ELSEVIER
Geomorphology
21 (1997) 17-38
The internal structure of scrolled floodplain deposits based on
ground-penetrating radar, North Thompson River, British
Columbia
Rene F. Leclerc ay*, Edward J. Hickin b
a Department of Geography and the Institute for Quatemary Research, Simon Fraser University, Bumaby, B.C. VSA lS6, Canada
b Departments of Geography and Earth Sciences, Simon Fraser University, Bumaby, B.C. V5A IS6, Canada
Received 8 May 1996; revised 22 January
1997; accepted 30 March 1997
Abstract
A study of floodplain deposits using ground-penetrating
radar (GPR) was conducted on a confined meander of the North
Thompson River near Kamloops, British Columbia. A survey grid consisting of 20 individual GPR profiles from 200 m to
900 m in length was constructed on a 0.9 km2 section of the floodplain. All GPR profiles were collected using a pulse
EKKO” IV GPR system with 100 or 200 MHz antennae, dependent on the thickness of floodplain sediments. Floodplain
sediments consist of sand and gravel deposits approximately 7 to 14 m thick that overlie silts and clays interpreted to be
glaciolacustrine in origin. Vibracore and auger data obtained at nine locations in the GPR survey grid show that changes
from medium sand to gravel do not produce distinct radar reflections in GPR profiles but that the water table may locally
produce a distinct, high-amplitude radar reflection. Area well-log data show that rapid signal attenuation at depth in all GPR
profiles coincides with underlying silt and clay sediments. GPR profiles were, therefore, useful in mapping 3-D variability in
the thickness of floodplain deposits. All GPR profiles were divided into macroscale (> 150 m2 in 2-D cross-section) radar
facies, delineated on the basis of the nature and orientation of reflections relative to surface scroll topography. Radar
stratigraphy suggests that the floodplain is composed of a single, complex lateral accretion deposit with periodic channel fills
located between surface scroll ridges. Dip direction profiles, normal to surface scroll ridges, exhibit mainly inclined
stratification (IS) and/or preserved ridge-and-swale
@AS) deposits dipping in the direction of floodplain accretion.
Well-developed
RAS architecture in 200 MHz GPR profiles displays mainly wavy reflections conforming with surface
topography and little inclined stratification. These reflectors are interpreted to be the result of processes of vertical accretion
associated with floodplain formation. Preserved scroll ridge deposits exhibit asymmetry at depth, dipping more steeply in the
direction of floodplain accretion. Profiles of strike direction, parallel to surface scroll ridges, show either parallel or slightly
inclined (l-2”) reflections dipping in the downstream direction. 0 1997 Elsevier Science B.V.
Keywords: floodplain
sedimentology;
ground penetrating
radar; scroll bars; meandering
* Corresponding
author. Present address: Northwest Hydraulic Consultants,
95691-3430, USA. Fax: + 1 (916) 371-7475; E-mail: rene@nhc-sac.com
0169-555X/97/$17.00
0 1997 Elsevier Science B.V. All rights reserved
PII SO169-555X(97)00037-8
rivers
3950 Industrial
Boulevard
lOOC, West Sacramento,
CA,
1. Introduction
Traditionally, studies of the sedimentology of meandering rivers have focused on two-dimensional
(2-D) sections (usually I or 2 channel widths) of the
floodplain, typically located near the river channel
(Sundborg, 1956; Frazier and Osanik. 1961; Harms
et al., 1963; McGowen and Garner, 1970: Bluck.
1971; Jackson, 1978; Nanson, 1980). These studies
use direct observation techniques including coring.
trenching, and logging of cutbank sections as primary means of data acquisition. Unfortunately, these
techniques yield limited information on the channelscale, three-dimensional
(3-D) characteristics of alluvial floodplain deposits. In spite of this fact, tluvial
sedimentology has, during the last 10 years, shown
an increasing trend away from 2-D and toward the
3-D description and classification
of channel-scale
sedimentology in the floodplains of meandering rivers
(Bridge, 1985, 1993; Hickin, 1993: Miall. 1996).
At present. and primarily because of the limitations of traditional data collection techniques, very
few studies have focused on large-scale. 3-D floodplain sedimentology; exceptions include Fisk (1952).
Brierley (19891, Brierley and Hickin (1991, 1992).
Stephens (1994), and Bridge et al. (1995). As a
result, comparatively little information exists on the
appearance
and strata1 variability
of large-scale
(across 2 or 3 channel widths) floodplain deposits in
varying 2-D orientations.
This study examines the channel-scale, 3-D structure of a scrolled floodplain of a meandering river,
specifically a square kilometer of the North Thomp\on
River floodplain
near Kamloops,
British
Columbia. Sedimentologic
data are acquired largely
by a survey with ground-penetrating
radar (GPR), a
geophysical tool only recently applied to geomorphic
problems in meandering streams (Gawthorpe et al.,
1993; Bridge et al., 1995).
2. Study area
The study area is on a single meander bend of the
North Thompson River, located about 10 km north
of Kamloops, British Columbia, and approximately
300 km northeast of Vancouver (Fig. I). The North
Thompson River Valley lies in the Interior Plateau,
an area characterized by a series of rolling uplands
separated by deep valleys (Bostock, 1948).
Drainage area of the North Thompson River is
approximately
20,000 km’. Based on 32 years of
record (1959- 1991), the maximum annual discharge
at McLure Station, 15 km north of the study site, is
1860 m”/s (Environment
Canada, 1992). The time
of peak discharge is controlled by snow melt and
Cl study area
Legend
\ ,
terrace
,>
road
‘*., valley wall
modern floodplain
C
Pi-C’ I Location
map ot the study area.
concave bank-bench
R.F. Leclerc, E. J. Hickin / Geomorphology 21 (I 997) 17-38
usually occurs in early June. Minimum flow discharge is reached in February and averages 63.3
m3/s whereas mean annual discharge over the 32
years of record is 428 m3/s (Environment Canada,
1992).
Bedrock in the study area, and in much of the
lower North Thompson Valley, is covered by a
variable mantle of post-glacial alluvium resulting
from the Fraser glaciation (Fulton, 1963, 1967).
