Original Article
Landslides
DOI 10.1007/s10346-006-0039-y
Received: 21 December 2005
Accepted: 13 March 2006
© Springer-Verlag 2006
Marten Geertsema . John J. Clague
1,000-year record of landslide dams at Halden Creek,
northeastern British Columbia
Abstract Large, rapid, low-gradient landslides are common in
clay-rich glacial sediments in northeastern British Columbia. Many
of the landslides create upstream impoundments that may persist
for years in small watersheds in the region. We have documented
such events in the Halden Creek watershed, 60 km southeast of
Fort Nelson. The events are recorded geologically in two ways.
First, trees are drowned in lakes dammed by the landslides and
subsequently buried by deltaic sediments, where they are protected
from decay. Bank erosion later exhumes the drowned trees.
Second, landslide deposits with entrained wood are exposed along
stream banks. We have reconstructed the recent history of
landslide damming at Halden Creek by performing radiocarbon
dating on exhumed trees and wood in and beneath landslide
deposits at 13 sites in the watershed. Drowned trees range in age
from 169±59 to 274±49 14C year BP. Wood in and below landslide
deposits yielded radiocarbon ages ranging from modern to 965±49
14
C year BP.
Keywords Landslide dam . Drowned forest . Landslide .
Radiocarbon dating . Canada . British Columbia . Halden
Creek . Fort Nelson
Introduction
Large, low-gradient landslides are common in clay-rich glacial
sediments in northeastern British Columbia (BC), Canada. They
commonly dam small streams, forming lakes that may persist for
years (Geertsema et al. 2006). These landslides are members of a
suite of slope failures that block drainage and impound upstream
lakes in mountainous areas throughout the world (Swanson et al.
1986; Costa and Schuster 1988, 1991; Clague and Evans 1994; Hejun
et al. 1998; Casagli and Ermini 1999; Korup 2002, 2004, 2005).
Halden Creek (Fig. 1) in northeastern BC is an example of a
stream subject to recurrent landslide damming. In this paper, we
document a recent landslide dam in the Halden Creek watershed
and examine geological evidence for past landslides and landslide
dams in the basin.
Geological and geomorphological setting
The Halden Creek watershed (156 km2) is located on the Alberta
Plateau (Holland 1976), 60 km southeast of Fort Nelson, within the
Black and White Spruce biogeoclimatic zone (Meidinger and Pojar
1991) and the zone of discontinuous permafrost (Brown 1973).
Bedrock comprises a flat-lying to gently dipping Cretaceous clastic
sequence including, from youngest to oldest, Dunvegan sandstone,
Sully marine shale, and Sikanni sandstone (MacIntyre et al. 1998).
The Dunvegan sandstone underlies an old plantation surface and
caps remnant steep-walled mesas and buttes, bordered by steep
slopes (Edwards and Scafe 1996). The mesas have arcuate
embayments formed by landslides and perhaps glacial erosion.
The mesas are covered by till deposited by the former
Laurentide Ice Sheet, with distinctive crystalline erratics derived
from the Canadian Shield hundreds of kilometers to the northeast
(Mathews 1980). Most of the surface glacial sediments in the
Halden Creek watershed have been displaced by earth slides and
flows and are therefore designated as colluvium (Fig. 2).
Halden Creek drains to the east from a bedrock-walled
amphitheater into Tenaka Creek, a tributary of Prophet River.
The waters eventually reach Beaufort Sea via Mackenzie River.
Materials and methods
The study involved examining a recent landslide, its dam and lake,
as well as evidence of older landslides and landslide-dammed
lakes. Landslides and terrain types were mapped from aerial
photographs and through helicopter overview flights. They were
verified through field examination of exposures. Landslides were
classified according to the system of Cruden and Varnes (1996).
