Holocene lateral channel migration and incision of the Red River,
Manitoba, Canada
Gregory R. Brooks*
Geological Survey of Canada, 601 Booth Street, Ottawa, ON, Canada K1A 0E8
Received 1 June 2002; received in revised form 1 November 2002; accepted 29 November 2002
Abstract
The Holocene evolution of the shallow alluvial valley occupied by the Red River was investigated at two successive river
meanders near St. Jean Baptiste, Manitoba. A transect of five boreholes was sited across the flood plain at each meander to
follow the path of lateral channel migration. From the cores, 24 wood and charcoal samples were AMS radiocarbon dated. The
dates from the lower half of the alluvium in each core are interpreted to represent the age of the lateral accretion deposits within
the flood plain at the borehole sites. The ages of these deposits increase progressively from f 900 to 7900 and 1000 to 8100 cal
years B.P. along each transect, respectively, from the proximal to distal portions of the flood plain. At the upstream meander, the
average rate of channel migration was initially 0.35 m/year between f 7900 and 7400 cal years B.P., then decreased to 0.18 m/
year between f 7400 and 6200 cal years B.P., and subsequently varied between 0.04 and 0.08 m/year. Net channel incision of
the river since 8100 cal years B.P. is estimated to have ranged between 0.4 and 0.8 m/ky. The pre-6000-years-B.P. interval of
greater channel migration is hypothesized to reflect a higher phase of sediment supply that was associated with the
establishment of the river system on the former bed of glacial Lake Agassiz. Since 1000 years B.P., the outward migration of the
meanders has caused a gradual enlarging of 0.7­2% in the cross-sectional area of the shallow valley at the two meanders. When
considered proportionally over timescales of up to several centuries, the widening of the valley cross-section is very low to
negligible and is deemed an insignificant factor affecting the modern flood hazard on the clay plain.
Crown Copyright D 2002 Published by Elsevier Science B.V. All rights reserved.
Keywords: Red River; Flood plain; Lateral channel migration; Channel incision; Sediment supply; Alluvial chronology
1. Introduction
Studies investigating fluvial geomorphic change
over timescales of many thousands of years have
provided insights into the long-term temporal character
and controls on modern rivers. As a general context
to the contemporary alluvial landscape, many major
rivers in temperate, mid-latitude areas during the late
Quaternary experienced a general transition from
braided to meandering planforms because of changes
in discharge and sediment supply regimes associated
with deglaciation (e.g., Starkel, 1991; Knox, 1995, and
references therein). Lower intensity perturbations in
fluvial activity have continued during the Holocene
and have been driven by variations in climate, shifts in
vegetation ecozones, base level and tectonic changes,
0169-555X/02/$ - see front matter. Crown Copyright D 2002 Published by Elsevier Science B.V. All rights reserved.
doi:10.1016/S0169-555X(02)00356-2
* Tel.: +1-613-996-4548; fax: +1-613-992-0190.
E-mail address: gbrooks@gsc.nrcan.gc.ca (G.R. Brooks).
www.elsevier.com/locate/geomorph
Geomorphology 54 (2003) 197­215
anthropogenic effects and/or local factors (see Knox,
1983, 1995, 2000; Starkel, 1991). Studies of geomorphic
change over longer timescales have also
provided insights into processes where geomorphic
development is not apparent over the short term (years,
decades; see, for example, Brakenridge, 1985; Tinkler
et al., 1994). Consideration of long-term variations in
the intensity and spatial pattern of fluvial processes
have aided the interpretation of North American
archaeological records (see Bettis, 1995). Of increasing
importance, however, is the consideration of longer
term geomorphic change to modern environmental
issues or geological hazards. Such studies have proved
insights into, for example, flood frequency­magnitude
and climate change (Knox, 1993; Nott and Price,
1999), factors controlling bank-breach floods (Jiongxin,
2001), paleohydrology (O'Connor et al., 1994)
and the occurrence of prehistoric river impoundments
by landslides (Brooks and Hickin, 1991; Reneau and
Dethier, 1996).
The Red River in Manitoba, Canada (Fig. 1),
experiences extreme spring floods that form a broad,
shallow flood zone and rise and fall slowly over a
period of up to 4­6 weeks. A key geomorphic factor
contributing to the flood hazard is that the river
occupies a shallow, low-gradient alluvial valley that
has insufficient capacity to contain large flows (LeFever
et al., 1999; Brooks and Nielsen, 2000). During
such floods, river waters overtop the margins of the
valley and spread across the adjacent flat clay plain.
For example, the flood of April­May 1997 (see
Rannie, 1998; Todhunter, 2001), the largest Red River
flood in southern Manitoba since 1852, inundated an
area of about 2000 km2
and up to 40 km wide
between the Canada/USA border and Winnipeg (Fig.
1; data from Manitoba Natural Resources). An important
consideration is whether geomorphic processes
relating to the development of the shallow alluvial
valley are altering the cross-sectional area sufficiently
to affect the contemporary flood hazard on the clay
plain. While unlikely to be relevant at annual to
decadal timescales, the change in valley cross-sectional
area may be significant at a century timescale
and thus represent a background process that could be
either aggravating or easing the long-term trend of the
flood hazard.
This paper summarizes the results of a borehole
investigation into the alluvial deposits of the shallow
valley occupied by a sinuous section of the Red River
near St. Jean Baptiste, southern Manitoba (Fig. 1).
The objectives are to: (i) document the geomorphic
Fig. 1. Maps showing (A) the Red River drainage basin and (B) the
location of the study area near St. Jean Baptiste, Manitoba.
G.R. Brooks / Geomorphology 54 (2003) 197­215198
evolution of this valley by reconstructing lateral
channel migration and channel incision since the
early­mid Holocene; (ii) identify the major control(s)
on this evolution; (iii) assess the significance of the
resulting change in valley cross-sectional area to the
contemporary flood hazard on the clay plain.
2. Red river
The north-flowing Red River drains an area of
290,000 km2
(Fig. 1A) that traverses the flat and
gently sloping clay plain of the Red River valley
(Upham, 1895). The clay plain is composed of glaFig.
2. Oblique aerial photographs of the Red River showing (A) the irregular meandering planform of the river and (B) the downstream
meander at the study site where the 99RR1 transect of boreholes are located. The shallow alluvial valley occupied by the river is approximately
the width of the meander belt, but is imperceptible from the flat clay plain in both photographs. The view in (A) is downstream looking toward
the study area meanders that are located in the background (marked by arrow), while (B) is looking west across the alluvial valley.
