The 1939 Montagneuse River landslide, Alberta

The 1939 landslide on the west wall of the Montagneuse River valley, 1300 m long, 1400 m wide, and 80 m thick with a volume of 76 x 10(6) m(3), is the largest historic rapid landslide on the Interior Plains of Canada. It lasted about a minute, dammed the river, and formed a reservoir 1.5 km long. The reservoir was filled before 1952 by lake deposits, which were eroded by the river by 1988. The landslide contained two major blocks. The downslope block, 950 m long, kept intact without obvious tilting, indicating that the block moved almost horizontally on a comparatively deep rupture surface. The displaced material included part of the preslide river channel so the Montagneuse River was pushed eastward. The toe of the block moved up and dammed the river. The upslope block, with a length of 300-350 m, slid with little rotation but broke up into at least five smaller blocks. Most trees on the upslope block postdate the slide. Two sag ponds on the landslide, visible in 1952, had enlarged by 1988, probably a result of water running off from the recently cleared farmland behind the scarp. The main scarp has been substantially modified by earth flows and slides. Landslides in the Montagneuse River valley coincide with the Shaftesbury buried preglacial channel of the Peace River. The surfaces of rupture of five historic landslides in the Peace River Lowland follow clays deposited in preglacial channels, confirming that these deposits may be hazardous when eroded.