The extent and volume of the most recent landslide that dammed
Halden Creek in 1996 (Fig. 3) were estimated from digital elevation
models of pre- and post-failure surfaces obtained from Province of
British Columbia aerial photographs (30BC79176: 122–124 and
30BCC97102: 97–98, respectively). Two sediment samples were
collected and analyzed for particle size, Atterberg limits, elemental
chemistry, and clay mineralogy using standard techniques. Crosssectional discs were collected from tree stems in landslide debris
and drowned trees in growth position. Samples taken from the
discs were AMS radiocarbon dated at the NSF Arizona AMS
Facility at the University of Arizona. Radiocarbon ages were
calibrated using the program OxCal version 3.10 (Bronk Ramsey
2005). Ages obtained from outer rings of trees killed by the same
event were combined statistically. Radiocarbon ages on different
sets of annual rings from the same tree were wiggle-matched
(Bronk Ramsey et al. 2001) to more tightly constrain kill dates of
the trees. Ages of inner and outer wood from single trees, as well as
from different trees of the same cohort, were also compared
statistically.
Landslides
The steep mesa walls that border the basin consist of competent
Dunvegan sandstone overlying deformable Sully shale. These
slopes have been sites of rock fall, rotational bedrock landslides,
and translational rockslides, many of which have low gradients.
Slopes below the mesa walls are completely covered by landslide
deposits. In at least one instance, a bedrock landslide triggered an
earth flow in unconsolidated sediment. Lateral ridges that extend
Landslides
Original Article
Fig. 3 Orthophotograph of lower Halden Creek watershed, showing landslide scarps,
exhumed drowned forests, dated landslide deposits, and the 1996 landslide dam and
lake. The lettered sites are those in Table 3
till plain, and bedrock is exposed in places along the stream. All
slopes along this reach of the stream have been affected by
landslides of different ages. Most of the landslides are wide,
translational slides and spreads with transverse horst-and-graben
morphology and minor evidence of true flow. Smaller earth slides
(Fig. 4) and flows occur frequently as reactivations of parts of
larger landslides.
Fig. 1 Shaded relief digital elevation model (DEM) of the region around Halden
Creek, 60 km southeast of Fort Nelson. The Halden Creek watershed is outlined in
white
downslope from a displaced bedrock ridge mark the lateral
margins of this earth flow. Similar earth flows triggered by
rockslides are common elsewhere in northeastern BC (Geertsema
et al. 2006).
The landslides that formed the dams discussed in this paper
happened on a till plain in the lower part of the watershed (Fig. 3).
The till is a cohesive diamicton, approximately 10 m thick,
overlying flat-lying shale bedrock. Halden Creek is incised into the
Fig. 2 Shaded relief DEM of Halden Creek watershed, showing locations of study
sites (see Fig. 3 for site numbers). The site outside the watershed boundary
(dashed line) is a landslide on Tenaka Creek
Landslides
Fig. 4 a Earth slide on a gently sloping surface underlain by till and incised by
Halden Creek. The landslide occurred sometime between July 22, 1967 and August
16, 1993. Reaction wood in conifers suggests the slide occurred in 1985. Halden
Creek flows from right to left; view to the southwest. b Recent reactivation of part
of a prehistoric landslide deposit at sites 14 and 15 (Fig. 8)
Fig. 6 Geologic section of the 1996 landslide, showing diamicton (till and
colluvium) overlying horizontally bedded shale. The movement was mainly
translational. Note the vertical exaggeration in the sketch
deposits cover 47 ha and have a volume of more than 3 million
cubic meters. Shale is exposed along Halden Creek below the
landslide deposit.
