Natures proved man wrong time and time again
http://metrovancouver.org/Documents/SeymourRiverRockslide-updateDec8.pdf
Depends on the man … sure the unexpected could happen, but that was the conclusion from a geotechnical/hydrological survey of the area and water continues to drain over the top of the blockage. A bigger danger at the moment appears to be a storm surge from the sea.