Ice Age Floods
National Geologic Trail
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Around 12,000 years ago, at the end of our last ice age, a large ice dam created Glacial Lake Missoula, located in what is now modern Montana. The lake held almost half the amount of water of Lake Michigan, and covered 3,000 square miles. The ice dam holding back all the water eventually cracked due to water seeping into it, and sent the entire volume of the Glacial Lake Missoula roaring westward. It drained the entire lake in only a few days, sending a torrent of water, ice, dirt, and debris towards the Pacific Ocean at around 50 miles an hour. The water would sweep across valleys and low-lying areas, eventually affecting over 16,000 square miles of terrain leaving a lasting mark of dramatic and distinguishing features on the landscape of parts of the States of Montana, Idaho, Washington and Oregon..

After the flood, Glacial Lake Missoula would refill, only to break again and again. Some scientists think that over 40 floods once flowed across the western states. Evidence of these floods still remain in the form of coulees, high water lines, giant current ripples, and massive boulders transported across hundreds of miles.

Source: (2020)


Ice Age Floods National Geologic Trail — March 30, 2009

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Brochures ◆ Site Bulletins ◆ Trading Cards expand section


Flood Basalts and Glacier Floods: Roadside Geology of Parts of Walla Walla, Franklin, and Columbia Counties, Washington (HTML edition) State of Washington Department of Natural Resources Division of Geology and Earth Resources Information Circular 90 (Robert J. Carson and Kevin R. Pogue, January 1996)

Foundation Document, Ice Age Floods National Geologic Trail, Montana/Idaho/Washington/Oregon (Draft, September 2011)

Foundation Document Overview, Ice Age Floods National Geologic Trail, Montana/Idaho/Washington/Oregon (July 2014)

Long-range Interpretive Plan, Ice Age Floods National Geologic Trail (Gookrick Interpretive Group, June 2016)

Megaflood Adventure Junior Ranger Activity Book, Ice Age Flood National Geologic Trail (July 2019)

Reference List for the Ice Age Floods (2019)

Study of Alternatives and Environmental Assessment: Following the Pathways of the Glacial Lake Missoula Floods (Jones & Jones, February 2001)

The Channeled Scablands of Eastern Washington: The Geologic Story of the Spokane Flood USGS Information Circular 72-2 (Paul L. Weis and William L. Newman, 1976)

Washington's Channeled Scablands Washington Division of Mines and Geology Bulletin No. 45 (J. Harlen Bretz, 1959)

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Last Updated: 12-Aug-2021