LANDSLIDE DAMS, PEACE RIVER WATERSHED, ALBERTA AND BRITISH COLUMBIA

Large landslides that create dams are commonplace in the Peace River watershed. On small tributaries, the landslides tend to fully obstruct the valleys creating lakes that can persist for decades. The associated dams disrupt stream flow, damage timber, and threaten infrastructure both upstream and downstream. Six landslide dams on small tributaries are documented including the Eureka (Miller and Cruden 2001, 2002, 2008), Montagneuse (Cruden et. al. 1997), Saddle (Cruden et al. 1993), and Spirit (Miller and Cruden 2001, 2008) River dams, and Fox (Kim et al. 2010) and Hines (Lu et al. 1998) Creek dams.

The landslides that created the dams ranged in size from 20 to 78Mm³. Each was composed of Quaternary sediment originally deposited within the pre-glacial Peace River valley. This sediment records the up-valley advance of the Laurentide ice sheet and a down-valley ice front retreat. These events filled the pre-glacial topography with up to 200m of advance-phase glaciolacustrine sediment, till, and retreat-phase glaciolacustrine sediment.

Each landslide's rupture surface extended beneath the stream channel, causing the channels to be thrust upwards and forming the dams. The Eureka, Saddle, and Montagneuse rivers, and Fox and Hines creeks had dams between 19 and 30m in height; the Spirit River dam was 9m. New channels formed around the toes of the landslides, abandoning the pre-landslide channels. Despite the pre-landslide streambeds being armoured with coarse alluvium (cobbles and boulders), the new channels are mostly devoid of coarse alluvium, leaving them susceptible to stream incision.

The landslide lakes diminished over several years through stream incision into the dams and sediment infilling. The dams on the Eureka and Saddle rivers persisted for up to 23 years. Other dams have persisted for less time. Several factors can delay draining of the lakes including continued landslide activity, log jams, beavers, the new channel intersecting the armoured pre-landslide channel, and coarse alluvium from the old channel entering and armouring the new channel as a result of bank instability. Once the lakes have completely drained, several decades can pass before the channels fully mobilize the associated lacustrine, alluvial and colluvial sediment and return to their pre-landslide profiles.