These sediments, termed Kamloops Lake Drift by
Fulton and Smith (1978), are comprised mainly of
glaciolacustrine silt and clay deposited while stagnant ice blocked the valley immediately to the south
of the study area during deglaciation (Fulton, 1967;
Fulton and Smith, 1978). The upper 10 m of these
deposits have been heavily reworked by the North
19
Thompson River and, to some extent, by paraglacial
alluvial fan channels stemming from steep erosional
gullies that bisect the valley walls (Ryder, 1971).
Consequently, reworked surficial deposits contain a
coarse-grained component, consisting mainly of sand
and gravel that form a cap over glaciolacustrine silt
and clay. Well-logs from the study area show this
contact, located 7 to 14 m below the surface (Leclerc,
19951, as the base of the floodplain.
The lower North Thompson River is characterized
by a series of meanders that are confined between
the valley walls, approximately 2.0 km apart within
the study area. The river varies between 150 and 300
m in width. The thalweg depth at bankfull stage is
unusually deep, between 9 and 10 m, reaching 14 m
in a narrow section of the channel east of the study
Fig. 2. The study site showing (a) topographic variability of the floodplain and (b) surface scroll topography. Dark bands in (a) follow along
the tops of steps (steeper slopes) on the floodplain. Thicker dashed lines in (b) indicate changes in the orientation of surface scroll
topography.
site (Fig. 1) where the channel abuts the valley wall
(Ministry of Environment,
1982a). Concave bank-benches (Hickin, 1979; Nanson and Page. 1983) are
located on the upstream bank at sharp bends in the
river channel (‘C’ locations in Fig. 1).
The floodplain was selected for study because of
its suitability for GPR work. It presents a fairly level
ground surface with relatively few obstructions and
is readily accessible. The dimensions of the study
area are approximately 1.1 km by 0.8 km. yielding a
total area of 0.9 km’, roughly 3 by 5 channel widths
in size. The floodplain displays a stepped topography
(Fig. 2a), in addition to several changes in the orientation of surface scroll bars (Fig. 2b), interpreted as
different stages of floodplain growth.
Application of models qf jloodplain architrcturcd
to the study Nreu. Given the absence of detailed
stratigraphic information
of floodplain deposits in
the study area, examples from other scrolled tloodplains of meandering rivers are used instead to characterize the expected internal architecture
of the
study site (Fig. 3). Generalized
channel-normal
cross-sections by Sundborg (1956) and Nanson and
Croke (1992) (Fig. 3a,b) show vertically preserved,
streamward-dipping
scroll ridge deposits that overlie
inclined stratification.
Sections ‘a’, ‘b’, and ‘d’ in
Fig. 3 derive from streams that have incised into
lacustrine silt and clay, similar to this study, and
have high suspended sediment loads, a major source
material of vertically accreted scroll ridge deposits.
Coarser units of material appear at the base of some
vertically accreted ridge deposits in Fig. 3a, designated by the ‘U’ and termed unit bars by Sundborg
( 1956). Similar coarser units were also described at
the base of ridge deposits in Fig. 3d by Nanson
( 1980). To the authors’ knowledge, no large-scale
cross-sections of scroll bar deposits in channel-parallel orientation have been documented, although Allen
( 1963) described
the genetically
similar epsilon
cross-stratification
(ECS) as vertically stacked, parallel bedding. Based on GPR, core, and historical data,
Bridge et al. (1995) identified similar large-scale
strata in channel-parallel
orientation that exhibited a
ytreamward dip toward the proximal and distal ends
of the scroll bar.
3. Methods
3.1. GPR methodology
Similar in principle to shallow seismic profiling,
GPR systems use electromagnetic
energy to provide
high-resolution
(decimeter-scale)
imagery of subsur-
(a >
0
5
10
15
Distance
20
25
(no scale)
(m)
Distance
(m)
Distance
(m)
Fig. 3. Large-scale strata1 trends in channel-normal
crowbecttons
of laterally accreting, scrolled floodplains: (a) generalized model from
Sundborg (1956); (b) generalized model from Nanson and Croke (1992): (cl pit section from Kinoshita (1987): (d) cutbank exposure from
Nanson (1980). No vertical exaggeration. See discussion in text
R.F. Lderc,
E.J. Hickin/ Geomorphology 21 (1997) 17-38
face deposits, as deep as 50 m in Quaternary environments (Jol, 1995). Whereas a display of radar
data superficially resembles that of a seismic reflection profile, a primary difference between the two is
scale; radar is limited to the shallow subsurface
( < 50 m in soft sediments) but is capable of imaging
a higher resolution (decimeter-scale) of detail than
shallow seismic profiling (meter-scale).
Recent studies have refined the application of
GPR to geomorphic environments, utilizing the
strong relationship between the electrical and physical properties of a material to identify the location
and properties of physical boundaries such as (i)
geologic interfaces, (ii) the water table, and (iii)
strata1 contacts (Davis and Annan, 1989; Smith and
Jol, 1992; Stephens, 1994). Although strata1 contacts
are generally weaker reflectors when compared to
types (i) and (ii), they are the most common sources
of radar reflections, and are the main focus of geomorphic studies where the meter-scale radar stratigraphy of a sediment body is of primary interest.
Large-scale patterns of radar stratigraphy are shown
to be reliable indicators of subsurface strata1 trends
in sand and gravel deposits through GPR testing
along exposed sections (Smith and Jol, 1992;
21
Huggenberger, 1993; Pilon et al., 1994; Leclerc,
1995).
Reflection patterns on radar profiles that coincide
with sedimentary interfaces are used to characterize
the radar stratigraphy of subsurface deposits, defined by Jo1 (1993) as “the study of stratigraphy and
depositional facies as interpreted from radar data
using seismic stratigraphic principles.” A radar faties is identified as “a mappable, three-dimensional
sedimentary unit composed of reflections whose parameters differ from adjacent units” (Jol, 1993).
These parameters include signal amplitude, continuity, waviness, and inclination of reflections. Examples of different types of reflection patterns, including the classification and interpretation, are given by
Beres and Haeni (1991), although almost all techniques of radar stratigraphic interpretation are taken
directly from the seismic literature (e.g., Mitchum et
al., 1985). Recent applications of radar stratigraphic
interpretation to river deposits are given by
Gawthorpe et al. (1993), Stephens (1994), and Bridge
et al. (1995).
