Water Stewardship Information Sources

Citation Church, M and Ham, D. 2004. Atlas of the alluvial gravel-bed reach of Fraser River in the Lower Mainland showing channel changes in the period 1912-1999. UBC.
Organization UBC
URL http://ibis.geog.ubc.ca/fraserriver/reports/morphatlas.pdf
Abstract/Description or Keywords This atlas documents morphological changes along the
alluvial gravel-bed reach of Fraser River in the Lower
Mainland during the period 1912-1999 - essentially,for the
duration ofthe 20 century. The reach extendsfromHunter
Creek (River km 153) to the Mission gauge (River km 86),
a distance of 67 km. Within thisreach the river depositsthe
gravel portion of the sediment load that it carries out of the
upstream canyons. Deposition of the gravel is caused by
the reduction in river gradient when it enters the Lower
Mainland and approaches marine base level at the Strait of
Georgia.The reduction in gradientreducesthe power ofthe
riverto transportsediments onward.
Gravel is the first part of the river's sediment load
(consisting also of sand and finer sediments) to be
deposited because it is the largest material and requires the
greatest power to maintain its onward transport. Because it
is heavy material, it is transported along the bottom of the
channel and deposited in bars within the channel, in
contrast to most sand, silt and clay, which are borne in
suspension by the river currents and may be deposited
wherever the water flows. The river subsequently isforced
to flow around the gravel bars, thereby creating an
irregularly sinuous and divided (braided) channel pattern
and islands. This habit is called a
(Desloges and Church, 1989) because of the irregular
pattern of lateral channelshifting that occurs. The resulting
network of perennial, seasonal and abandoned channels
represents aquatic habitat of exceptionally high quality.
Hence, thisreach is ofspecial ecological value.
The alluvial gravel-bed reach can be divided into five
major subreaches with somewhat distinctive morphology
(Table 1; Figure 1). Gradient declines by 10x between the
upstream and downstream limits of the reach. This atlas
further subdivides the reach into 13 subreaches for more
detailed discussion.
The period of study is determined by the availability of
reliable documentation of river morphology. The primary
materials chiefly are air photographs taken between 1928
and 1999 by federal and provincial survey agencies. The
1912 map of the river (
) is also used as a primary reference. Reference is
made to earlier information, but the quality of early surveys
is not sufficient for mapped comparisons to be made. The
time between mappings is approximately 10 years from
1949, but it is variable in the earlier period according to the
availability of photos. In addition, the "1940" coverage is
assembled from several photographic projects executed
between 1938 and 1943. In the later period, actual intermap
periods also vary according to the availability of the
best photos, which should be obtained at low flow in order
to reveal the greatest detail of the channel morphology.
Distances quoted in the atlas are river kilometres above
Sand Heads, the mouth of the river. The distances are based
on the main channel position in 1971 and may not be exact
for other dates because of changes in channel position.
Furthermore, the distances may not correspond exactly
with independently reckoned distancessince different lines
might be adopted for measurements along the channel or
channel zone. The adopted distances are standard in work
reported from the UBC Department of Geography.
In the atlas, descriptions are facilitated by the adoption of
recognized names for river localities. These are mainly the
names of the principal bar/island groups. Such features
attract local names, but persistent islands and side channels
also carry official names and the two may not coincide.
Hence, some names used in the atlas may vary from local
usage, although attempts have been made to prefer the
latter. Names are given on the 1999 base map presented for
each reach. In the atlas, the terms “left bank” and “right
bank” are used. These terms conventionally refer to the
banks of the river when the observer isfacing downstream.
Hence, the “right bank” is, in general, the bank on the north
side ofthe river or of an individual channel.
