reaches of Squamish River
indicate that this extreme flood had little geomorphic impact
here, thus supporting the notion that extreme floods do not
necessarily achieve geomorphic work that is disporportionately large with respect to their frequency. Furthermore, these
recent observations are consistent with the photographic
record of the last 4 decades. The fact that good correlations
have been obtained here between changes in floodplain surface
area and the number of days that flow exceeds 700 m3 s-',
regardless of the size of that excess, implies a minor role for
flood magnitude provided it is above the threshold for bank
erosion and sediment transport. This result accords with recent
observations (Martinson 1984) made on the meandering
Powder River in Montana.
None of this discussion is meant to imply, of course, that the
October 1984 flood was not a disastrous event in terms of its
human impact. The flood destroyed roads and bridges and
inundated many Squamish Valley homes, causing millions of
dollars in damage.
Acknowledgments
We would like to thank Gary Brierley (Simon Fraser University) for field assistance and Godfrey Stevwanti (University
of Zambia) for assistance with the preparation of the illustrations. The data reported in this study are part of a more extensive data set fully documented in the M.Sc. thesis of the
second author (Sichingabula 1986) from the Department of
HICKIN AND SICHINGABULA
Geography, Simon Fraser University. W e would also like to
thank Derald Smith and Andre Roy for their helpful reviews
of this paper.
This work forms part of a n ongoing Simon Fraser University project on the morphodynamics of Coast Mountains rivers
funded by the Natural Sciences and Engineering Research
Council of Canada. W e would also like to acknowledge the
support of the University of Zambia through its award of a
Staff Development Fellowship to the second author.
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