G.R. Brooks / Geomorphology 54 (2003) 197­215 199
ciolacustrine sediments that aggraded within glacial
Lake Agassiz during the late Pleistocene and early
Holocene (see Teller and Clayton, 1983). In Manitoba,
the river became established on the lake bed
between 7800 and 8200 14
C years B.P., as the lake
waned and receded northward (Fenton et al., 1983;
Teller et al. 1996). The river has since eroded a
shallow valley into the clay plain, up to 15 m deep
and 2500 m wide, that contains the genetic flood plain
of the river. Reflecting the northward drainage and
development on the lake bed, the river does not
occupy a valley eroded by melt water under a glacially
influenced or glacial-lake-influenced hydrological
regime.
Within Manitoba, the Red River is a single-channeled,
meandering river with an irregular sinuosity
and a low average valley gradient of 0.0001 (Figs. 1B
and 2). The flood plain and the contemporary riverbanks
are composed predominantly of silt alluvium
(Brooks, submitted for publication), and thus, the
river represents an example of a mud-dominated
stream. A curvilinear pattern of ridge and swale topography
commonly marks past channel positions
across the valley bottom and reveals that the flood
plain has been formed by the lateral migration of
meanders. This topography also reveals no obvious
change in meander geometry, which would be indicative
of a significant alteration in the discharge and/or
sediment supply regimes or a threshold response in
the river planform (see Schumm, 1977). Sequential
aerial photographs and the comparison of mid-19th
and late-20th century maps reveal no significant
lateral channel migration along the river.
At Emerson, Manitoba (Fig. 1B), 28 km upstream
of the study area and with a contributing drainage area
of 104,000 km2
, the mean annual discharge of the Red
River is 98 m3
/s, and the lowest and highest mean
monthly flows are 22 (February) and 362 m3
/s (April),
respectively (Water Survey of Canada data, 1912­
1995). Bankfull discharge, defined as a flow with a 2year
return period, is approximately 600 m3
/s. The
annual hydrograph typically consists of a freshet flow
in April or May arising from snowmelt runoff, while
secondary peaks generated by rainfall can occur
between June and October. The flood of record at
Emerson occurred on 27­28 April 1997, peaking at
3740 m3
/s (data from Manitoba Water Resources).
The suspended sediment load of the river consists of
over 90% silt and clay, regardless of sediment concentration
and river discharge (Glavic et al., 1988).
The annual suspended sediment load at Emerson
varies between 0.4 and 1.5 million tonnes with minimum
and maximum sediment concentrations ranging
from 10­20 (December to February) to 500­1900
mg/l (April to July), respectively (1978­1986 data;
Glavic et al., 1988).
Postglacial crustal rebound resulting from the isostatic
depression of the landscape by the Laurentide
Ice Sheet during the late Pleistocene has affected the
regional landscape of the eastern Prairies. This is
readily exemplified by the differential tilting of Lake
Agassiz shorelines in Manitoba, eastern North Dakota
and western Minnesota (Upham, 1895; Johnston,
1946; Teller and Thorleifson, 1983). Isobase maps
defined by the deformed shorelines indicate that
greater uplift occurred toward the NE in the direction
of Hudson Bay (Johnston, 1946; Teller and Thorleifson,
1983). Although the rates of uplift have diminished
over the Holocene (see Tackman et al., 1998),
regional tilting is active in southern Manitoba, as
revealed by contemporary lake gauge and geodetic
data (Lambert et al., 1998; Tackman et al., 1999).
3. Methods
Data for this study were obtained by coring the
flood-plain alluvium of the Red River across the
valley bottom at two successive river meanders
located about 2.5 and 3.5 km S­SE (upstream) of
St. Jean Baptiste, Manitoba (Figs. 1B, 2B and 3).
These sites were chosen because of the relatively
broad width and locally uniform morphology of the
meander belt, easy access onto the flood plain via
cultivated fields and a preserved ridge and swale
pattern on the flood plain. These meanders are typical
of the Red River in southern Manitoba, notwithstanding
local variations in channel sinuosity. At the study
area, the channel sinuosity is 2.8, the valley gradient is
0.00006 and the bankfull channel is about 75 m wide
and 6­9.5 m deep. The boreholes were positioned in
two transects (99RR1 and 99RR3) along the east and
west sides of the valley bottom, with each transect
consisting of five boreholes (label A to E in Fig. 3).
They were sited to follow the path of lateral migration
of the channel, as revealed by the ridge and swale
G.R. Brooks / Geomorphology 54 (2003) 197­215200
pattern. The heights of the boreholes along each
transect were surveyed relative to an arbitrary datum.
Brooks et al. (2001) described in detail the coring and
core logging methods.
Twenty-four wood and charcoal samples were
submitted to Beta Analytic (Miami, FL) for AMS
radiocarbon dating. These samples were selected carefully
to favour ``fresh-looking'' samples to avoid
dating reworked materials. The radiocarbon ages were
calibrated to calendar years by Beta Analytic following
Talma and Vogel (1993) and using the calibration
data set of Stuiver et al. (1998).
4. Core stratigraphy
The cores contain two distinct units: a lower unit of
glaciolacustrine deposits that aggraded within Lake
Agassiz and an upper unit of flood-plain alluvium
(Fig. 4). The presence of glaciolacustrine deposits
indicates that the entire vertical sequence of the
alluvial deposits is represented in the 10 cores. As
summarized from Brooks (submitted for publication),
the Red River alluvium in the 10 cores ranges from
15.3 to 21.6 m thick and is composed primarily of silt.
The deposits consist of three main lithofacies; cosets
of massive silt, thick (>0.4 m) massive silt and
deformed and disrupted silt. Sand beds and interlaminated
sand and silt form relatively minor deposits,
principally within the lower half of the alluvium. Thin
beds of medium­coarse sand and pea gravel are
present in two cores within the lower meter of the
alluvium. Authigenic CaCO3 precipitate and Fe-oxide
mottling are present in the alluvial deposits and
generally increase in vertical extent and lateral concentrations
from the younger (A) to older (E) cores.
CaCO3 precipitate is present predominantly in the
upper half of the alluvium and forms irregularly
shaped, whitish agglomerates up to 2 cm across and
occasionally nodules. The Fe-oxide mottling is
present in the lower half of the alluvium. The mottles
are irregularly shaped, generally several millimeters in
scale and tend to be developed in zones of slightly
finer silt sediments.
Organic material is present throughout the alluvium
as small ( < 1 mm) disseminated organic mateFig.
3. Map showing the locations of the 10 boreholes along the two transects and the ridge and swale pattern across the flood plain.
