The Rocky Mountains have a high hazard potential from rapid slope failures, subsequent damming of drainage systems, and catastrophic rapid drainage of the landslide-dammed lake. Most landslide dams eventually fail by overtopping. This paper examines the history of, and the potential for catastrophic failure and high discharges from, a pair of landslide-dammed lakes in Glacier National Park, Montana, USA. Tree-ring and lichen data from the surface and margins of the rockfall-avalanche dams indicate contemporaneous deposition occurred in 1910, with additional deposition in 1946. The rockfall-avalanche dams overlie Cretaceous shale capped with late Pleistocene glacial till. The deposits are therefore inherently unstable. They also leak and show evidence of lubrication along the base of one dam. The smaller of the two lakes undergoes dramatic fluctuations in lake level. Additional rockfall activity into the lakes is likely, and could lead to catastrophic overtopping and dam failure. Lake surface areas and measured depths, as well as rockfall dam heights and volumes, provide data for calculating maximum discharges (Qmax) in the event of dam failures. The Qmax estimates range from 82 to over 380 cubic meters per second, sufficient to disrupt local transportation corridors and endanger lives and property.