Major landslides and tributary geomorphology in the Peace River Lowland, Alberta, Canada
D.M. Cruden (University of Alberta), Z.Y. Lu (Lu-Star Consulting), and B.G.N. Miller*
(Earth and Atmospheric Sciences, University of Alberta, Edmonton, T6G 2E3, [email protected])
Figure 1: The 1990 Eureka River Landslide and Dam
Introduction
The last 60 years have seen at least seven major landslides in tributaries of the Peace River in the
Peace River Lowlands that rank among the largest in Alberta’s history. Five of these landslides
occurred in the past decade: the 1990 Saddle River landslide (Cruden et al., 1993), the 1990 Eureka
River landslide (Lu et al., 1998; Miller, 2000), the 1995 Spirit River landslide (Miller, 2000), the 1990
Hines Creek landslide (Lu et. al., 1998), the Vessall Creek landslide (1993-97). Earlier landslides
include the 1939 Montagneuse River landslide (Cruden et al., 1997), and the 1959 Dunvegan Creek
landslide (Brooker, 1959).
All of the above landslides are reactivated, retrogressive or enlarged, translational earth slides. Their
volumes range from about 30 Mm3 on the Spirit River to about 80 Mm3 on the Montagneuse River.
In every case, a landslide dam and reservoir was created.
In this paper, we discuss the interaction between fluvial processes and landslide activity in the Peace
River Lowland. We begin with an examination of the effects of surficial stratigraphy on landslide
properties and an examination of landslide occurrence in relation to stream longitudinal profile. Then,
we examine the effects of landslides on the fluvial geomorphology of Spirit River, Hines Creek,
Eureka River, and Montagneuse River, considering successively larger landslides. Our nomenclature
for the landslides follows Cruden and Varnes (1996).
Stratigraphy and Landslides
The genesis of the surficial stratigraphy of the Peace River Lowlands was the up-drainage advance of
the Laurentide ice sheet followed by the down-drainage ice front retreat. These events deposited preglacial lacustrine, till, and postglacial lacustrine sediments, atop the preexisting channel and
floodplain sediments, filling pre-glacial valleys. This sedimentary sequence is thought to be thickest
and most complete within the pre-glacial Peace River valley. Where tributaries reoccupy pre-glacial
valleys, or where they impinge on the pre-glacial Peace River valley, landslides are common.
Landslides and tributary geomorphology
Numerous dormant and abandoned large landslides have been recognized within each, or most, of the
sedimentary units, within the Spirit River, Hines Creek, Eureka River, and Montagneuse River
watersheds. As each of the surficial units has distinct properties, instability within each unit is
distinguishable by depths of rupture surface, size, and affect on river morphology.
Field inspection of the Spirit River confirmed the presence of dormant and abandoned landslides
within the till, and dormant landslides within the pre-glacial lacustrine deposit. The 1995 landslide is
a reactivated and retrogressive earth slide, with the rupture surface at the top of the pre-glacial
lacustrine deposit, 60 m below the Lowland plains. The landslide also reactivated a higher rupture
surface within the till. Landslides downriver of the 1995 landslide, evident on aerial photographs,
likely also had rupture surfaces within the pre-glacial lacustrine deposit.
In the Hines Creek watershed, abandoned shallow landslides were recognized within the post-glacial
lacustrine deposits, and dormant landslides were recognized within the till. The 1990 Hines Creek
landslide is a reactivated earth slide with the rupture surface within the till, 76 m below the Lowland
plains (Lu and Cruden, 2000).
In the Eureka River watershed, abandoned landslides were recognized within the post-glacial
lacustrine deposits, and dormant and abandoned landslides were recognized within the pre-glacial
lacustrine deposits. The 1990 landslide was an enlarged or retrogressive earth slide with the rupture
surface within the pre-glacial lacustrine deposit, approximately 120 m below the lowland plains.
