glacial lake missoula was a preshistoric proglacial lake that formed periodically at the end of the most recent ice age, between 15,000 and 13,000 years ago. the lake measured about 3000 square miles and contained about 500 cubic miles of water, making it as large as lakes erie and ontario combined. glacial lake missoula was formed behind an ice dam on the clark fork river at the eastern end of lake pend oreille in the idaho panhandle. the dam, reaching heights of 2000 feet, was the southern encroachment of an arm of the cordilleran ice sheet. the lake backed up as far as 200 miles upstream into the watersheds of western montana, the largest ice-dammed lake known.
periodic rupturing of the ice dam resulted in the missoula floods -- cataclysmic floods that swept across eastern washington and the columbia river gorge approximately 40 times during a 2000 year period. the cumulative effect of the floods was to excavate 50 cubic miles of loess sediment and basalt from the channeled scablands of eastern washington, transporting it downstream. the floods also produced canyons and other large geologic formations, by cataclysmic rather than gradual erosional processes.
an excellent PBS NOVA documentary on glacial lake missoula, and the detective work that went into deducing its causes and the effects of flooding, can be foung at this link. (as always, click on images to enlarge for full effect.)