Fig. 5 a Photographic stereopair showing the 1996 landslide and impoundment
(Province of British Columbia aerial photographs 30BCC97102: 97–98). b Drowned
forest upstream of the 1996 landslide (September 2000). The shoreline of the lake
is marked by a dashed blue line. Arrows indicate direction of movement of the
landslide that dammed the creek
The most recent large landslide (Fig. 5) occurred some time
between August 16, 1993, which is the date of a LANDSAT 7 image
on which the slide is absent, and August 6, 1997, the date of an
aerial photograph that shows the slide. Tree-ring dating suggests
that the failure occurred after the 1996 growing season. Precipitation at the nearest climate station (Fort Nelson) was above
average in 1996 (Egginton 2005), perhaps contributing to the
instability of the slope. Movement was translational along a nearly
horizontal rupture surface, accompanied by spreading (Fig. 6). The
landslide occurred in clayey till with less than 15% stones. Its
Material properties of the 1996 landslide
Representative sediment samples collected from exposures of the
1996 landslide deposit along Halden Creek were analyzed for
physical, chemical, and mineralogical properties (Tables 1 and 2).
The diamicton consists mainly of clayey silt with about 20% sand.
The sediment plots in the CL range of the standard plasticity
diagram, indicating that it is a low-plasticity inorganic clay. It has a
low activity (less than 0.65) and relatively high salinity and pH,
perhaps due to its derivation from marine shale. The clay-size
fraction has abundant smectite, illite, and kaolinite, and a
moderate amount of quartz (Table 2). One of the samples contains
traces of chlorite. The silt-size fraction contains abundant quartz
and common feldspar. Plagioclase is more abundant than
orthoclase. These properties are similar to those of other translational landslide deposits in northeastern BC (Geertsema et al.
2006).
Landslide dams and lakes
Clague and Evans (1994) recognized four types of dam-forming
landslides in the Canadian Cordillera on the basis of source
material: (1) bedrock failures, (2) failures in dissected Quaternary
fills, (3) failures of thin sediments overlying bedrock, and (4)
Table 1 Physical and chemical properties of sediment associated with the 1996 Halden Creek landslide
WLa
36.6
40.5
Sample
Halden
Halden
WPa
17.5
19.8
I Pa
19.1
20.7
Activitya
0.63
0.65
Sand (%)
22.0
21.0
Silt (%)
47.5
47.0
Clay (%)
30.5
32.0
Salinity (g/l)
1.63
1.40
pH
8.2
7.9
a
Atterberg limits
Analyses done by J.K. Torrance, Carleton University, Ottawa
Table 2 Clay mineralogy of sediment associated with the 1996 Halden Creek landslide
Sample
Halden 1
Halden 1
Halden 2
Halden 2
clay
silt
clay
silt
Smectite
Abundant
Illite
Abundant
Abundant
Abundant
Chlorite
Trace
Kaolinite
Abundant
Abundant
Quartz
Common
Abundant
Common
Abundant
Feldspar
Common
Common
X-ray diffraction was performed by J.K. Torrance, Carleton University, Ottawa
Landslides
Original Article
Fig. 7 Sketches of stratigraphy at site 11 (see Fig. 4a for location), showing
radiocarbon ages and sample numbers. Note fluvial sand and silt separating
modern and prehistoric landslide debris
from failure of thick Quaternary sediments in northeastern BC and
adjacent Alberta have been described by Cruden et al. (1993, 1997),
Evans et al. (1996), Lu et al. (1999), Fletcher et al. (2002), Miller and
Cruden (2002), and Geertsema et al. (2006).
The Halden Creek landslide dams fall within the type II
category of Costa and Schuster (1988)—they span the entire valley
floor and commonly extend onto the opposite slope. The longevity
of such dams depends on the rate of inflow to the impoundment
and the size, shape, and geotechnical properties of the dam itself
(Costa and Schuster 1988). Geertsema et al. (2006) found that many
landslide dams consisting of cohesive diamicton and impounding
small streams in northeastern BC are stable. Although the dams are
incised, the lakes may remain in place for decades, likely due to
armoring of overflow channels by clasts in landslide debris.
