Glacial Lake Missoula was a prehistoric proglacial lake in western Montana that existed periodically at the end of the last ice age between 15,000 and 13,000 years ago.
The lake was the result of an ice dam on the Clark Fork River caused by the southern encroachment of a finger of the Cordilleran ice sheet into the Idaho Panhandle. The height of the ice dam typically approached 2,000 feet, flooding the valleys of western Montana approximately 200 miles eastward.
Glacial LakeMissoula, an immense water body dammed in the upper Clark Fork valley by the Purcell Trench lobe, was the source of floodwater that catastrophically swept across the Channeled Scabland (Bretz, et.al., 1956; see also Baker, 1982).
LakeMissoula shorelines etched across the sharp Fraser-age terminal moraines of alpine glaciers that flowed from mountains on the east side of the lake (Alden, 1953; 197-13) similarly indicate that the alpine-glacial maximum there occurred before or during the higher stands of the lake.
Because the lake was ponded near the terminus of the Purcell Trench lobe, the long interval of ponding implies that the lobe maintained its near-maximal position for millennia.
In the rugged terrain outside of Missoula, Montana, are clues indicating that sometime in the last 10,000 years or so an enormous lake filled the valleys of western Montana.
On the hills around Missoula are numerous straight, horizontal ledges called "lap marks" that mark the shorelines as the level of the lake filled to different heights and drained.
Glacial Lake Spokane and Glacial LakeMissoula are indicated on the map (above) in medium blue.
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