G.R. Brooks / Geomorphology 54 (2003) 197­215 201
Fig. 4. Summarized lithostratigraphic diagram of the cores showing the depth locations of the 24 AMS radiocarbon ages. The boreholes are arranged from the distal to proximal areas
of the flood plain with the sequencing intended to reflect the relative ordering at successive river meanders. The age at the bottom of each core (except 99RR1D) is the representative
calibrated age of the lateral accretion deposits, as explained in the text (also see Table 2). The dates used in this determination of this representative age are flagged with an asterisk (*).
G.R.Brooks/Geomorphology54(2003)197­215202
rial, fine charcoal and to a lesser extent, shell fragments.
Whole shells, larger charcoal and wood fragments
are present infrequently. Buried paleosols,
several centimeters thick, occur in the upper 2 m of
99RR1D, 99RR1E, 99RR3B, 99RR3D and 99RR3E.
All but one of the 24 organic samples submitted for
Table 1
Listing of AMS radiocarbon ages from the boreholes
Borehole Location
(latitude/longitude)
Depth
(m)
Sample
number
Material Radiocarbon age
(14
C years B.P.)
13
C/12
C
ratio (x)
Calibrated age
and age range at
1r (cal years B.P.)
99RR1A 49j14.8VN 97j18.5VW 4.11 Beta-144952 wood 460 F 40  26.1 515 (500­525)
8.16 Beta-144953 probably
Betulaa
680 F 30  29.1 655 (650­665)
11.19 Beta-144954 deciduous
wooda
980 F 40  29.0 925 (910­940)
11.37 Beta-144955 wood 1020 F 40  27.8 940 (925­955)
12.25 Beta-144956 Salixa
1260 F 40  27.4 1185 (1165­1260)
14.97 Beta-144957 deciduous
wooda
1140 F 40  28.8 1055 (980­1075)
18.24 Beta-144958 deciduous
wooda
1050 F 40  27.0 950 (935­970)
99RR1B 49j14.7VN 97j18.6VW 9.88 Beta-144959 deciduous
wooda
2200 F 40  29.0 2160, 2260, 2300b
(2140­2300)
10.78 Beta-144960 deciduous
wooda
2310 F 40  26.1 2340 (2325­2350)
99RR1C 49j14.6VN 97j18.7VW 12.27 Beta-144961 wood 4760 F 40  28.0 5480, 5530, 5575b
(5465­5585)
14.19 Beta-144962 Salixa
4750 F 40  26.5 5475, 5540, 5570b
(5465­5585)
99RR1E 49j14.5VN 97j19.1VW 0.41 Beta-144963 charcoal
(wood)
1190 F 30  24.5 1080 (1065­1165)
11.38 Beta-144964 wood 9540 F 60  26.1 10,755, 10,985,
11,030b
(10,710­
10,870 and
10,935­11,080)c
16.68 Beta-144965 wood 7290 F 50  28.4 8060, 8090, 8110b
(8020­8160)
99RR3A 49j14.3VN 97j19.5VW 11.62 Beta-144966 deciduous
wooda
940 F 40  26.1 910 (790­925)
16.22 Beta-144967 deciduous
wooda
1000 F 40  26.4 930 (915­945)
99RR3B 49j14.3VN 97j19.4VW 9.78 Beta-144968 wood 2470 F 70  27.1 2490, 2645, 2700b
(2360­2730)
99RR3C 49j14.2VN 97j19.2VW 14.40 Beta-144969 Salixa
5430 F 90  25.1 6270 (6175­6300)
14.77 Beta-144970 Salixa
5310 F 60  27.5 6020, 6070, 6105b
(5990­6185)
99RR3D 49j14.2VN 97j19.1VW 11.40 Beta-144971 Salixa
6550 F 60  25.7 7440 (7425­7485)
16.15 Beta-144972 Salixa
6360 F 70  27.0 7275 (7240­7330)
99RR3E 49j14.2VN 97j18.9VW 13.68 Beta-144973 Salixa
6990 F 70  26.3 7805 (7725­7865
and 7900­7920)c
16.44 Beta-144974 wood 7060 F 60  27.6 7865, 7900, 7920b
(7815­7945)
16.69 Beta-144975 deciduous
wooda
7850 F 70  27.8 8610 (8565­8715)
a
Identified by C. Keith (written communication, 1999).
b
Multiple ages represent separate intercepts of the mean radiocarbon age with the calibration curve.
c
Two age ranges represent separate intercepts of the mean radiocarbon age at 1r with the calibration curve.
G.R. Brooks / Geomorphology 54 (2003) 197­215 203
radiocarbon dating yielded post-glacial Lake Agassiz
ages, which is consistent with a fluvial origin of the
deposits (Table 1).
Identifying specific depositional environments in
the alluvium is complicated by the lack of obvious
vertical changes in texture or lithofacies within the
deposits. Based on the topography and geomorphology
of the flood plain, the presence of organic matter
in the deposits and the character of modern sedimentation
along the riverbanks, the alluvium is interpreted
to be composed of overbank deposits from 0 to 2­3 m
depth, oblique accretion deposits from 3­5 to 8­12 m
depth and oblique accretion/channel deposits below
8­12 m (Brooks, submitted for publication). Lag
deposits composed of thin beds of medium to coarse
sand or pea gravel can be present locally in the lower
1 m of the alluvium.
5. Flood-plain chronology
The radiocarbon ages are depicted stratigraphically
in Fig. 4. At least one date was obtained from each of
the cores with the exception of core 99RR1D.
Twenty-one dates are from the lower halves of the
alluvial unit within the cores (Fig. 4). The within-core
variability of these dates can be assessed from core
99RR1A from which seven organic samples were
dated, five from the lower half of the alluvial unit
(Fig. 4). The radiocarbon ages of these latter samples
range from 980 F 40 to 1260 F 30 14
C years B.P.
(Fig. 4). Although some transposition in the vertical
sequencing occurs (Fig. 4), the ages are statistically
indistinguishable at an error range of 2r. Geomorphically,
this similarity of age indicates that vertical
sedimentation over the lower half of the alluvial unit
was relatively rapid and probably occurred over a few
hundred years.
This consistency of radiocarbon ages in the lower
halves of the alluvial unit is duplicated by the pairs of
dates from cores 99RR1B, 99RR1C, 99RR3A,
99RR3C and 99RR3D and from two of the three
dates in core 99RR3E (Fig. 4). Two dates, 9540 F 60
and 7850 F 70 14
C years B.P. from cores 99RR1E and
99RR3E, respectively, are anomalously old relative to
the other dates at nearby depths. The organic materials
that yielded these dates are interpreted to have been
reworked and thus unrepresentative of the age of the
encapsulating sediments. The 9540 F 60 14
C years
B.P. age clearly is too old since it predates the final
recession of glacial Lake Agassiz in southern Manitoba,
while the 7850 F 70 14
C years B.P. age is
substantially older than two other dates sampled at
approximately similar depths within core 99RR1E.