The 1996 landslide dam is substantially incised, but it still
impounds a lake. The lake was originally 1,100 m long and had a
surface area of 13.5 ha. About 3.8 m of deltaic sediment had
accumulated on the pre-dam floodplain when we visited the site in
September 2000. The original high-water level, evident from
detritus in trees, was an additional 7 m above the delta surface,
indicating an original lake depth of about 11 m. Projecting this
water level to the dam gives an original overflow height of 13–15 m.
failures of mine tailings. The Halden Creek landslides belong to the
second group, as they involve relatively thick Quaternary
sediments incised by Halden Creek. Other landslides resulting
Fig. 8 Stratigraphy at sites 14 and 15 (see Fig. 4b for location), where a recent,
small landslide has occurred within an older, larger deposit. The graph of calendric
ages shows a wiggle match (black area) for four outer ring and one pith
radiocarbon ages (area delineated by line)
Landslides
Fig. 9 Landslide debris at site 12 overlies fluvial sand, silt, and pebble gravel.
Radiocarbon ages were obtained on two roots in the fluvial sediment and on the
pith and outer rings of the cottonwood (Populus balsamifera var. tricocarpa) stem
projecting from the stream bank. The graph illustrates the increased precision
obtained by from wiggle matching the calibrated inner and outer ring ages
(black area) compared to the calibrated outer wood age alone (area delineated by
line)
Table 3 Radiocarbon ages of wood in landslide debris
Site
5
5
5
6
7
Sample, laboratory
number
1, AA40525
2, AA40526
3, AA40527
1, AA40529
1, AA40530
2, AA40531
6,7
Radiocarbon age
(14 C year BP)
112±34
Weighted mean radiocarbon
age (year BP)
(32%) 1670–1780
AD
(64%) 1800–1940
AD
(27%) 1640–700
205±34
121±39
175±39
AD
(18%) 1920–1960
AD
(36%) 1670–1780
AD
(59%) 1790–1950
AD
231±28
AD
(37%) 1620–1700
AD
(39%) 1720–1820
AD
(13%) 1920–1960
AD
(78%) 1650–1890
AD
(18%) 1910–1960
AD
(9%) 1520–1590
233±39
AD
(39%) 1620–1690
AD
(37%) 1720–1810
AD
(12%) 1920–1960
AD
(47%) 1630–1690
AD
(28%) 1730–1810
AD
(11%) 1930–1960
AD
8
1, AA40532
139±37
(95%) 1660–1950
AD
11
1, AA40537
post-bomb
(95%) 1950–1955
AD
11
2, AA40538
231±43
(10%) 1510–1600
AD
(36%) 1610–1700
AD
(37%) 1720–1820
AD
(12%) 1910–1960
AD
11
3, AA40539
post-bomb
(95%) 1950–1955
AD
11
4, AA40540
post-bomb
(95%) 1950–1955
AD
11
5, AA40541
184±51
(95%) 1640–1960
AD
11
6, AA40542
381±43
(95%) 1440–1640
AD
11
7, AA40543
1,251±51
(95%) 660–890
Kill date from wiggle matching
2 sigma (95.4%)
AD
(50%) 1720–1820
(6%) 1520–1580
229±39
6-1 and 7-2
Calendar age range
AD
Landslides
Original Article
Table 3 (continued)
Site
11
Sample, laboratory
number
8, AA40544
12
1, AA40545
1 pith, AA46360
Radiocarbon age
(14 C year BP)
post-bomb
Weighted mean radiocarbon
age (year BP)
Calendar age range
(95%) 1950–1955
AD
644±46
(95%) 1270–1410
AD
724±52
(79%) 1200–1330
AD
(16%) 1340–1400
AD
12
Wiggle, 103 rings
644±−46
14
1, AA40549
867±41
(95%) 1040–1260
14
1 pith, AA46361
965±52
(95%) 980–1210
14
2, AA40550
922±49
(95%) 1020–1220
AD
15
1, AA42070
929±47
(95%) 1020–1210
AD
15
3, AA42071
938±59
(95%) 990–1220
14, 15
14-1, 14-2, 15-1, 15-3
908±24
14-1I 14,15 Wiggle, 95 rings
1, AA40518
3757±42
Tenaka
1 pith, AA46362
3660±57
Tenaka
Wiggle 120 rings
Inversion
3757±42
The dam is about 900 m wide and has a volume of about 1 million
cubic meters, much less than the more than 3 million cubic meters
of material involved in the landslide, most of which remained on
the slope. The size of the contributing catchment is 120 km2.
