ABSTRACT: Major landslides and tributary geomorphology in the Peave River Lowland, Alberta, Canada.

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Abstract

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 Creeklandslide (Brooker, 1959). All of the above landslides are reactivated, retrogressive or enlarged, translational earth slides. Their volumes range from about 30 Mm 3 on the Spirit River to about 80 Mm 3 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 landslideproperties 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 nomenclaturefor the landslides follows Cruden and Varnes (1996). 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 pre-glacial lacustrine, till, and postglacial lacustrine sediments, atop the preexisting channel andfloodplain 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.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 isdistinguishable 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-glaciallacustrine 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 andpost-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, andshales. 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 equilibriumand 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. 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.

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