Radar reflections do not always represent sedimentological boundaries. Changes in the electrical
properties of subsurface materials resulting from var-
Fig. 4. The location of radar survey lines and core sites in the study area. Continuous, undashed black lines identify GPR survey lines taken
with 100 MHz antennae; dashed black lines are those taken with 200 MHz antennae. Rectangular black boxes are ginseng fields that could
not be GPR profiled. White dashed lines indicate the orientation of surface scroll topography.
22
R.F. Ldrrr~,
E.J. Hickin / Gromorpholo,q,t
ious factors including relative changes in porosity,
grain size, organic content, and moisture conditions
within the vertical column (Greenhouse et al.. 1987;
Davis and Annan, 1989; Bridge et al.. 19951 will
also produce radar reflections. Thus, any interpretation of radar reflection profiles must recognize that
not all GPR reflections directly represent bedding in
geologic deposits (Greenhouse et al., 1987; Leclerc,
1995). Furthermore, above ground objects such as
trees, overhead wires and telephone poles are also
potential sources of radar reflections, and should be
noted when the GPR profile is created (Young and
Sun, 1994).
3.2. GPR suruey design
A grid consisting of 20 GPR profiles was imposed
on a large section (0.9 km’) of the floodplain (Fig.
4). 100 MHz GPR survey lines were spaced approximately 100 m apart and aligned to follow existing
cultural boundaries such as fences and the main road.
200 MHz GPR survey lines were oriented along the
dip and strike of surface scroll bars. The goal of
antennae selection was to penetrate the full thickness
of floodplain deposits with the highest resolution
possible, the former being the primary objective.
Thus, 200 MHz antennae were used in the southern
portion of the study area (see Fig. 4) because the
thickness of floodplain deposits generally was less.
For a review of the relationship between antennae
frequency and depth penetration, see Jo1 (1995).
All radar reflection profiles were obtained using a
pulseEKK0 TMIV ground-penetrating
radar system in
common-offset,
single-fold reflection profiling mode
with the antennae placed in a perpendicular broadside position (Sensors and Software, 1992). Antennae separation and station spacing were set at 1.0 m
for 100 MHz antennae and 0.5 m for 200 MHz
antennae. Radar profiles were collected with either
64 or 128 stacks per trace and at the recommended
sampling interval (Sensors and Software, 1992). Signal processing of each GPR profile included the
following five steps: (i> signal saturation and dewow,
(ii) timezero adjust, (iii) Automatic Gain Control
(AGC = IOOO), (iv) signal filtering (running average
3, mix 31, and (v> topographic correction. Migration
of signal returns was not performed in this study, but
all reported dip angles of radar reflections are cor-
21 (lYY71 17-38
rected for migration. A vertical depth axis for each
profile was based on velocity estimates made from
common mid-point (CMP) profiles performed on
location, a process explained by Jo1 and Smith (1991).
Parameter settings for CMP profiles at the study
site were the same as those for reflection profiling.
Six CMP profiles taken at evenly spaced intervals in
the GPR survey grid were used to calculate an
average velocity of 0.16 m/ns (1.6 X lo8 m/s) for
radar waves through floodplain sediments (Leclerc,
19951. Individual CMP profile results varied from
0.130 m/ns to 0.175 m/ns although both mean and
median values are 0.16 m/ns. This result is greater
than typical velocity values for dry and saturated
sands which are 0.15 and 0.06 m/ns, respectively
(Davis and Annan, 1989). Nonetheless, a velocity
estimate of 0.16 m/ns results in GPR profiles that
exhibit floodplain depths between 14 and 18 m
adjacent to the river channel. This is between 1.5 and
3 times the bankfull depth of the modem thalweg
(between 9 m and 10 ml, a difference accounted for
by additional channel bed scour during high flows
(Wolman and Leopold, 1957; Palmquist, 1975).
3.3. Collection @topographic
data
Each GPR survey line represents a series of connected radar profiles each about 100 m long (Fig. 4).
After a 100 m GPR transect line was completed,
topographic data along the transect line were collected using an Abney level and survey staff at an
interval of minimum 10 m. Most topographic data
were collected at intervals of 5 m or less, depending
on the topographic features encountered.
Error in
Abney level readings was f 3 cm for intervals of 10
m and + 1.5 cm for intervals of 5 m. Because these
data could lead to significant error over kilometer
distances, they were used in conjunction with 1 : 5000
scale orthophoto maps of the study area, labeled with
contour intervals of 1.0 m and over 100 point elevations accurate to the nearest 0.1 m (Ministry of
Environment,
1982b).
For topographic correction of GPR survey lines,
orthophoto maps were first used to create a base map
of large-scale topographic variability along each GPR
survey line. Abney level data were then used to fill
in small-scale topographic variability between contour lines, such as the location of minor ridges and
R.F. L.eclerc, E.J. Hickin/Geomorphology
swales. This system provided a rapid means of acquiring reasonably accurate, highly detailed topographic data for more than 10 km of GPR profiles in
the survey area. Maximum error in topographically
corrected GPR profiles was estimated at 0.3 m.
21 (1997) 17-38
23
grain size chart and a pocket magnifying glass.
Auger holes were sampled every 15 cm and vihracorer sections were sampled every 10 cm.
4. Results
3.4. Core sampling technique
Unsystematic coring of the river floodplain identified grain size variability within the vertical column
and correlated the location of the water table and
major geologic boundaries with individual radar reflections. Coring was done with a hand-powered
auger and a vibracorer. The hand auger was able to
penetrate to a maximum depth of 5.0 m and was
used in cases where only a shallow core was needed
above the level of the water table. The vibracorer
was used to access material below the level of the
water table where augering was hampered by continual sediment slumping. All core data were subject to
grain size analysis in the field using a hand-held
This section is divided into three parts: (i) comparison of lithologic cores from the floodplain with
corresponding radar profiles; (ii) interpretation of
100 MHz and 200 MHz GPR profiles from the study
area; and (iii) identification of the 3-D distribution of
selected radar facies. The locations of lithologic
cores and radar profiles taken in the study area are
shown in Fig. 4.
