Missoula floods

The Missoula floods were repeated Ice Age outburst floods caused by the failure of an ice dam holding back glacial Lake Missoula. They carved the Channeled Scablands and reshaped parts of the Pacific Northwest.

Glacial Lake Missoula: A large Ice Age lake in western Montana, impounded by the Purcell Lobe of the Cordilleran Ice Sheet. Its repeated formation and drainage supplied the water behind the floods.
Channeled Scablands: A deeply eroded region of eastern Washington marked by channels, coulees, cataracts, and streamlined hills. The floods carved its distinctive network of channels and stripped away much of its soil.
J Harlen Bretz: An American geologist who argued that catastrophic floods carved the Channeled Scablands. His fieldwork established the flood hypothesis despite strong initial resistance.
Columbia River Basalt Group: A vast sequence of flood-basalt lava flows erupted across the Pacific Northwest during the Miocene Epoch. The floods cut through these basalt layers, exposing the rock structure of the Scablands.
Uniformitarianism: The principle that processes observable today can explain geological features formed in the past. Bretz’s catastrophic floods challenged gradual interpretations, though they relied on familiar erosion processes at exceptional scale.
Purcell Lobe: An arm of the Cordilleran Ice Sheet that advanced into the Purcell Trench of northern Idaho. It blocked the Clark Fork River and created the lake’s ice dam.
Columbia River Gorge: A deep canyon through the Cascade Range, forming part of the boundary between Washington and Oregon. Floodwater passed through this constriction on its route toward the Pacific.
Joseph T. Pardee: An American geologist who identified evidence for a large glacial lake in Montana and its catastrophic drainage. His work connected Lake Missoula to the flood source Bretz had proposed.
Loess: A wind-deposited sediment dominated by silt, often derived from glacial or floodplain material. Wind reworked fine sediment left by floodwaters into extensive deposits across the region.
Scabland hypothesis: The hypothesis that catastrophic floods carved the Channeled Scablands of eastern Washington. This was the central geological explanation developed from evidence of the Missoula floods.

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