Whitefish History & Museums: The Missoula Flood

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Missoula Flood

The Missoula Flood refers to floods that occurred periodically from the geological wonder of Glacial Lake Missoula, etching the landscape into what we see today in northwestern Montana. Read More

  • The Missoula Floods would sweep periodically across western Montana
  • Glacial Lake Missoula was an inland sea of about 2,000 feet deep
  • Tons of water from Glacial Lake Missoula escaped when the ice dam broke
  • J. Harlen Bretz was the first to see the evidence of these floods

The Missoula Floods are also known as the Spokane Floods or the Bretz Floods. These cataclysmic floods would sweep periodically across western Montana, eastern Washington, western Oregon and down the path of the Columbia River. The Lake Missoula Flood was one of the largest floods in Earth’s history. It was the result of a sudden rupture of an ice dam on the Clark Fork River.

Pleistocene Epoch Ice Age
About two and a half million years ago, glaciated ice sheets covered much of northern Montana, Washington and Idaho. The Cordilleran ice sheet of North America was one of these.

Peak of the Last Ice Age
As a section of the Cordilleran ice sheet moved south, a huge ice dam came about and blocked the Clark Fork River, forming a huge lake that was 2,000 feet deep – Glacial Lake Missoula. The lake contained more than 500 cubic miles of water.

End of the Last Ice Age
The Glacial Lake Missoula was an inland sea, stretching for 200 miles. It had more water than Lake Ontario and Lake Erie combined. The ice dam that held it back finally broke from the pressure, flooding down the Columbia River, sweeping what are now Montana, Idaho and Washington State. It is said that the flow was ten times the flow of all rivers in the world put together. A towering cascade of water and ice stripped away soil, cut canyons, moved boulders and shook the world as it raced almost 65 miles per hour across and through the land.

Over the next 2,500 years, this Cordilleran ice sheet continued to move south and the Clark Fork River became blocked over and over again, creating the Glacial Lake Missoula again and again. The ice dam would rupture periodically, with floods rushing once more through Montana, Washington and Idaho.

J. Harlen Bretz, a geologist, was the first to see the evidence of these floods. Over an eight-year period, he thoroughly researched the phenomenon and developed the theories published today.

Other geologists finally confirmed Bretz’ theories after studying the area and finding overwhelming evidence that what he said was true.

Today we are able to view the ways in which these floods have affected the landscape. They carved out miles of earth, piled up gigantic mountains of gravel and moved 200-ton boulders.