4.1. Core log comparisons
with GPR profiles
A total of nine cores were extracted from the
study site. Grain size variability and water table
location (where present) at each core site are shown
in Fig. 5. All cores exhibit a vertical upward-fining,
C6
Fig. 5. Cores from the study area show mainly fine, medium, and coarse sands with occasional clay and gravel. All cores exhibit a vertical
fining-upward
trend. The location of the water table (WT) is indicated beside each core, where applicable. Core numbers refer to their
location in Fig. 4.
typically from medium sand to fine-grained sand and
silt. Corresponding
radar profiles of each core are
shown in Fig. 6 with the location of major lithologic
changes and the water table indicated beside each
GPR profile. Not all radar profiles are of the same
frequency; radar profiles Cl and C3 through C6 are
from 100 MHz GPR surveys and C3, C7. C8, and
C9 are based on 200 MHz surveys. The water table
depth from Fig. 5 is indicated in only four radar
profiles in Fig. 6 (C 1, C2, C7. and C8) because the
remaining GPR profiles were obtained 12 months
after the cores were taken. No core exceeded 7 m in
depth because of the inability of the vibracorer to
penetrate through coarse-grained sand and gravel in
the floodplain. Poor core recovery did not allow for
the documentation of internal stratification.
The ability of major changes in grain size and the
water table to produce distinct radar reflections in
GPR profiles varies considerably
from one GPR
profile to another. Gravel lenses in profiles C2. C7.
and C9 do not produce distinguishable radar returns,
although individual reflections coinciding with the
depth of gravel lenses suggest that they may act as
radar reflectors (Fig. 6). A gravel lens in core C4
does correspond with a strong radar reflection (Fig.
6). but it is also a major source of well water on the
property (M. Tessier, pers. commun., 1994). Hence,
signal amplitude is most likely influenced by the
increase in saturation conditions within the gravel
lens rather than by the gravel lens itself (Davis and
Annan. 1989). A thin (IO cm) cobble layer in core
C I lay immediately above a clay layer that inhibited
further penetration by the vibracorer. The radar profile of core Cl (Fig. 6) shows different characteristics in radar reflections above and below this clay
boundary.
Radar returns above the clay exhibit
stronger individual reflections, with single peaks and
fewer radar signatures between reflections. In contrast, radar returns below the clay boundary exhibit
multiple peaks and greater signal noise between each
reflection. This could be attributed to the fine laminations observed in the clay sample taken from the
base of the vibracore because each laminated bed
could produce multiple weak signal returns.
All core sites exhibit rapid radar signal attenuation below 7-14 m depth. This attenuation is attributed to an increase in silt and clay concentrations
at these depths, identified in well logs from the
region (Leclerc, 1995). It follows, therefore, that the
depth of rapid signal attenuation in radar profiles
CORE
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Fig. 6. Locations of the water table and lithologic boundaries from core data (Fig. 5) are shown on corresponding
frequency is shown in the lower right corner of’each radar profile
radar profiles. Antennae
pp. 25-28
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Fig. 7. Original (top) and interpr
profile is referred to in the text.
with GPR profiles A-G in Fig.
Fig. 8. Original (top) and interpr
profile is referred to in the text. ’
with GPR profiles 1-7 in Fig. 4
-
800
H
800
R.F. Leclerc, E.J. Hickin / Geomorphology 21 (1997) 17-38
may be used as an indicator of the basal contact
between coarser floodplain sediments and underlying
Kamloops Lake Drift sediments.
Of the four cores taken at the same time as radar
profiles (Cl, C2, C7 and C8), only the water table
reflector in core Cl produced a reflection distinguishable from surrounding radar returns. This reflection exhibits a streamward dip in the water table
that is attributed to throughflow from the terrace
along the western border of the study site (see Fig.
2a). A high-amplitude reflection at 2.5 m depth in
GPR profile C3 (Fig. 6) does correspond to the water
table in core C3 (Fig. 5) but the two cannot be
correlated with certainty because the core and GPR
profile were taken 12 months apart.
Oblique clinoform radar facies are common in
these floodplain sediments and two lithologic cores
(C5 and C6) were taken at representative sites to
identify the vertical grain size distribution. Both
cores show upward-fining from medium sand to silt
within each radar facies.
Core C3, a 2.5m-deep auger hole drilled to confirm reports from a local resident that clay lay within
the top 1 m of the floodplain at this location (see
‘C3’ Fig. 41, revealed no clay at this site. Nonetheless, signal attenuation below a depth of 4 m suggests that clay deposits may be nearer the surface
here than at other core locations. Hyperbolic and
inclined reflections below 6 m in this profile (C3 in
Fig. 6) are interpreted as surface reflections caused
by a tree and storage shed within 5 m of the GPR
survey line.
In summary, GPR profile Cl exhibits the most
distinctive radar reflection, at the level of the water
table, a reflector which, based on radar signal amplitude and lateral continuity, could be interpreted in
the surrounding area without the need of additional
coring. Profile C4 showed clear correlation between
a strong radar reflection and a gravel layer but local
residents have identified this gravel bed as a major
source of well water, suggesting that this reflection
could also be attributed to the water table. The water
table could not be identified solely on the basis of
radar reflections in any of the 200 MHz GPR profiles. Cobble and gravel lenses in the remaining
cores did not produce distinctive radar reflections. In
contrast, the approximate depth of basal silt and clay
underlying the floodplain could be consistently de-
29
termined based on the rapid signal attenuation exhibited in all radar profiles of core sites (Fig. 6).
4.2. Interpretation
of GPR profiles
All radar profiles in the GPR survey grid are
identified in Fig. 4 where GPR survey lines are
labeled alphabetically along one axis and numerically along the other. Selected 100 MHz and 200
MHz GPR profiles from the survey grid are presented in this section in original and interpreted
form. Original profiles show two-way travel time
(TWIT) in the vertical axis and distance (m) in the
horizontal. Interpreted profiles replace TW’IT with a
depth (m) axis, derived from CMP profiles of the
study area (see Section 3). Interpreted profiles identify and interpret macroscale radar facies, arbitrarily
defined as a radar facies exceeding 150 m2 on a
two-dimensional (x,y) GPR profile. Radar facies are
distinguished by patterns of reflections whose appearance differs from that of adjacent units, based
upon a visual assessment of differences in parameters noted in Section 3.1. Radar reflections which
make up a radar facies are described using terminology adapted from seismic reflection profiling for
GPR by Beres and Haeni (1991). The genetic interpretation of radar facies is based on the expected
internal architecture of a scrolled floodplain of a
meandering river (see Section 2).
