Ancient Lake Missoula shorelines, giant current ripples and dropstone on lake floor.
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Express Tour - Section II

Ice Dam Blocks the Clark Fork River

Summary of the spectacular catastrophic floods that raged through eastern Washington and the Columbia Gorge at the end of the most recent Ice Age

Glaical Lake Missoula Map
Ice Dam that blocked the Clark Fork River creating Glacial Lake Missoula is shown above. The body of water west of the Ice Dam is Glacial Lake Columbia.


ICE DAMS BLOCK MOUNTAIN VALLEYS, CREATING SUPER-RESERVOIRS

Glacial Lake Missoula Ice Dam Map by Bruce Bjornstad. Map by Bruce Bjornstad showing Ice Dam that blocked the Clark Fork River. Note outline of modern day Lake Pend Oreille.

As the ice sheets pushed southward from Canada they interrupted normal stream flows in the deep valleys of the mountainous Pacific Northwest. One protrusion, the Okanogan Lobe, created Lake Columbia, which was a super-sized version of modern Lake Roosevelt. An ice mass clogging the Purcell Valley of Idaho's panhandle blocked the outflow of the Clark Fork River, forming Glacial Lake Missoula. Other impoundments included a lake near Spokane, Wash. Scientists believe that additional lakes existed in Washington, Montana and British Columbia.

Clark Fork River.
Clark Fork River


LAKE MISSOULA BECOMES A CATALYST FOR A CATASTROPHE

Little Bitterroot Valley in western Montana.

Image shows Little Bitterroot Valley in western Montana. Huge valleys in western Montana combined to store over 500 cubic miles of water.

The ice blockage in the Purcel Valley gradually collected tremendous volumes of water in the deep valleys of western Montana, creating a gigantic reservoir which attained a depth of 2,000 feet and impounded over 500 cubic miles of water--equivalent to the combined volumes lakes Erie and Ontario. The natural ice dam periodically failed, which caused a catastrophic emptying of Lake Missoula. After each dam failure, the southward moving ice sheet then created a new one, and the cycle repeated itself. At least 40 major flood episodes originated from Lake Missoula.


Glacial Lake Missoula strandlines above the Mission Valley.
Faint horizontal strandlines above the Mission Valley floor in western Montana mark the ancient shorelines of Glacial Lake Missoula.



Ice Age Floods Express Tour: Continue

Section I
Lava Flows
Ice Age
Section II
Ice Dam
Lake Missoula
Section III
Scabland Floods
Wallula Gap
Section IV
Columbia Gorge
To the Pacific
Section V
Evidence
Solving the Mystery


All photos by Tom Foster unless otherwise noted.

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