The atlasis organized into 4 sections, each depicting two or
three subreaches. Each section consists of a sequence of
maps, each of which is a georeferenced air photo mosaic at
1:50 000 scale depicting the river morphology at the photo
date. The channel margin from the preceding map is
superimposed on each display so that channel changes in
the intervening period can be observed. The earliest
mosaic/map is based on the 1928 air photography (the first
on the river), and the channel margins depicted on the map
of the 1912 survey are superimposed on that display. In
addition, at the end of each section there is a mapped
comparison of the 1949 and 1999 channels, showing the
net change overthe 50-year period ofmore reliable record.
The maps locate cross-sections at 1 km spacing for which
section surveys are variously shown for 1952, 1984 and
1999. These sections are based on channel surveys from
those dates. Each section of the atlas is accompanied by a
brief text summarizing the main changes observed in each reach. In those descriptions, references to specific sites are
annotated by (xxA), (xxB), etc., where xx is the date of the
base airphoto map (e.g., 19 ), and the letter corresponds
with a letter on the airphotomap.
Apeculiarity of the presentation isthat reaches,subreaches
and river cross-sections are presented and discussed in
reverse enumerated order (from subreach 13 to subreach 1,
for example). That is because our survey numbering
systems proceeds upstream from Mission gauge, but the
logical order for interpretive discussion proceeds
downstream,with the flow.
Additional information presented in the atlas includes
talweg (deep channel) position on each of the survey dates,
the long profile along the talweg line at each date, and
information of gravel texture through the gravel reach,
collected between 1982 and 2001.
It must be recognized that each of the photomaps depicts
the river at a different flow, so that part of the apparent
change in bar deposits observed from map to map may be
due to water level changes. The superimposed channel
margins, however, represent vegetation edges, so that they
are consistently comparable with vegetated channel
margins seen on the later map. Comparisons before 1949
are probably less accurate than later ones because the 1912
map may not be entirely reliable for features within the
channel zone (in a few places distinctly unusual island
features are shown). Furthermore, georeferencing of the
1928 photosmayhavebeenlesssuccessfulthanoflatersets
both because of extreme distortion in those photos and
because of a relative lack of identifiable reference points at
that early date . Finally, the "1940" map is a collage from
several dates.
In interpreting the maps further, the flood history of the
river (Figure 2) should be kept in view. Major floods
transport much more sediment than the smaller ones and so
are capable of effecting major changes on the river. The
primary gauging record along the river is from the Hope
gauge (Water Survey ofCanada station 08MF005; drainage
area 217 000 km ) and is continuousfrom 1912. The largest
floods within the period ofstudy occurred (in rank order) in
1948, 1972, 1950, 1964, 1955, 1997, 1921, 1999, 1920,
1967 and 1974. However, the highest known flow on the
river occurred in 1894, before the first reliable surveys but
near enough the early record to possibly have had special
influence over it. Analysis of the flood records
demonstrates that above average and below average floods
tend to be clumped into recognizable periods. Hence, the
period before 1925 was one of dominantly above-average
floods while the following period to 1947 had mainly
below average ones. From 1948 to 1976 floodstended to be
very high (7 of the 11 highest flows occurring in this
period), and then low to near the end of the record (see
Figure 3).
Finally,some circumstances of the sedimentation history of
the river may have a significant bearing on observed river
channel changes. During the 18 and early 19 centuries,
the period ofrelatively cool climate known asthe “Little Ice
Age” may have created conditions of increased sediment
supply from certain mountain tributaries of Fraser River.
Perhaps more significantly, the latter half of the 19 century
witnessed major sediment disturbances along the Fraser
itself and certain of its principal tributaries, including
extensive placer gold digging on the river bars and terraces
and the major railway building projects along the Fraser
and Thompson rivers. Engineering disturbances continued
until after the mid-20 century with railway and highway
construction. Gravel and cobbles introduced into the river
and its tributaries by these activities would require many
decades to pass downstream into the Lower Mainland.
River channel changesin the 20 century may in partreflect
themovement and deposition ofthesematerials.
Information Type report, mapping
Regional Watershed Lower Fraser
Sub-watershed if known Fraser River
Aquifer #
Project status complete
Contact Name Michael Church
Contact Email [email protected]