GPR survey lines are examined in two parts. The
first part identifies radar facies along the GPR profile
and includes some facies-level interpretation. The
second part, entitled ‘interpretation’, is reserved for
the genetic interpretation of radar facies as they
relate to the orientation of surface ridge-and-swale
(RAS) topography. The inclusion of some interpretation in the first part, such as the identification of
potential water table reflections, hyperbolic diffractions, multiples, and lithologic boundaries is undertaken in order to minimize repetition between ‘description’ and ‘interpretation’ sections.
No attempt is made to formulate a set of radar
facies for use in the description of GPR profiles in
the survey grid. Rather, each GPR survey line is
examined and interpreted independently. Thus, the
number assigned to a radar facies in a GPR profile
refers to that profile only. That is, radar facies 1
from GPR survey line 7, for example, has no relation
to radar facies 1 identified in GPR survey line C; the
numbering system exists only to differentiate
facies in an individual GPR profile.
radar
4.2. GPR sun~ey line 7
Fig. 7 shows the original and interpreted profiles
of GPR survey line 7 (location shown in Fig. 4).
Macroscale patterns of radar reflections in this profile can be divided into four radar facies. Radar
facies 1 is a zone located from 0 to 800 m distance
in the upper 6 m of the floodplain and is distinguished predominantly
by continuous and semi-continuous reflections dipping from 0” to 8” toward the
south. Radar facies 2 exhibits northward-dipping,
semi-continuous
radar reflections atop a concave-up
reflection at its base. Radar facies 3 identifies a zone
from 215 to 850 m distance with few radar returns
below a depth of 6-7 m, appearing as horizontal,
laterally continuous reflections. An increase in signal
returns below a depth of 6 m from 220 m to 200 m
distance occurs along a diffuse boundary, indicated
by a vertical dashed line separating radar facies 3
and 4. Reflections in the upper half of radar facies 4
are mainly semi-continuous
with inclined radar returns dipping from north and south toward the middle of radar facies 4. Deeper radar reflections (lo- 15
m depth) exhibit a southerly dip of 1” to 4”, although
a few show no apparent dip. A prominent reflection
fluctuating in depth from 10 to 15 m in radar facies 3
and 4 is identified by a dashed line in Fig. 7.
marking the deepest radar return from this section of
the floodplain. Reflections indicated by dashed lines
at 520 and 610 m distance in Fig. 7 are attributed to
surface reflectors, in this case fence posts between 5
and 15 m distance from the GPR survey line.
Interpretcltion. GPR survey line 7 follows the
strike of surface scroll bars (Fig. 4) except in its
southern third (O-290 m distance) where it departs
from true strike orientation by as much as 22”. This
departure is marked by an increase in the dip angle
of subsurface reflections in the southern portion (O290 m distance) of radar facies 1, in agreement with
the generally steeper appearance of beds as they are
viewed closer to channel-normal
orientation. Almost
all reflections in radar facies 1 dip toward the south,
in the direction of point-bar accretion. A high-amplitude, concave-up reflection in radar facies 2 is interpreted as a channel fill (CH). This deposit may be a
cause of overlying reflections that dip opposite the
direction of floodplain accretion, though the relationship is unclear. Signal attenuation and horizontal
radar reflections along the upper portion of radar
facies 3 are interpreted as the basal contact between
floodplain deposits and underlying Kamloops Lake
Drift. A lack of radar signal attenuation in radar
facies 4 is interpreted as a deepening of floodplain
sediments resulting from channel scour.
1.4. GPR surr~ey line C
Pig. 8 shows the original and interpreted profiles
of GPR survey line C (location in Fig. 4). Macroscale
patterns of radar reflections in this profile can be
divided into six radar facies. Radar facies 1 exhibits
wavy and subparallel,
semi-continuous
reflections
overlying continuous reflections dipping slightly toward the east. Radar facies 2 is characterized by
semi-continuous
and discontinuous,
subparallel and
hummocky reflections. The water table is apparent as
a high-amplitude,
laterally continuous reflection at a
depth of 4-5 m from 0 to 166 m distance (‘WT’ in
Fig. 81, confirmed by a water table depth of 4 m at
core Cl (Fig. 4). Radar facies 3 is distinguished by
semi-continuous
and discontinuous
reflections that
show a consistent dip from 1” to 6” toward the east
and exhibits surface scroll ridges (‘S’ in Fig. 8).
Radar facies 4 delimits a series of eastward-dipping
reflections beneath a low-lying area of the floodplain
surface that lacks well-defined scrolled topography.
The base of radar facies 4 follows a concave-up
lower boundary. A deepening trend of floodplain
deposits is observed along the base of radar facies 5
from 330 m to 450 m distance, revealed by increased
radar reflections to a maximum depth of 16 m at 454
m distance in Fig. 8. The water table is identified
from 510 to 580 m distance, based on a strong,
laterally continuous reflection at a depth of 6-7 m.
Radar facies 5 manifests
semi-continuous,
hummocky and subparallel clinofonns
that exhibit an
eastward dip of 0” to 5” from 380 to 490 m distance,
with no consistent dip in hummocky reflections further east. Radar facies 6 is distinguished
by rapid
signal attenuation and an overall lack of radar reflections.
Hyperbolic reflections at 225 m distance are interpreted as surface reflections from a stand of trees
R.F. L.eclerc, E.J. Hickin / Geomorphology 21 (1997) 17-38
adjacent to this site. Hyperbolic reflections at 382,
585, and 612 m distance are attributed to buried
objects because no surface objects were located in
these areas.
Interpretation.