Precise dimensions of prehistoric large landslide dams on
Halden Creek are unknown, but the widths of the dams (width =
distance along long axis of stream) can be determined from the
widths of the landslides. Large dams were long-lived, as were the
lakes they impounded. Deltaic sediments deposited in these lakes
partially bury drowned forests that grew on the pre-slide floodplain. In contrast, small landslides produced much smaller dams,
which were probably short-lived. Although trees were likely killed
by inundation, they are unlikely to have been preserved by burial
in deltaic sediment.
Landslide and dam ages
Large and small landslides dam Halden Creek. Trees and other
organic debris are preserved in and under the deposits of both
large and small landslides and are visible in exposures along
Halden Creek. Only the large landslides produce long-lived lakes in
which deltas are formed. Over time, the drowned trees disappear,
by decay and stand-destroying wildfires with an average return
period of about 100 years (British Columbia Ministry of Forests
1995). Thus, only the buried portions of the trees are preserved.
These buried remnants may later be exposed by bank erosion. Two
types of datable landslide records are thus preserved in the study
Landslides
(95%) 1365–1410
AD
(95%) 1080–1220
AD
(16%) 2180–2120
BC
(79%) 2110–1970
BC
AD
AD
AD
(95%) 1030–1210
AD
908±24
Tenaka
Kill date from wiggle matching
2 sigma (95.4%)
(95%) 2290–2030
BC
(95%) 2200–1890
BC
area: (1) plant debris within and under landslide deposits and (2)
remnants of rooted drowned trees upstream of landslide dams.
Both records are amenable to radiocarbon and tree-ring dating.
Landslide record
No portion of the studied section of Halden Creek has escaped
landsliding (Fig. 3). Therefore, new landslides involve the reactivation of older landslide deposits. Seven exposures of landslide
debris with entrained or subjacent wood were found and examined
along Halden Creek, and two exposures were examined along
Tenaka Creek just upstream of its confluence with Halden Creek
(Fig. 2). The landslide deposits at these sites consist of massive
clayey diamicton with low percentages of clasts. They overlie
fluvial sand and gravel or bedrock. Logs, oriented in the direction
of movement, protrude from the base of the landslide deposits, just
above the basal shear surfaces (Figs. 7, 8, and 9). More than one
layer of logs were found in landslide debris at three sites (sites 5, 7,
and 11).
Twenty-two radiocarbon ages were obtained on buried wood
within and below the landslide deposits from the nine exposures.
Some of the radiocarbon ages pertain to smaller landslides that
have reactivated within a much larger landslide (sites 6, 7, and 8;
Fig. 3). Correlative landslide deposits are exposed at sites 6 and 7;
two logs from these sites yielded a weighted mean radiocarbon age
of 231±28 14C year BP (Table 3).