In the Montagneuse River watershed, abandoned landslides were recognized within the till and
post-glacial lacustrine deposits, and dormant landslides were recognized within the pre-glacial
channel deposits. The 1939 Montagneuse River landslide is a reactivation and retrogression of a
dormant slide, with the rupture surface within the pre-glacial flood plain deposit. (Lu and Cruden,
2000)
Longitudinal Profiles and Landslide Activity
Following the eastward shoreline retreat of the postglacial lake, the Peace River rapidly cut down
through the Quaternary sediments and into the underlying Cretaceous sandstones, siltstones, and
shales. At present, the Peace River Valley has eroded as much as 275 m below the prairie level in the
region. The tributaries of the Peace River were unable to erode at the same pace as the Peace River,
and many developed convex longitudinal profiles similar to those of other tributary streams in Alberta
(Rains and Welsh, 1988; Rains et al., 1994).
Longitudinal profile convexity has been identified in the Spirit River (Cruden et al., 1993), Saddle
River (Cruden et al., 1993), Montagneuse River (Cruden et al., 1997), Eureka River, and Hines Creek
watersheds (Lu and Cruden, 2000). The Montagneuse and Saddle Rivers have longitudinal profiles
that can be divided into three contiguous reaches: a gentle lower reach that has reached equilibrium
and in which few or no landslides are occurring, a steeper intermediate reach where most of the
landslides are occurring, and a gentler upper reach with little or no instability. The Spirit and Eureka
Rivers have longitudinal profiles that can be divided into two contiguous reaches, a steep lower reach
in which most landsliding is occurring, and a gentle upper reach with little or no instability. The
Eureka and Spirit Rivers are tributaries of Clear and Saddle Rivers respectively. Therefore, the
evolution of the Eureka and Spirit Rivers is linked to the evolution of the Clear and Saddle Rivers.
Interactions Between Fluvial Processes and Landslide Activity
As the tributary streams incise into each sedimentary deposit, the volumes of the landslides increase,
as do the effects of the landslides on stream processes.
Landslides and tributary geomorphology
Initial instability is within the post-glacial lacustrine deposits. This instability includes earth slides
and earth flows, and can be frequent. These events have a marginal effect on the tributary maturation
as relatively small volumes of material are displaced during any one event. Following incision
through the post-glacial lacustrine deposits, earth slides within post-glacial lacustrine deposits become
dormant, then abandoned. Earth flows may continue, though associated with deeper-seated slides.
Landslides within the till occur following stabilization of the upper lacustrine deposits. The depth of
rupture surface of these slides is generally between 30-80 m. Instability in this deposit substantially
impacts river processes as landslide dams form. These dams include Types 2 (rupture surface daylighting above the river and a zone of accumulation that spans the entire valley floor) and 6 (one or
more rupture surfaces that extend under the stream or river valley and emerge on the opposite valley
side from the landslide) (Costa and Schuster, 1988). The Spirit River dam is both a Type 2 and 6
dam, with rupture surfaces within the till and the pre-glacial lacustrine deposits. The Hines Creek
landslide dam is a Type 6 dam, with the rupture surface within the till. Lacustrine deposits form
behind these landslide dams, which may enhance slope toe support, thus temporarily curtailing
landslides. The lives of the dams are generally only a few years. In Hines Creek, the stream cut a
new channel around the toe of the landslide and, as the new channel is free of bed armour, incision
has been rapid. The Spirit River landslide also broke the streambed armour, though the new channel
intersects with the old channel in two locations. The drainage of the Spirit River reservoir caused
small landslides adjacent to the stream by rapid draw-down. After the landslide dam has been cut
through, the lacustrine deposits are quickly eroded, and support at the toe of the slope is removed, and
the landslide is prone to reactivation. This cycle may be repeated several times.
After channel incision through the till, most of the landslides within the till become dormant and then
abandoned. Landsliding then occurs within the underlying pre-glacial lacustrine deposits. The
39Mm3, 1990 Eureka River landslide (Figure 1) is an example. The rupture surface of the Eureka
River slide extends 20 m below the previous channel, 120 m below the Lowland plains. A Type 6
landslide dam was formed, with a reservoir 5 km in length. The Eureka River has cut a new channel
around the landslide toe and, as this new channel is free of bed armour, incision has been 20 m in 9
years. As the banks of this new channel are steep, instability adjacent to the channel, both flows and
slides, is frequent. Continued instability rearmours the new channel with the alluvium of the old
channel. The Eureka River landslide dam still exists with a reservoir 2 km long. The landslide is
dormant.