A bend at about 490 m distance in
GPR survey line C and three abrupt changes in the
orientation of surface scroll bars (Fig. 4) result in
significant variability in profile orientation. Radar
facies 1 is oriented 30-35” from the dip direction of
surface scroll topography and exhibits two weakly
developed scroll ridges, labeled ‘S’ in Fig. 8. A steep
rise at 220 m distance in Fig. 8 suggests erosion to
the west, although distinct evidence of channel fill
deposits in radar facies 1 are not apparent to suggest
a process.
The base of radar facies 2 shows rapid signal
attenuation that is interpreted as fine-grained silt and
clay sediments underlying the floodplain. Reflections
in radar facies 2 are more discontinuous than in radar
facies 1 and appear to identify sediment deposited in
a higher-energy environment; however, this does not
correlate with the limited evidence of clay deposits
observed in this facies (core Cl in Fig. 5).
Although the orientation of GPR survey line C
from 220 to 490 m distance lies between 50” and 60”
from true dip direction, radar facies 3 clearly illustrates surface scroll topography and inclined stratification dipping in the direction of lateral accretion of
the floodplain. Radar facies 4 is interpreted as a
preserved channel fill, a hypothesis supported by
evidence of a lower concave-up boundary in addition
to surface evidence of a low-lying area bounded by
well-developed scrolled topography.
A deepening trend in radar reflections in radar
facies 5 from 330 to 450 m distance is interpreted as
the result of channel scour, a trend also observed in
GPR survey line 7. Greater inclination of reflections
in radar facies 5 from 380 to 490 m distance is
attributed to steeper channel banks, the result of
channel deepening as the river migrated eastward.
Rapid signal attenuation along the upper boundary of
radar facies 6 is interpreted as the interface between
floodplain sediments and Kamloops Lake Drift.
4.5. GPR survey line dip1
Fig. 9 shows the original and interpreted profiles
of GPR survey line dip1 (location in Fig. 4). Patterns
31
of radar reflections can be divided into two radar
facies, based on signal attenuation within the radar
profile. Radar facies 1 exhibits laterally continuous,
subparallel and wavy reflections from 0 to 160 m
distance that tend to mirror surface topography. Reflections in radar facies 1 from 160 to 195 m distance are less continuous and more hummocky, exhibiting radar reflections that dip to the north and
south. Patterns of reflections in radar facies 1 dipping to the north are distinguished by shaded areas
and appear in sections dominated by wavy and inclined reflections. The locations of prominent surface
swales and vertically preserved scroll ridges (at ‘S’
locations) are indicated in Fig. 9. Macroscale inclined surfaces that exhibit a southerly dip of up to
4” at some surface scroll ridge locations are also
apparent and identified by dashed lines in Fig. 9.
Radar facies 2 is identified by a region of multiple
signal reflections from 0 to 80 m distance and by
rapid signal attenuation from 80 to 192 m distance,
increasing to a depth of 18 m at the south edge of the
profile. Evenly spaced weak reflections that mirror
surface topography from 0 to 80 m distance at a
depth of 7-10 m in radar facies 2 are identified as
multiples caused by a lithologic change from sand
and gravel to silt and clay at a depth of 7 m. This
form of ringing and coupling is most prominent
when ground conductivity is high (i.e. saturated silts
and clays; Telford et al., 1977).
Interpretation. This profile is oriented parallel to
the dip direction of surface scroll bars (Fig. 4). Thus,
radar stratigraphy in Fig. 9 shows a channel-normal
view of lateral accretion architecture in the scrolled
floodplain. Floodplain deposits from 0 to 160 m
distance exhibit preserved ridge-and-swale topography at depth, with some variability between individual scroll bars. Some scroll bars exhibit more steeply
inclined strata at depth, dipping in the direction of
floodplain accretion (18 m, 62 m, and 91 m distance
in Fig. 91, whereas others do not (33 m, 48 m, 107
m, and 151 m distance). Visual examination of RAS
deposits in the northern half of Fig. 9 reveals a
pronounced lack of inclined radar reflections, and
suggests poorly developed inclined stratification.
Some large sections of the radar profile exhibit no
inclined reflectors at all, such as an 18 m length from
68 to 86 m distance that show parallel reflections
interpreted as a vertically accreted swale fill.
s
33
R.F. L.eclerc, E.J. Hickin / Geomorphology 21 (1997) 17-38
Rapid signal attenuation between radar facies 1
and 2 is interpreted as the base of floodplain deposits. An increase in the depth to radar facies 2
between 100 and 160 m distance in Fig. 9 is interpreted as the result of channel scour in response to
confinement at the concave bank-bench immediately
upstream (see Fig. 1). Dashed lines in Fig. 9 identify
large-scale reflections of inclined stratification, dipping in the direction of floodplain accretion. The
southern portion of the floodplain (150-192 m distance) exhibits more of these large-scale inclined
strata and a lack of scroll topography which may be
associated partly with the formation of an island and
adjacent chute channel in the river next to the profile
(see Fig. 4). Shaded regions dipping opposite to the
direction of floodplain accretion are uncharacteristic
and may also be associated with these features although the relationship is unclear.
4.4. GPR survey line strike
Fig. 10 shows the original and interpreted profiles
of GPR survey line strike (location in Fig. 4). Patterns of radar reflections in this profile can be divided into three radar facies. Radar facies 1 exhibits
laterally continuous, parallel reflections, some of
which exhibit a slight (1”) dip in the direction of
floodplain accretion (west). Radar facies 2 is distinguished from radar facies 1 by less continuous,
subparallel and hummocky reflections, some of which
exhibit a westward dip of up to 6”. Radar facies 3
exhibits rapid signal attenuation below 7 m and
displays a series of equally spaced reflections that
are identified as multiples caused by a sand and
gravel contact with underlying silt and clay at a
depth of 7 m.
Interpretation. GPR survey line strike follows
along the top of a surface scroll ridge (Fig. 4). Radar
facies 1 is interpreted as parallel and inclined strata.
Less continuous inclined reflections from 40 to 110
m distance at a depth of 3.5-7 m in radar facies 2
are interpreted as downstream accretion of the pointbar. Remaining reflections in radar facies 2 are
interpreted as parallel beds that show no overall dip.
Based on signal attenuation, radar facies 3 is interpreted as Kamloops Lake Drift below a depth of 7-8
m.