Table 4 Inferred dates of drowning of forest
Site
1
Sample/laboratory
number
1, AA4021
Radiocarbon age
(14 C year BP)
258±35
Weighted mean radiocarbon
age (14 C year BP)
Calendar age range
(76%) 1510–1680
AD
(15%) 1760–1800
AD
(4%) 1930–1960
5a
1, AA40528
1, 5a
AD
(84%) 1460–1680
274±9
263±28
AD
(9%) 1720–1810
AD
(3%) 1930–1960
AD
(31%) 1520–1600
AD
(54%) 1610–1670
AD
(10%) 1780–1800
AD
(1%) 1940–1960
AD
(72%) 1640–1670
AD
6-1,
(22%) 1780–1800
AD
7-2
(1%) 1940–1960
1, 5a
247±20
2, AA40533
169±59
(95%) 1640–1960
10
1 pith, A40534
226±46
(9%) 1510–1600
1-2, AA40535
210±43
AD
AD
(32%) 1610–1700
AD
(38%) 1720–1820
AD
AD
(13%) 1910–1960
10
(1%) 1520–1550
Combined from Tables 3
and 4
AD
8
(2%) 1830–1880
Kill date from wiggle
matching
AD
AD
(28%) 1630–1700
AD
(45%) 1720–1820
AD
(4%) 1830–1880 AD
(16%) 1910–1960 AD
10
3 pith, AA40536
268±43
(82%) 1480–1680
AD
(10%) 1760–1810
AD
(3%) 1930–1960
AD
10
2 pith, AA46359
201±50
(95%) 1630–1960
AD
13
1, AA40548
196±43
(71%) 1640–1820
AD
(7%) 1830–1890
AD
(17%) 1910–1960
AD
Landslides
Original Article
Table 4 (continued)
Site
Sample/laboratory
number
10, 13
10
pith, 1,2,3
Radiocarbon age
(14 C year BP)
Weighted mean radiocarbon
age (14 C year BP)
203±30
235±27
Calendar age range
(27%) 1640–1690
AD
(51%) 1720–1810
AD
(18%) 1920–1960
AD
(53%) 1630–1690
AD
(34%) 1760–1810
AD
(9%) 1930–1960
10, 13
Wiggle, 157 rings
203±30
Sites 5, 7, and 11 contain woody debris at several levels within
and below landslide deposits. Multiple layers of logs in landslide
debris do not necessarily indicate multiple events. Radiocarbon
ages from site 5 are inverted, but they overlap statistically. Thus, the
logs may be of similar age or they may represent reworked
material. The ages at site 7 are in proper order, but they too overlap
statistically. Site 11 (Figs. 4a and 7), however, does have landslide
units of different ages. Based on aerial photographs and a satellite
image, the most recent slide occurred between 1967 and 1983. Four
of the five uppermost radiocarbon samples yielded modern (postbomb) ages, consistent with a post-1967 landslide (Fig. 7). A root at
the base of a sand–silt unit underlying the landslide deposit also
yielded a modern date. However, a twig in a 6-cm thick peat bed
near the top of a sand–silt unit gave a radiocarbon age of 381±43 14C
year BP, which is significantly older than ages from two lower wood
samples. This inversion can be explained by fluvial erosion,
transport, and deposition of the twig. A log underlying a second,
lower landslide deposit gave a radiocarbon age of 231±43 14C year
BP, and a twig from near the base of an underlying silty clay unit
yielded an age of 184±51 14C year BP. Although inverted, these two
ages are statistically equivalent. The oldest age in the sequence at
site 11 came from the lowest sample. Twigs and roots from a 2-cm
thick peat bed between fluvial gravel units gave an age of 1,251±51
14
C year BP.
Precise dating of the Halden Creek landslides is challenging,
given the radiocarbon age inversions, statistical overlap in calendar
dates, and the possibility of reworking of older material. Greater
precision can be obtained by combining radiocarbon ages, where
there is reasonable certainty of a common kill date, and by wiggle
matching. The weighted means of radiocarbon ages of the outer
rings of logs at sites 6 and 7 and at sites 14 and 15 were calculated
(Fig. 8; Table 4) to reduce the age uncertainty. The combined kill
date at sites 14 and 15 was then wiggle-matched with a pith date for
an even tighter constraint. Wiggle matching also reduced the age
uncertainty at site 12 (Fig. 9) and at Tenaka Creek (Fig. 2).
At least eight landslides are indicated by the radiocarbon ages.
All of the dated landslide deposits along Halden Creek yielded
calendar ages within the last millennium (Table 3). The landslide
Landslides
Kill date from wiggle
matching
AD
(40%) 1640–1680
AD
(43%) 1760–1800
AD
(12%) 1930–1960
AD
deposit at Tenaka Creek, however, gave a much older age,
approximately 4,000 calendar years BP.
