Glacial Lake Missoula

During the last ice age about 18,000 years ago, an enormous glacier pushed down from British Columbia and blocked the Clark Fork River in northern Idaho, forming an ice dam that created Glacial Lake Missoula, the largest known glacial lake. Its waters backed up into the river’s drainage in western Montana, creating a body of water comparable to today’s Lake Ontario. As the lake filled and the water at the ice dam deepened, the lighter glacial ice floated and eventually broke up, unleashing floods of epic proportions that scarred eastern Washington and created scablands that still define the landscape. The geologic record shows that the lake filled and emptied cyclically over about two thousand years, with evidence of at least 36 separate fillings preserved in many Interstate 90 road cuts, along with ancient ice age shorelines on the mountains around Missoula.

Montana · United States