4.7. Three dimensional (3-D) distribution of selected
radar facies
A summary of the three-dimensional (3-D) extent
of radar facies exhibiting ridge-and-swale (RAS) and
inclined stratification (IS) in the GPR survey grid is
presented in Fig. 11. These radar facies account for
the majority of observed radar stratigraphy in the
GPR survey grid. RAS and IS deposits in the floodplain are not mutually exclusive; RAS structures
typically dip in the direction of floodplain accretion
(Sundborg, 1956; Nanson, 1980; Kinoshita, 1987).
Thus, cross-sections showing the location of interpreted RAS and IS radar facies from 100 and 200
MHz GPR profiles may - and in this study almost
always do - exhibit RAS and IS at the same
location.
Fig. 11 shows that IS and RAS deposits are the
dominant characteristics of floodplain deposits in the
meander bend. Whereas RAS deposits tend to become indistinct with increased deviation from channel-normal orientation, IS deposits persist even in
true strike orientation, such as upper scroll ridge
deposits in Figs. 7 and 10, where IS is interpreted as
the downstream accretion of the floodplain.
Poor correlation was found between radar facies
boundaries at intersection points in the GPR survey
grid; however, the interpreted basal contact between
floodplain alluvium and underlying silt and clay
showed much better correlation. Of the 50 intersection points on the GPR survey grid, 30 (or 60%)
showed good (5 1 m> 3-D correspondence in the
interpreted depth of this basal contact. Boundaries of
radar facies not associated with the basal contact,
Fig. 9. Original (top) and interpreted bottom) 200 MHz GPR profile of survey line dip1 (location in Fig. 4). Terminology
profile is referred to in the text. Vertical exaggeration is 1.6 X
Fig. 10. Original (top) and interpreted (bottom) 200 MHz GPR profile of survey
interpreted profile is referred to in the text. Vertical exaggeration is 1.6 X
line strike (location
in the interpreted
in Fig. 4). Terminology
in the
34
R. F. Leclerc,
I%../. Hickin / ~;ronwrpholo~y
Fig. I I. Locations of ridge and swale (RAS) and inclined stratification
necessarily imply RAS and IS within the entire vertical column.
however, exhibited much poorer correspondence.
Of
50 intersection points, only 5 (or 10%) showed good
3-D correspondence
(+ 1 m between radar facies
boundaries). The 3-D identification
of sedimentary
units is a persistent problem in GPR interpretation
and also in logging genuine floodplain
sections
(Wolman and Leopold, 1957; Brierley, 1991).
Rapid signal attenuation at depth, because of its
consistent appearance in all radar profiles, was a
useful indicator of the interface between floodplain
alluvium and underlying silt and clay. GPR profiles
7, C and dip1 all contribute evidence of a gradual
reduction in radar signal attenuation toward the south,
from about 6-8 m to about 15 m in depth. These are
interpreted as the result of channel scour attributed to
confinement of the bend immediately upstream, creating a deep scour hole (14 m) at this location
(Ministry of Environment,
1982a).
5. Discussion
This section is divided into three parts: (i) observation of inclined stratification (IS), (ii) geometry of
RAS deposits, and (iii) limitations of GPR methodology.
21 (1997) 17-38
(IS) deposits in 100 MHz and 200 MHz
.i. I. Observation
radar
profiles. This does not
of inclined stratijcation
For the most part, RAS structures from this study
exhibit a geometry similar to those described in the
literature (see Fig. 3). Some degree of inclined stratification was observed in almost all GPR profiles of
dip direction (Fig. 111, in agreement with large-scale
models of lateral accretion architecture from the
literature (Nanson and Croke, 1992; Miall, 1996). A
notable exception to this trend was observed in the
northern half of GPR survey line dip1 (Fig. 9>,
where vertically preserved RAS structures exhibit
only weakly developed inclined stratification. They
appear as individual large-scale reflections dipping
in the direction of floodplain accretion. Although
similar large-scale inclined strata are observed in
channel-normal
sections of other scrolled floodplains
(see Fig. 31, inclined strata in Fig. 9 are atypical
because they are so infrequent, separated by regions
of parallel and wavy radar reflections that exhibit no
streamward dip. This observation is notably different
from the successive inclined strata characterizing
generalized models (Nanson and Croke, 1992; Miall,
1996) as well as results from other GPR applications
in the floodplains of meandering rivers (Gawthorpe
et al., 1993; Bridge et al., 1995).
R. F. Leclerc, E. J. Hickin / Geomorphology 21 (1997) 17-38
Genetically, inclined bounding surfaces, such as
those observed in Fig. 9, could be interpreted as
separating successive floodplain-forming events,
similar to those described by Puigdefabregas (1973).
Although no clear association exists between these
events and the formation of individual scroll bars,
some scroll bars exhibit these inclined strata and
some do not. In addition, the description of floodplain accretion by Puigdefabregas (1973) is the only
one to suggest the possibility of inclined strata dipping away from the river channel. This feature is
largely observed in the southern section of Fig. 9 and
not at all in the 100 MHz GPR survey grid (see Fig.
4), suggesting that it may be locally related to island
and chute channel formation adjacent to the floodplain near profile dipl.
Frequent parallel reflections that separate largescale, channelward dipping reflections in Fig. 9 present a deviation from generalized models of lateral
accretion architecture that seems best explained by
vertical accretion processes. All surface scroll ridges
exhibit some form of preservation within the vertical
column. The example located at 20 m distance in
Fig. 9 bears a close resemblance to vertically accreted scroll-bar deposits observed by Nanson (Fig.
3d). Although it has been suggested that vertical
accretion has a minor role in the formation of scrolled
floodplains (Wolman and Leopold, 1957), this may
not be true for all meandering rivers, specifically
those that have actively incised through materials
which supply a high concentration of fines (Sundborg, 1956; Nanson, 1980; and this study).
Because predominant inclined stratification is described in models and observations of scrolled floodplains (Fig. 31, GPR may not accurately reveal all
the inclined strata that are indeed present. Nonetheless, GPR data from other studies of floodplains of
meandering rivers show that inclined strata can easily be identified using GPR (Gawthorpe et al., 1993;
Bridge et al., 1995). Similar observations of inclined
stratification in large-scale studies of deltas also
demonstrate the ability of GPR to resolve IS deposits
&nith and Jol, 1992). Thus, we are inclined to
believe that radar stratigraphy indicating only weakly
developed or absent inclined stratification is an accurate portrayal of underlying stratal trends.