Landslide dam record
Five exposures of deltaic sediments deposited in landslidedammed lakes were found along lower Halden Creek (Figs. 2
and 3). Rooted trees truncated at or near the modern ground
surface were preserved in the sediments and were later exposed by
bank erosion (Figs. 5 and 10). Figures 11 and 12 show remnants of
drowned forests exposed in stream banks composed of silt and
sand at sites 1 and 13, respectively. The fossil trees at site 1 are
truncated below the sediment surface, whereas those at site 13
protrude above the surface. The difference may relate to the
younger age of the forest at site 13 (Table 4). Alternatively, an upper
sediment unit may have been removed by erosion at that site.
Nine radiocarbon ages were obtained on outer and pith rings of
drowned trees at five sites. As in the case of the landslide ages,
greater precision was achieved by combining radiocarbon ages and
by wiggle-matching ages on inner and outer rings (Table 4).
Fig. 10 Deltaic sediments partially burying drowned forest in the 1996 landslidedammed lake (September 2000). About 3.8 m of sediment had accumulated above
the pre-slide floodplain when this photograph was taken
Fig. 11 a Stratigraphy and photograph of an exhumed forest in deltaic
sediment deposited in a former landslide-dammed lake (site 1). b Graph
showing calibrated radiocarbon age of
drowned tree at site 1. c Combined
calibrated ages of outer rings of
drowned trees at sites 1 and 5a. d
Increased precision is achieved by
combining the ages of drowned trees
at sites 1 and 5a, with ages of wood in
the landslide deposit at sites 6 and 7
Kill dates of the drowned trees are less than 300 14C year BP
(Table 4) and, thus, span only the most recent third of the dated
Halden Creek landslide period. The radiocarbon ages and locations
of the samples suggest that trees at four of the five sites constitute
two populations. Based on proximity and geomorphic setting, trees
at sites 1 and 5a form one group, and those at sites 10 and 13 form a
second group. The 1996 landslide-dammed lake, which is more
than 1 km long, is a modern analogue for large prehistoric lakes in
the basin. This analogue suggests that the grouped sites are well
within the limits of possible impoundments.
Only one dated landslide possibly corresponds to a drowned
forest, suggesting that our event record is incomplete. The ages on
the landslide at sites 6 and 7 are statistically equivalent to those of
the upstream, drowned forests at sites 1 and 5a (Fig. 3). Assuming
Fig. 12 a Stratigraphy and photograph of exposure at site 13. A
drowned tree extends above the
modern ground surface. b Graph
showing calibrated radiocarbon age of
drowned tree at site 13. c Combined
calibrated ages of outer rings of
drowned trees at sites 10 and 13. d
Graph showing a wiggle match of the
three combined pith ages at site 10
and the combined outer wood age
shown in c (Table 4)
Landslides
Original Article
Fig. 13 Halden Creek landscape evolution model. a. Pre-slide condition
with stream bordered by floodplain
forest. b. Landslide blocks stream and
drowns the floodplain forest. Deltaic
sedimentation begins, partially burying
the drowned forest. c,d Progressive
incision of landslide dam; continuing
deltaic sedimentation. e Exposed portion of buried forest either decays or is
burned. f Halden Creek migrates,
exhuming drowned forest
the four radiocarbon ages at the four sites record the same event, we
obtain a calibrated age range of 1640–1670 AD (72% probability) for
the landslide (Table 4). This 30-year interval is much shorter than
the window of 320 years (1640–1960 AD) provided by the single
sample at site 8.
Discussion and conclusions
Wood within and beneath landslide deposits is commonly used to
date mass movements (e.g., Lang et al. 1999). However, to our
knowledge, our study is the first to use drowned trees preserved in
deltaic sediments of landslide-dammed lakes to date these events.