Excluding the two anomalously older dates, the
radiocarbon dates from the lower half of the alluvium
within a given borehole are interpreted to represent the
age of the lateral accretion deposits at that location of
the flood plain. The age of these deposits thus reveals
a past position of the inner bank of a meander at a
specific time, as the deposits would have aggraded
immediately adjacent to the river channel. To better
Table 2
Summary listing of the single or averaged calibrated radiocarbon
ages representing the age of the lateral accretion deposits at each
borehole
Borehole Number
of dates
used in
determining
the average
agea
Mean age
(cal years
B.P.)
1r error Age range
at 1r (cal
years B.P.)
99RR1A 5 1010b,c
 80/ + 140 930­1150
99RR1B 2 2290b
 100/ + 80 2190­2370
99RR1C 2 5530b
 90/ + 80 5440­5610
99RR1D no data ­ ­ ­
99RR1E 1 8090d
F 70e
8020­8160
99RR3A 2 920  120/ + 20 800­940
99RR3B 1 2610d
 250/ + 120 2360­2730
99RR3C 2 6170b
F 120 6050­6290
99RR3D 2 7360  30/70 7330­7430
99RR3E 2 7850b,f
 110/ + 130e
7740­7980
a
See Table 1 for listing of calibrated radiocarbon ages.
b
Where there are multiple calibrated ages for an individual
radiocarbon date (see Table 1), this date is represented by the
average of these ages in the determination of the overall calibrated
average age of the lateral accretion deposits in this borehole.
c
The calibrations of the radiocarbon ages 460 F 40 and
680 F 30 14
C years B.P. from this borehole are not included in
the average calibrated age.
d
Based on the average of the intercepts on the calibration curve
from a single radiocarbon date (see Table 1).
e
Where the calibration of a radiocarbon date has produced two
separate ranges of error at 1r (see Table 1), the overall maximum
and minimum of these ranges is used in the determination of the age
range at 1r for the overall calibrated average age for the lateral
accretion deposits in this borehole.
f
The calibration of the radiocarbon age 7850 F 70 14
C years
B.P. from this borehole is not included in this average calibrated
age.
G.R. Brooks / Geomorphology 54 (2003) 197­215204
compare the radiocarbon ages between the boreholes,
the dates were calibrated as mentioned above (Table
1). Where a core contains two or more dates, the
calibrated ages were averaged to attain a representative
age for the lateral accretion deposits in that
borehole (Table 2). All further reference to the age
of the flood plain is made to the calibrated (and
averaged, where applicable) ages of the lateral accretion
deposits in the boreholes, unless otherwise speci-
fied.
Three radiocarbon dates are from the upper half of
the alluvial unit. Two of the three are from core
99RR1A, and the ages decrease toward the top of the
core (Fig. 4). The depth and age of the two dates reveal
that the sedimentation rate decreases toward the top of
the core. The third date, 1190 F 30 14
C years B.P., is
from core 99RR3E and was sampled at a depth of 0.41
m (Fig. 4). It indicates that the distal areas of the flood
plain experience a low rate of sedimentation, which is
consistent with the presence of buried paleosols in
cores 99RR1D, 99RR1E, 99RR3B, 99RR3D and
99RR3E.
6. Lateral channel migration
The calibrated ages of the flood plain increase
sequentially along the two transects, becoming older
from the A to E cores (Fig. 4; Table 2). This ordering
traverses the proximal to distal portions of the flood
plain based on the ridge and swale pattern. The ages
of the two most distal cores (99RR1E and 99RR3E) are
only f 1000 14
C year younger than the final recession
of glacial Lake Agassiz, which occurred at f 8000
14
C years B.P. (f 8900 cal years B.P.; Teller et al.,
1996). This, in combination with the sequential ages
of the cores, indicates that the two study area meanders
have undergone only a single sequence of
expansion, an interpretation that is consistent with
there being only a single pattern of ridge and swale
topography across the flood plain (Fig. 3).
The temporal pattern of the channel migration was
reconstructed by interpolating the positions of the
inner bank at 1000-years-B.P. intervals between the
calibrated age of the lateral channel deposits and using
the pattern of the ridge and swale topography as a
Fig. 5. Isochrones depicting the location of the inner bank of the meanders across the study area at intervals of 1000 years B.P.
G.R. Brooks / Geomorphology 54 (2003) 197­215 205
guide to position isochrones (Fig. 5). The isochrones
reveal that the two meanders have extended outward
and rotated downvalley over time. The spacing of the
isochrones, however, also reveals that the rate of
lateral channel migration was greater prior to about
6000 years B.P. than afterward. This trend is well
illustrated at the upper meander, where the 99RR3
transect of boreholes closely follows the meander
growth across the flood plain, the lateral pattern of
meander growth is simple, and all five boreholes
contain radiocarbon-dated organic matter. At the
downstream meander, no dates were obtained from
borehole 99RR1D, thus, the position of the 6000-,
7000- and 8000-years-B.P. isochrones have been
interpolated from the ages and spacing of boreholes
99RR1E and 99RR1C. Nevertheless, the migration
pattern at both meanders reveals that approximately
half of the flood plain formed between f 6000 and
8900 cal years B.P. (the final recession of glacial Lake
Agassiz), which represents about one-third of the
post-Lake Agassiz time interval.
The average rates of channel migration for the
upstream meander were calculated using ages and
straight-line distances between the 99RR3 transect
boreholes (Table 3 and Fig. 6A). The error ranges
shown for the migration rates are based on the 1r
uncertainty of the calibrated radiocarbon ages for the
lateral accretion deposits (Table 3). All of the rates
along the 99RR3 transect are considered to reasonably
approximate the maximum rate of migration at the
different time intervals because the borehole locations
generally follow the path of maximum meander
growth (or the erosional axis of the meander; see
Hickin, 1974).
As shown in Fig. 6A, the average rates of channel
migration along the upstream meander were initially
0.35 m/year between f 7900 and 7400 cal years B.P.,
then decreased to 0.18 m/year between f 7400 and
6200 cal years B.P. and varied between 0.04 and 0.08
m/year since f 6200 cal years B.P. Significantly, Fig.
6A indicates that the upstream meander has been
migrating laterally at 0.04 m/year after f 900 cal
years B.P. This implies that the river presently is
undergoing long-term lateral migration at a low rate,
despite there being insignificant change in the channel
position between mid-19th and late-20th century maps
or in sequential aerial photographs. Similarly, a low
rate of lateral channel migration occurred post-1000
cal years B.P. along the downstream meander, as
indicated by the average rate of migration between
borehole 99RR1A and the riverbank (Table 3).
