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Glacial Lake Missoula

Natural Area, Geologic Feature
Artist Bryron Pickering\'s illustration of Glacial Lake Missoula looks east across the Mission Valley. – Byron Pickering

During the last ice age, 13,000 to 15,000 years ago, an ice dam in the Clark Fork River near Sandpoint, Idaho collapsed and re-established itself every 30 years or so. Each time it opened, it released raging torrents from a 2,000-foot (610-meter) deep lake in western Montana. Now, thanks to a March 2009 Act of Congress, the Ice Age Floods National Geologic Trail will tell the story about this tremendous force of nature as a unit of the National Park Service.

The 600-mile Ice Age Flood Trail will include interpretive centers, roadside signs and markers, allowing motorists to track the course of the floods from Flathead Lake to the mouth of the Columbia west of Portland.

The repeated filling and collapse of the ice dam would drain an inland sea the size of Lake Erie and Lake Ontario in two days. At its greatest extent, Glacial Lake Missoula stretched eastward a distance of some 200 miles. When the lake rose high enough, it burst through the ice dam. When the highest of these ice dams failed, lake water burst through in torrents that would carry massive boulders hundreds of miles.

Over approximately 2,500 years, the lake, ice dam and flooding sequence was repeated dozens of times, leaving a lasting mark on the landscape.

Today we can see how the floods impacted the landscape. They carved out more than 50 cubic miles of earth, piled mountains of gravel 30 stories high, created giant ripple marks the height of three-story buildings, and scattered 200-ton boulders from the Rockies to the Willamette Valley. Grand Coulee, Dry Falls, Palouse Falls -- all were created by these flood waters, as were the Missoula and Spokane ground-water resources, numerous wetlands and the fertile Willamette Valley and Quincy Basin.

The legacy of these floods, which are recalled in Salish-Pend d’Oreille Coyote stories, is evident in Missoula, MT where the lines from the lake’s waves can be seen on Mount Jumbo. The “scablands” of eastern Washington are so-named because the floods washed away the top soil, leaving “scabs” – outcroppings of basalt bedrock.

Ripple marks 30 feet high can be seen in Camas Prairie, and giant boulders, from the Lake Missoula basin, can be found in Washington and Oregon – even in the Willamette Valley, hundreds of miles from Lake Missoula. The great Columbia River Gorge was not formed by the erosion of the Columbia River, but was scoured out by the repeated floods of Glacial Lake Missoula.

In many ways, the story of the Floods is also the story of J Harlen Bretz (1882-1981), who proposed the theory that the Channeled Scablands of eastern Washington, and much of the Northwest as we know it today, were formed by catastrophic flooding.

Bretz began his field research in the Channeled Scablands of central Washington during the summer of 1922, and it quickly became clear to him that neither glaciation nor ordinary stream erosion explained the Scablands. The following year Bretz made his two presentations to the Geological Society of America on the Scablands.

The first paper provided a detailed physiographic description of the Scablands; the second suggested that it would have taken a massive volume of water to create the degree of channel erosion that had occurred. Bretz's second paper on the Scablands also discussed the mounded gravel deposits that were scattered throughout the area.

He proposed the idea of a catastrophic flood and included the first detailed geological map that included all of the Scablands and showed the extent of the floods. Bretz used the name "Spokane Flood" because he assumed the source of the water for this flood was somewhere near Spokane, Washington.

In the scientific community, Bretz' reward for developing this theory was scorn and derision.

Nevertheless, Bretz was confident that a flood had occurred, but was unable to figure out where the water had come from. Originally, he proposed that the water was the result of increased runoff from melting glaciers. But even Bretz had a tough time imagining any significant volume of water melting rapidly enough to have such devastating impact.

Not until 1930 did Bretz consider Glacial Lake Missoula as the possible source of water he was searching for. But the geologic evidence was elusive, and he did not fully embrace the idea until 1956.

His theory and findings about the great floods finally received general scientific vindication and braod acceptance in the 1950s after years of acrimonious debate.

As part of its ongoing program to educate the public about the uniqueness of the area, the Lower Flathead Valley Community Foundation commissioned seascape artist Byron Pickering to create a painting of Glacial Lake Missoula, which is featured at the top of this page.

The Mission Mountains’ valleys were filled with glaciers from which large icebergs broke off as the water level fluctuated, as depicted in Mr. Pickering's painting. The painting's perspective is looking east from the national Bison Range, approximately 1,000 feet above the valley floor, and extends from Mount Harding on the left to the Mission Falls valley on the right. A reproduction of this painting is on a permanent outdoor display at the Glacial Overlook.

You may order a print of this beautiful painting at

A broad coalition of groups and communities have joined together to propose the creation of an Ice Age Floods National Geologic Trail as a unit of the National Park Service. The best statement of how the story of the Ice Age Floods can be told regionally is contained in the report of a major Special Resource Study completed by the National Park Service (NPS) in 2001 (available at the National Park Service web site to the right.)

The proposed trail designation is pending in Congress (as of March 2009) as part the Omnibus Lands Management Act of 2009.

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