The 1939 Montagneuse River landslide has its rupture surface within the pre-glacial flood plain
deposits. The 80Mm3 volume of the landslide exceeds all the other historic landslides. The landslide
formed a Type 6 landslide dam. Lacustrine deposits, as a result of the landslide dam, are seen in the
channel of the Montagneuse River in the 1952 aerial photographs. The lacustrine deposits below the
north flank of the landslide formed a depositional terrace up to 80 m wide and 270 m long (Cruden et
al., 1997). From the extent of the lacustrine deposits in the channel, the height of the landslide dam
could be estimated to being 30 m. By 1988 these deposits were eroded away, as was a substantial part
of the toe of the landslide.
Conclusion
At least seven major landslides have occurred on tributaries of the Peace River, in the Peace River
Lowlands, within the last 60 years. All the landslides occurred in a steep lower reach of the convex
streams. Landslide activity is frequent along the pre-glacial drainages in the Peace River Lowlands.
The impact of the landslides on fluvial processes depends on the sedimentary deposit in which the
landslide’s rupture surface is located. Landslides with rupture surfaces in the uppermost post-glacial
lacustrine unit have little impact on stream processes. Landslides with rupture surfaces in
successively lower depositional units have progressively greater impacts on stream processes, as dams
of progressively greater volume and lifespan are formed. The landslide dams temporarily obstruct the
Landslides and tributary geomorphology
progress of the landslide front up the tributaries. As each sedimentary unit is eroded through,
landslide activity within that unit becomes dormant. However, as slopes that have already failed are
easily reactivated, routes and structures crossing these valleys should be located and monitored with
exceptional care.
References
Brooker, E.W., 1959. Dunvegan landslide. Alberta Research Council, Edmonton, Open File Report
1959-6.
Costa, J.E. and Schuster, R.L., 1988. The formation and failure of natural dams. Geological Society
of America Bulletin, 100: 1054-1068.
Cruden, D.M., Keegan, T.R. and Thomson, S., 1993. The landslide dam on the Saddle River near
Rycroft, Alberta. Canadian Geotechnical Journal, 30: 1003-1015.
Cruden, D.M., Lu, Z.Y. and Thomson, S., 1997. The 1939 Montagneuse River landslide, Alberta.
Canadian Geotechnical Journal, 34: 799-810.
Cruden, D.M. and Varnes, D.J., 1996. Landslide types and processes. In Landslides: investigation
and mitigation. Edited by A..K. Turner and R.L. Schuster. Transportation Research Board,
Special Report No. 247, pp. 36-75.
Lu, Z.Y., and Cruden, D.M., 2000. Fluvial processes and landslide activity in the western Peace River
Lowlands, Alberta, Canada. Proceedings, 8th International Landslide Symposium, Cardiff, U.K.
Lu, Z.Y., Cruden, D.M. and Thomson, S., 1998. Landslides and preglacial channels in the western
Peace River Lowland, Alberta. Proceedings, 51st Canadian Geotechnical Conference, Edmonton,
Vol. 1, pp. 267-274.
Miller, B.G.N., 2000. The Spirit and Eureka River landslides, Peace River Lowlands, Alberta.
Master of Science Thesis, Department of Earth and Atmospheric Sciences, University of Alberta,
Edmonton (in progress).
Rains, R.B., and Welsh, J. 1988. Out-of-phase Holocene terraces in part of the North Saskatchewan
River basin, Alberta. Canadian Journal of Earth Sciences, 25: 454-464.
Rains, R.B., Burns, J.A., and Young, R.R. 1994. Postglacial alluvial terraces and an incorporated
bison skeleton, Ghostpine Creek, southern Alberta. Canadian Journal of Earth Sciences, 31: 15011509.
Biographical Note
Brendan Miller B.Sc., G.I.T. (B.C.)
Brendan Miller graduated from the University of British Columbia in 1996 with a B.Sc. in Physical
Geography. Since that time, he has been employed in environmental consultancy, primarily for the
forestry industry. Presently, he is completing a M.Sc. degree in the Department of Earth and
Atmospheric Sciences, University of Alberta, under the supervision of Dr. Dave Cruden.