The ``apparenť' rates of lateral channel migration
between the boreholes along the 99RR1 transect are
also listed in Table 3. However, except for the rates
between boreholes 99RR1B, 99RR1A and the modern
riverbank, the meander growth is poorly represented
by these rates compared to the upstream meander.
Reasons for this are that the meander growth has
occurred in two phases within a single sequence of
meander expansion (see Hickin, 1978); more imporTable
3
Average rates of lateral channel migration between the 99RR1 and 99RR3 transect boreholes and the contemporary river bank
Locations Distance
(m)
Ages of bank­borehole
or borehole­borehole
(cal years B.P.)a
Difference in
averaged age
(cal years B.P.)
Upper and lower
difference in
age range at 1r
(cal years B.P.)a
Average rate
of channel
migration
(m/year)
Range of
average rate
of channel
migration at
1r (m/year)
Bank to 99RR1A 40 0b
­1010 1010 930­1150 0.04 0.03­0.04
99RR1A to 99RR1B 152 1010­2290 1280 1040­1440 0.12 0.11­0.15
99RR1B to 99RR1C 208 2290­5530 3240 3070­3420 0.06 0.06­0.07
99RR1C to 99RR1E 541 5530­8090 2560 2410­2720 0.21 0.20­0.23
Bank to 99RR3A 38 0b
­920 920 800­940 0.04 0.04­0.05
99RR3A to 99RR3B 127 920­2610 1690 1410­1930 0.08 0.07­0.09
99RR3B to 99RR3C 191 2610­6170 3560 3320­3930 0.05 0.05­0.06
99RR3C to 99RR3D 215 6170­7360 1190 1040­1380 0.18 0.16­0.21
99RR3D to 99RR3E 171 7360­7850 490 310­650 0.35 0.26­0.55
a
The errors at 1r are listed in Table 2.
b
The age of the contemporary river bank is taken as 0 cal B.P. (i.e., A.D. 1950).
G.R. Brooks / Geomorphology 54 (2003) 197­215206
tantly, boreholes 99RR1E, 99RR1D and 99RR1C are
poorly aligned with the path of maximum meander
growth, and borehole 99RR1D lacks radiocarbondated
organic material.
7. Channel incision
The depth of contact between the alluvium and
glaciolacustrine deposits in the boreholes is depicted
in Fig. 7. The heights of the boreholes along each
transect are scaled to the topography of the flood plain
relative to the height of the E boreholes. In Fig. 7, the
depth of alluvium-glaciolacustrine contact varies
between 15.7 and 23.5 and 17.0 and 25.1 m along
the 99RR1 and 99RR3 transects, respectively. At the
two oldest boreholes (99RR1E and 99RR3E), the
depth of contact is between 19 and 20 m; whereas
in the adjacent D and C cores, the contact is up to
several meters shallower (15.7­18.7 m deep). The
depths in the E boreholes indicate that the main phase
of river incision into the clay plain occurred prior to
the lateral migration of the channel past the locations
of the D and C boreholes. If this incision had still been
underway, the contact in the E boreholes would be
distinctly shallower than the D (and possibly the C)
boreholes, with the contacts thereby delineating a
buried slip-off slope. Based on the age of the lateral
accretion deposits in the oldest borehole (99RR1E),
the main phase of channel incision was completed
betweenf 8900 and 8100 cal years B.P. This period
of incision is consistent with chronostratigraphic data
from Winnipeg (Fig. 1B), where wood, buried at 10.7
m depth in alluvium near the mouth of the Assiniboine
River valley, produced a radiocarbon age of
7490 F 80 14
C years B.P. (or f 8300 cal years B.P.;
Nielsen et al., 1993).
Along both transects, the net depth of contact deepens
by 4.4 and 5.6 m between boreholes 99RR1E and
99RR1A and 99RR3E and 99RR3A, respectively
Rate based on average calibrated age
Rate range based on 1 error of the average
calibrated age
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1000 2000 3000 4000 5000 6000 7000 8000
Age (cal YBP)
Rateofmigration(m/yr)
A
B1
2
3
4
5
6
7
8
Rm/w
Fig. 6. (A) Average rates of lateral channel migration between the locations of 99RR3 boreholes and contemporary river bank. The time interval
depicted for each rate is based on the representative calibrated radiocarbon ages for the boreholes (Table 2), while the error zones reflect the 1r
uncertainty in these calibrated ages (see Table 3). (B) Plot of Rm/w ratio at 1000-years-B.P. intervals for the 99RR3 (upstream) meander, based
on the isochrones in Fig. 5.
G.R. Brooks / Geomorphology 54 (2003) 197­215 207
(Table 4). This deepening probably represents the
gradual net incision of the river channel into the clay
plain, since it also corresponds approximately to the
slope of the flood plain (Table 4). However, along both
transects, the depth of contact is up to 3.4 m shallower
within the D and C boreholes than in the E boreholes,
suggesting that perhaps there has been a period of
channel aggradation. However, the lack of backswamp
areas on the distal areas of the flood plain in the study
area (or elsewhere along the river), which would be
indicative of valley bottom aggradation, do not support
this interpretation. The shallowing of the contact may
instead reflect a decrease in the maximum depth of
scouring between boreholes rather than the aggradation
of the channel. Whether this was due to, for example, a
climatically controlled change in the flow regime or the
transitory formation of deep scour holes as the channel
migrated laterally is not known.
The average rates of incision between the E and A
boreholes are 0.62 and 0.81 m/ky for the 99RR1 and
99RR3 transects, respectively, based on the difference
in the depth of contact and the calibrated flood plain
ages (Table 4). These rates are marginally greater
(40­60%) than those rates based on the difference
in the flood-plain topography between the E and A
boreholes (0.42 and 0.49 m/ky for the 99RR1 and
99RR3 transects, respectively; Table 4). The floodplain
topography may indicate that the average rate of
incision was greater in the post-6200­5500-cal-yearsB.P.
interval because the slope of the flood plain is
greater between the C and A than the E to C boreholes
Table 4
Rates of lateral channel incision
Transect Between
boreholes
D
calibrated
age (cal
years)a
D depth
of alluvial/
glaciolacustrine
contact (m)b
D
topography
of flood
plain (m)b
Rate of
incision
(m/ky)
99RR1 E and A 7080 4.4 na 0.62
E and A 7080 na 3.0 0.42
E and C 2560 na 0.5 0.20
C and A 4520 na 2.5 0.55
C and B 3240 na 1.7 0.53
99RR3 E and A 6930 5.6 na 0.81
E and A 6930 na 3.4 0.49
E and C 1680 na 0.3 0.18
C and A 5250 na 3.1 0.59
a
See Table 2.
b
See Fig. 7.
