Glacial Lake Missoula
Near Missoula, Montana
The hills above Missoula bear horizontal lines that look like bathtub rings, and that is essentially what they are. Between about fifteen thousand and thirteen thousand years ago, a lobe of the Cordilleran Ice Sheet repeatedly dammed the Clark Fork River in what is now Idaho, creating a lake that filled much of western Montana. At its largest, Glacial Lake Missoula was nearly one thousand feet deep at the site of present-day Missoula and held as much water as Lake Erie and Lake Ontario combined.
When the ice dam failed, the lake drained in a matter of days, sending a catastrophic wall of water across the Columbia Plateau and carving the channeled scablands of eastern Washington. The dam reformed and burst dozens of times. Geologist J Harlen Bretz proposed this theory in nineteen twenty-three and was ridiculed for forty years before the evidence became undeniable.