The Halden Creek watershed is subject to frequent landslides
and upstream inundation due to blockage of the drainage. The
landslides are translational, moving on nearly horizontal surfaces
and involving spreading. Their widths are typically 300 to 1,000 m,
and the volumes of the larger failures range from 1 to 5 million
cubic meters. These large landslides produce relatively long-lived
dams. Smaller slides and flows occur due to reactivation of older,
larger landslide deposits and may expose sediments left by the
older events. The smaller landslides are less than 100 m wide and
involve 10,000–50,000 m3 of sediment. They are too small to
produce long-lived dams.
A model of basin evolution involving landsliding, lake development, delta formation, and subsequent incision is illustrated in
Fig. 13. A stream flowing along a forested floodplain is dammed by
Landslides
a large landslide. The resulting lake drowns the forest, and deltaic
sediments begin to accumulate in the lake. Over time, the lake
lowers as the dam is partially incised. Exposed portions of trees
disappear through decay and fire, but buried portions are
preserved in the deltaic sediment. Eventually, the landslide dam
is entirely breached and the stream channel is reestablished at its
pre-slide base level. The stream erodes the banks of the delta and
exposes the drowned pre-slide forest.
The documented landslide record of lower Halden Creek spans
the last millennium. From this, one might conclude that the
landslides are a response to climate change over the last thousand
years. However, the modern climate regime, which triggered the
expansion of muskeg in northeastern BC, was achieved about
5,000–6000 years ago (MacDonald and McLeod 1996). A more
likely explanation is that the lower Halden Creek system is so
active that landslide deposits older than about 1,000 years are
removed from the basin or are buried by younger landslides. The
ca. 3,800-year event recorded at Tenaka Creek, a much larger
watershed than that of Halden Creek, indicates that landslides have
been occurring for more than just the last thousand years.
The age range of the drowned forests spans only the last
300 years (500 years at the extreme limits of 2-sigma age ranges), in
contrast to the 1,000-year record of landslides. The drowned
forests date landslides, but the record is shorter. This difference
may relate to the greater erodibility of the deltaic sand and gravel
in comparison to the cohesive landslide diamicton. In addition, the
drowned forests are located on the Halden Creek floodplain,
whereas the landslide deposits occur on adjacent slopes. The
drowned forests are thus subject to more frequent fluvial erosion
and are removed from the sedimentary record sooner than the
landslide deposits.
Implications for forest management
Episodic landslides and impoundments in lower Halden Creek
place forests in the watershed at risk. Our data indicate a minimum
of eight landslides in the last millenium and three landslideinduced forest drownings in the last 300 years over a 6-km reach of
Halden Creek. Furthermore, all slopes in the study area have been
modified by landslides and are unstable. With an 80- to 100-year
forest-harvesting rotation period, it is likely that some forests will
not reach maturity due to landsliding or impoundment. Thus, the
probability of landslides and landslide damming are factors that
should be considered in economic models evaluating the hazards
of harvestable working forests.
Acknowledgement
The project was funded by the British Columbia Ministry of Forests
and Range and Forest Renewal British Columbia. We thank Ljiljana
Knezevic, Chad Seigel, and Barbara Wohlfarth for assistance in the
field. Theo van Asch, Rens van Beek, and Steven de Jong reviewed
an early draft of this manuscript. Mauri McSaveney and an
anonymous referee reviewed the paper for the journal. Sean Barry,
Marlene Flannery, and Richard Franklin drafted the figures.
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M. Geertsema ())
Research Geomorphologist, B.C. Ministry of Forests and Range,
1011 4th Ave.,
Prince George, BC V2L 3H9, Canada
e-mail: Marten.Geertsema@gov.bc.ca
Tel.: +1-250-5656923
Fax: +1-250-5654349
J. J. Clague
Department of Earth Sciences, Simon Fraser University,
8888 University Dr.,
Burnaby, BC V5A 1S6, Canada
Landslides