Fig. 7. The vertical positions of the boreholes along the 99RR1 and
99RR3 transects relative to the heights of the ``E'' boreholes. Also
shown is the level of the observed river surface on the day of
surveying. The depths to the alluvium/glaciolacustrine contacts are
based on the core logs. The representative calibrated ages of the
lateral accretion deposits within the boreholes (where available) are
shown below each borehole label.
G.R. Brooks / Geomorphology 54 (2003) 197­215208
along both transects (Table 4). These rates range from
0.55 to 0.59 m/ky (99RR1 and 99RR3 transects,
respectively) between the C to A boreholes, and from
0.20 to 0.18 m/ky for the E to C boreholes (Table 4).
Based on both the difference in depth of contact and
the flood-plain topography, 0.4­0.8 m/ky is likely the
right range of magnitude for the post-8100-cal-yearsB.P.
rate of lateral incision of the Red River channel
into the Lake Agassiz clay plain.
8. Discussion
8.1. Long-term controls on lateral channel migration
The flood plain in the study area represents a
continuous and substantial temporal record of the
fluvial geomorphic activity. The flood-plain chronology
indicates that the rate of lateral channel migration
was faster prior to about 6 ka than afterward (Figs. 5
and 6). The long duration of this trend suggests that
the controlling factor is environmental rather than a
local perturbation, particularly because of a lack of
local candidate mechanisms for the latter.
Changes in the intensity of Holocene fluvial activities
can relate to variations in climate (e.g., Brakenridge,
1980; Knox, 1983, 1995, 2000; Starkel, 1995).
Work by Knox et al. (1981) and Knox (1984, 1985) in
the driftless area of Wisconsin in the Upper Mississippi
Valley (UMV), which is relatively nearby to
southern Manitoba, indicates that rates of fluvial erosion
and deposition varied in response to three general
Holocene climatic phases. These variations are attributed
to long-term fluctuations in the recurrence of large
floods. Notably in the period between f 7500 and
6000 14
C years B.P. (f 8300­6800 cal years B.P.),
Knox's work found that the erosion and deposition of
alluvial sediments was concentrated on the flood
plains and alluvial fans of small tributaries of less
than about 10 km2
. Little evidence of alluviation was
found along the flood plains of larger trunk streams
(Knox et al., 1981). The flood-plain stability of the
trunk streams is attributed to there being a generally
low frequency of larger floods in a climate that was
warmer and drier than present. Between f 6000 and
4500 14
C years B.P. (f 6800­5200 cal years B.P.),
climate was cooler and moister resulting in a higher
frequency of larger floods and, correspondingly, the
larger trunk rivers of the UMV experienced ``intensifieď'
lateral migration (Knox, 1985).
Recent paleolimnological evidence from within or
immediately adjacent to the Red River basin indicates
that the warmer­drier ``hypsithermaĺ' climatic interval
began as early as f 10,700­9500 cal years B.P.,
intensified after f 9200­7900 cal years B.P. and
ended atf 5400­4500 cal years B.P. (e.g., Bradbury
et al., 1993; Laird et al., 1996; Valero-Garcés et al.,
1997). This reveals that the period of higher lateral
channel migration and flood-plain construction in the
study area occurred entirely within a warmer­drier
climate, in contrast to the trunk streams of the UMV.
Also different is the lack of intensification of fluvial
activity along the Red River following the shift to a
cooler­moister climate after f 5400­4500 cal years
B.P. While this nonsynchronicity in fluvial geomorphic
activity and different response to climate may
simply reflect variability between separate watersheds
(see Taylor and Lewin, 1997), consideration of the
geomorphic history of the Red River basin suggests
that another factor(s) may control the temporal rates of
lateral channel migration in the study area.
The factors controlling the rate of lateral migration
(M) can be summarized as (following Hickin and
Nanson, 1984):
M  f x; b; G; h; Yb 1
where x represents unit stream power (a slope-discharge
product), b is a parameter expressing planform
geometry, G is the rate of sediment supply, h is
outer bank (or valley side) height and Yb the erosional
resistance of the outer bank material. Along
the study area, h has been relatively constant over the
pastf 8000 cal years B.P., as revealed by the depths
of alluvium­glaciolacustrine contact in Fig. 7. Particularly
noteworthy are the nearly identical depths of
contact at boreholes 99RR3E and 99RR3B, which are
situated at locations on the flood plain that experienced
markedly different lateral migration rates (Fig. 5). The
depth of scour at A boreholes along both transects are
marginally deeper than elsewhere (Fig. 7), but this
increase in depth does not correspond to a significant
decrease in the lateral migration rate between the
99RR3B and 99RR3A boreholes (Fig. 6). Yb also has
been constant because the valley side materials into
which the channel has laterally eroded are composed
G.R. Brooks / Geomorphology 54 (2003) 197­215 209
of homogenous glaciolacustrine deposits of the clay
plain.
Variations in b have occurred as the meanders
migrated laterally across the valley bottom, but these
changes are deemed an insignificant control of the
lateral migration rates. Expressing b as a ratio of the
radius of meander curvature (Rm) to channel width
(w), Hickin and Nanson (1975, 1984) have shown that
the rate of channel migration can vary significantly
with changes in Rm/w. Along the 99RR3 meander,
however, Rm/w is similar (5.0­5.3) at 8000 and 7000
years B.P. and at 2000 and 1000 years B.P. (Fig. 6B)
when the average rates of migration are relatively high
and low, respectively (Fig. 6B). In addition, the rates
of migration are similar throughout the entire post5000-years-B.P.
interval, despite a decrease in Rm/w
from 6.9­7.2 to 5.3 between 3000 and 2000 years
B.P.
From the context of lateral channel migration, unit
stream power (x) is primarily a function of discharge
( Q) and gradient (s). Based on a modern estimate of
the 2-year flow at Emerson and the average valley
gradient between Emerson and Morris (Fig. 1B), x for
the Red River is about 5.1 W m 2
(Table 5). Over the
past 8 ky, both Q and s have not been constant.
Regarding s, crustal rebound has caused the valley
gradient of the Red River to decrease over the postLake
Agassiz period. As summarized in Table 5, the
Emerson and Morris reach has lost about 24%, 37%,
45% and 50% of s at 8000 years B.P. by 6000, 4000,
2000 and 0 years B.P., respectively (0 B.P. = A.D.
1950). This implies that x has been greater in the past,
perhaps as great as 9.9 W m 2
at 8000 years B.P.
when s was highest, assuming a constant Q (Table 5).