Along a surface scroll ridge, inclined stratification
observed in GPR profile strike (Fig. 10) suggests a
35
downstream accretion process associated with scrollbar formation. Deeper radar reflections in radar faties 2 exhibit less continuous inclined strata consistent with deposition in higher-energy flows at this
depth. This distinction between upper and lower
point-bar deposits is in agreement with observations
in a similar GPR study by Bridge et al. (1995). No
evidence of preservation of proximal point-bar deposits in strike direction profiles exists, except for a
small (45 m) section at the northern extreme of GPR
profile 7, where the dip of reflections suggests
preservation of bedload swept onto the proximal
edge of the point bar.
5.2. Geometry of ridge-and-swale CRAS) deposits
Sundborg (1956) and Gibling and Rust (1993)
observed that preserved scroll ridges are asymmetric
and steeper on the streamward side. This study shows
the same trend in deeper-preserved scroll ridge deposits, such as those located at 18 m and 91 m in
GPR profile dip1 (Fig. 9). Surface ridges, however,
show no apparent trend. Some are asymmetric,
whereas some are not. Asymmetric forms show
steeper dips in variable directions, toward or away
from the river channel. Some surface ridge structures
show continuity to the base of the floodplain in
several locations in profiles C and dip 1 (Figs. 8 and
9>, appearing as lobate, convex-up radar reflections
near the base of floodplain sediments. These structures are similar in appearance and location to descriptions of unit bars made in similar environments
by several authors (Sundborg, 1956; Nanson, 1980;
Bridge et al., 1995). In addition, core C8 (located at
20 m distance in profile dipl) displays a coset of
alternating coarse- to finer-grained laminae from 1.4
to 1.6 m (see Fig. 5) which is in agreement with
Sundborg’s (1956) description of preserved scroll
ridge sediments.
Based on the width/depth ratio of the main channel near the distal end of the meander bend (Ministry
of Environment, 1982a), the average dip angle of
lateral accretion deposits in the river floodplain is
estimated as 4” to 5” (Leeder, 1973). This value is
comparable with observations from GPR profiles of
dip directions, although this is difficult to estimate
because the dip angle of inclined reflections varies
significantly about this range at any given location.
R.F.
36
o(m)
Leclerc,
50
E.J. Hickin
/ Geomorphology
100
21 (1997)
I7-3X
200
150
250
0
section
------+b
Vertically preserved RAS stratlficatm
with weakly observed IS
-.
*-
U
50
O(m)
100
150
Macroscale inclmed strata
Interpreted mt bar
200
250
0
c5
lo-I
--
_
strike sectton
f--------
Inclined strattfication
Fig. 12. Generalized schematic of radar stratigraphy and major featul-es interpreted
of floodplain accretion in each section is shown by the arrow
Some areas of the floodplain, however, exhibit
significantly steeper beds, such as the southern part
of radar facies 1 in GPR profile 7 (Fig. 7). This may
be the result of a stepped floodplain profile (see Fig.
2a) because steeply dipping IS deposits are located
on one of these steps. This location, where GPR
survey line 7 exhibits signs of channel scour in the
southern section of the radar profile (radar facies 1,
O-400 m distance), is likely a response to confinement and subsequent degradation of the river bed
immediately
upstream at the concave bank-bench
(see Fig. 1).
A representative
sample combining all the features discussed in dip and strike GPR profiles in this
section is shown in Fig. 12. Though not necessarily
occurring in this precise arrangement,
this model
summarizes large-scale radar stratigraphic trends and
features interpreted in the floodplain along the direction of dip and strike profiles.
5.3. Limitations
qf GPR methodology
The interpretation of alluvial stratigraphy in this
study was restricted by the following three important
methodological
limitations. (1) Profiles of radar reflection are only an indirect means of acquiring
subsurface data. The validity of results, based on
in dip and strike sections of GPR profiles. The direction
interpreted GPR profiles which differ from conventional models, may be subject to question. (2) GPR
survey design in this study focused only on largescale radar stratigraphic trends in the floodplain, and
restricted the amount of information
gathered on
smaller-scale structures. (3) A Cartesian GPR survey
grid design created complexity in the interpretation
of GPR profiles, because of the highly variable
orientation of GPR survey lines, and the appearance
of radar facies, relative to surface scroll topography.
Two problems that contribute to limitation (1) are:
(i) the changes in grain sizes, porosity, or strata1
variability that will result in partial reflection of the
radar signal cannot be predicted in an exposed section (Greenhouse et al., 1987), and (ii) GPR is, for
the most part, insensitive to variations in grain size
other than those which cause a significant change in
dielectric conductivity (Greenhouse et al., 1987). As
a result of the macroscale approach of the GPR
survey design (limitation 2), the resolution of GPR
profiles at the study site was fairly coarse. The
maximum vertical resolution of radar profiles was
0.4 m and 0.2 m for 100 and 200 MHz GPR profiles,
respectively. Horizontal resolution was less, 1.0 m
and 0.5 m for 100 and 200 MHz profiles, respectively. As a result, the internal architecture of microscale structures could not be clearly resolved.
R.F. Leclerc, E.J. Hickin/Geomorphology
Limitation (3) contributed to an inability to relate
radar facies across three dimensions in this study.
Good 3-D correlation of the interpreted boundary
between floodplain and glaciolacustrine sediments
was established; however, in large part because the
identification of basal silt and clay was not dependent on orientation of the GPR survey line but on the
dielectric properties of silt and clay, which cause
rapid signal attenuation at depth in all radar profiles.
21 (1997) 17-38
31
Acknowledgements
This study forms part of a project funded by the
Natural Sciences and Engineering Research Council
of Canada. Comments by anonymous reviewers contributed to improvements of the manuscript.
References
6. Conclusions
Radar stratigraphy of the North Thompson River
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scroll ridges. The lateral accretion deposit exhibits
inclined stratification (IS) and ridge-and-swale (RAS)
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