However, this estimate may be high as Q undoubtedly
was lower than present prior to 5400­4500 cal years
B.P. when climate was warmer­drier in the drainage
basin, as summarized above. Nevertheless, a range of
9.9­5.1 W m 2
probably is reasonably representative
of the magnitudes of x since 8000 years B.P. These
magnitudes are low in absolute terms; and variations
within this range are not deemed to have caused
significant change to the rate of fluvial erosion of
the cohesive glaciolacustrine sediments that form the
outer banks, regardless of whether x was higher pre6000
years B.P. than afterward.
The long-term change in sediment supply ( G)
through the study area is not known but can be
assessed qualitatively by consideration of experimental
drainage basin studies on sediment supply and the
scenario that led to the establishment of the Red River
in southern Manitoba. The river became established
during the final northward recession of glacial Lake
Table 5
Temporal change in valley slope and unit stream power, Emerson to
Morris section of the Red River
Age
(years B.P.)
Valley
slopea
Unit stream
power (W/m2
)b
% of unit stream
power at 8000
years B.P.
0 0.000064 5.1 50.8
2000 0.000070 5.5 55.1
4000 0.000080 6.2 62.7
6000 0.000097 7.6 76.2
8000 0.000127 9.9 100.0
a
The valley slope is based on the mean gradient of the
``normaĺ' water surface (about 750 m3
/s) between Emerson and
Morris (236 and 233.3 m asl, respectively, over a valley distance of
41.9 km), using data from Red River Basin Investigation (1951).
The change in gradient was determined by estimating the past
elevations of the start and endpoints of this water surface at 2000year-B.P.
intervals according to the equation:
Et  EpAet=s
 1Lewis et al:; 2000; after Peltier; 1994
and Tackman et al:; 1998
where Et is elevation at time t; Ep is present elevation (m); t is age
when elevation is desired (calendar years B.P.); s is relaxation time
(3500 years); A is site amplitude factor of uplift, defined according
to:
A  Ru=e10503=s
 1 after Lewis et al:; 2000
where Ru is site relative uplift since 10,503 cal years B.P. Relative
uplift for each gradient endpoint was determined using the difference
in present elevation of the Lower Campbell beach of Lake Agassiz
for the Emerson and Morris locations and the southern outlet of the
lake (288.11 m asl; Teller and Thorleifson, 1983). The relative uplift
values are 33.71 and 28.06 m for Morris and Emerson, respectively,
based on Lower Campbell shoreline elevations where Lower
Campbell isobases project through Morris and Emerson (Teller
and Thorleifson, 1983). The date of 10,503 cal years B.P. is the age
of the Lower Campbell beach (Teller et al., 1996). The relaxation rate
of 3500 follows Lewis et al. (2000). Valley slopes are shown to six
significant figures to demonstrate a progressive, albeit slight, loss of
slope occurring since 8000 years B.P.
b
Unit stream power (x) calculated according to x = Qscw 1
,
where Q is discharge [f 2-year flow (600 m3
/s) for Red River at
Emerson station (1883­1885, 1892­1896, 1898­1912­1997; data
from Manitoba Water Resources)], s is valley gradient, c is the
specific weight of clear water (9800 N/m3
) and w is channel width
(75 m).
G.R. Brooks / Geomorphology 54 (2003) 197­215210
Agassiz, which caused a progressive drop in river
base level and allowed the north-flowing ancestral
river system to extend onto and incise the lake bed in
NE North Dakota, NW Minnesota and southern
Manitoba. Studies on sediment supply from experimental
drainage basins described by Schumm (1977)
and Schumm et al. (1987) indicated that the rapid
lowering of base level results in a dramatic increase in
sediment yield rates at the mouth of the drainage
basin. These rates decline rapidly over time, but are
highly variable and eventually stabilize to form a
quasi-equilibrium. Variability in the rates can produce
intermittent secondary peaks in the sediment yield and
is attributed to the storage and periodic flushing of
alluvium from the tributary valleys within the experimental
drainage basins. Overall, however, the net
trend from the experimental work is that a drop in
base level produces sediment yield rates that are
initially high and generally wane rapidly over time.
This sediment yield model is considered generally
applicable to the Red River in southern Manitoba
because the basic process--the response of a fluvial
system to a drop in base level--is similar. Although
not located at the river mouth, the study area is
situated well within the area of lake bottom exposed
during the final recession of the lake. Thus, sediment
transported by the river in response to the drop in base
level would be routed through the study area reach,
and the basic pattern of sediment yield summarized
above should represent the sediment supply at this
specific location. Such a hypothesis is consistent with
the paraglacial sedimentation model described by
Church and Ryder (1972) that generally applies to
areas within or adjacent to formerly glaciated areas.
This model characterizes fluvial sedimentation rates
as being high during and immediately following a
glaciation because of the fluvial reworking of glacial
drift in the landscape. These enhanced rates decrease
over time as the supply of sediment that is readily
available to the fluvial system wanes but may last for
several thousand years (Church and Ryder, 1972). In
the Red River case, the high sedimentation rates
reflect the former presence of a large glacial lake
per se rather than the direct presence of glacial ice.
However, the existence and ultimately the demise of
Lake Agassiz was controlled by the Laurentide Ice
Sheet which represents a fundamental connection to
glaciation.
Flume experiments have demonstrated that the rate
of lateral migration of a meandering channel can be
increased by augmenting the sediment supply into the
flume (Hickin, 1988). Based on the experimental
sedimentation model and the hypothesized pattern of
sediment yield, an initial period of high lateral migration
rates should occur that wanes and eventually
stabilizes. This indeed is the general trend in the
lateral migration rates observed in Fig. 6A. The
long-term change in sediment supply thus provides a
reasonable explanation for the early phase of high
lateral channel migration and flood-plain construction.
An obvious question, however, is how an increased
sediment supply specifically enhances lateral accretion
along a mud-dominated river like the Red River
as opposed to sand- or gravel-bed rivers. Whether or
not the river experienced a complex response during
the pre-6000-years-B.P. period that resulted in episodes
of aggradation and degradation within the river
valley, as described by Schumm (1977), is not apparent
from the flood-plain cores or the basic morphology
of the valley bottom.
8.2. Relevance of valley widening to the flood hazard
The radiocarbon ages from the flood-plain cores
demonstrate that outward extension and downvalley
rotation of the meanders has gradually widened the
valley (Fig. 5). To assess the significance of this
widening to the modern flood hazard on the adjacent
clay plain, the cross-sectional area of the valley
bottom at 1000 years B.P. was estimated and compared
to that of the modern valley. The valley crosssections
were measured across the apex of both valley
meanders. The modern valley profile is based on a
1951 map of the alluvial valley where relief is
depicted at a 5-ft contour line interval (Red River
Basin Investigation, 1951). The valley profile at 1000
years B.P. was estimated by rescaling the modern
profile of both meanders to fit the position of the
inner bank as defined by the 1000-years-B.P. isochrone.
The modern channel shape was used in both
sets of profiles; thus, the variation in area occurs over
the flood-plain portion of the valley bottom. The rate
of incision of the channel has been ignored, as this is
low (see above).
The data reveal that the valley cross-section
increased by about 2% and 0.7% at the upstream
G.R. Brooks / Geomorphology 54 (2003) 197­215 211
and downstream meanders, respectively, since 1000
years B.P. (Table 6). In absolute terms, this represents
the addition of about 52 and 19 m2
to the valley crosssections
(Table 6). The limited increase reflects the
general low rate of lateral channel migration along
both meanders, but more importantly, the very shallow
depth of the flood-plain portion of the valley
bottom relative to the clay plain. Hence, only a small
amount of area was added to the valley cross-section
since 1000 years B.P. as the river meanders widened
the valley bottom. The valley widening produced a
decrease in hydraulic radius--the ratio of the crosssectional
area to wetted perimeter of the profiles and
a common hydraulic morphological parameter--of
2.64­2.56 and 2.01­2.00 m at the upstream and
downstream meanders, respectively, between 1000
and present (Table 6). These decreases represent a
change in hydraulic radius of about 3% and 0.5%
(Table 6).
The section of river valley within the study area
does not control the water surface of valley-full flows.
Nevertheless, the low rates of lateral channel migration
and incision along the study area meanders are
considered to be typical of meanders along the Red
River elsewhere in southern Manitoba. Thus, since
1000 years B.P., the river meanders have experienced
only a slight change in valley cross-section and hence
to the morphological parameter (i.e., hydraulic radius)
affecting the discharge capacity of the valley. When
considered proportionally over timescales of up to
several centuries--timescales that are directly relevant
to the modern flooding problem--the amount of
widening of the valley cross-section is very low to
negligible and the resulting change in total valley
conveyance would be undoubtedly within the error
of discharge measurement. Overall, therefore, valley
widening is deemed an insignificant factor affecting
the modern flood hazard on the clay plain, particularly
compared to other variables such as, for example,
climatic fluctuations (see Ashmore and Church, 2001;
St. George and Nielsen, 2002) and changes in valley
bottom roughness from agricultural land clearance.
9. Summary and conclusions
The radiocarbon age chronology of lateral accretion
deposits at the study area span f 8100 cal years
B.P. and reveals that the flood plain is the product of a
single sequence of meander expansion. The general
rate of channel migration has decreased over time and
about half of the flood plain formed in the initial third of
the post-Agassiz timescale. At the upstream meander,
the average rates of channel migration were initially
0.35 m/year between f 7900 and 7400 cal years B.P.,
then decreased to 0.18 m/year between f 7400 and
6200 cal years B.P., and varied between 0.04 and 0.08
m/year since f 6200 cal years B.P. Both meanders
in the study area have experienced about 0.04 m/
year of lateral channel migration since f 1000 years
B.P.
The interval experiencing the most rapid lateral
channel migration coincides with the warmest­driest
period of the Holocene in the eastern Prairies. The
higher phase of Red River alluviation is interpreted to
reflect an enhanced sediment supply to the study area
arising from the development of the river system on
the clay plain following the final recession of glacial
Lake Agassiz from NE North Dakota, NW Minnesota
and southern Manitoba. Other meandering river sysTable
6
Change in valley cross sectional area and hydraulic radius along the two study area meanders between 0 and 1000 years B.P.
Meander
(transect)
Time
(years B.P.)
Valley cross-
sectional
area (m2
)a
% of valley
cross-sectional
area at 1000
years B.P.
Wetted
perimeter
(m)a
Hydraulic
radius (m)b
% of hydraulic
radius at 1000
years B.P.
Upstream 0 2558 102 1000 2.56 103
(99RR3) 1000 2506 100 950 2.64 100
Downstream 0 2638 100.7 1321 2.00 100.5
(99RR1) 1000 2619 100 1300 2.01 100
a
Measured with a Ushikata X-PLAN360d planimeter.
b
Hydraulic radius = ratio of cross-section area to wetted perimeter.
G.R. Brooks / Geomorphology 54 (2003) 197­215212
tems also may have experienced a general decline in
the long-term rates of lateral channel migration in
response to waning post-glacial sediment supply, but
such declines could have gone unrecognized because
of a lack of sufficient dating control of the flood-plain
deposits, the lack of preserved (or very fragmentary)
older areas of the flood plain and/or the greater
relative importance of other variables controlling
channel migration.
The main phase of incision by the Red River into
the Lake Agassiz clay plain occurred between f 8900
and 8100 cal years B.P. Since f 8000 cal years B.P.,
gradual incision of the river has occurred at a rate
estimated to be between 0.4 and 0.8 m/ky.
The lateral migration of the river meanders is
causing the gradual widening of the alluvial river
valley. The resulting increase in valley cross-sectional
area since 1000 years B.P. is about 2% and 0.7% at the
two study area meanders, respectively. The change in
cross-section translates into a decrease in hydraulic
radius of about 3% and 0.5% at the two meanders.
When considered proportionally over timescales of up
to several centuries, the widening of the valley crosssection
is very low to negligible and is deemed an
insignificant factor affecting the modern flood hazard
on the clay plain, especially compared to other variables
(e.g., climate fluctuations and changes in valley
bottom roughness from land clearance).
This study demonstrates that a low-energy, muddominated
river can experience progressive, albeit
slow, lateral channel migration--the basic evolution
of which is consistent with other meandering river
types. However, for the geomorphic behaviour to be
fully displayed, the development of the flood plain
needs to be examined over a timescale of relatively
long duration (a ``geomorphic'' timescale of Hickin,
1983), which in the case of this study spans thousands
of years.
When comparing the Red River alluvial history to
other temperate, mid-latitude river systems, the late
Quaternary history and geomorphic setting of the
various systems needs to be considered carefully. In
the case of the Red River in southern Manitoba, the
mud-dominated river was established after the final
recession of a large glacial lake; it developed on a flat,
gently sloped clay plain that was the former lake bed,
and its past discharge regime lacks a glacially influenced
or glacial-lake-influenced stage.
Acknowledgements
Financial support for this project was provided by
the Red River Valley Flood Protection Program. I
thank M. Lewis for guidance with the crustal rebound
data, B. Medioli for assistance in coring and core
logging, M. Nixon for advice and assistance in coring,
E. Nielsen for logistical support and O. Lian for
assistance in core logging. Comments by M. Lewis,
B. Medioli, E. Nielsen, T. Hickin, S. Wolfe and R.
Marston (journal editor) on drafts of the paper are
sincerely appreciated. This paper represents Geological
Survey of Canada contribution number
2002026.
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