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Ice Age Floods

Ice Age Floods MapFloods that equaled half the volume of Lake Michigan but only lasting a week, tearing across the landscape at 80 miles per hour, transforming every thing in its path. Stripping some places to bedrock, while at the same time depositing the rich soils of the Willamette Valley. Nothing has shaped the Columbia Basin like the Ice Age Floods.

The last major geologic event to shape the Monument was the Ice Age Floods. During the freezes and thaws that occurred in the last Ice Age—the Wisconsin—ice dams formed and failed across several river systems in the Northwest. The largest and most frequent floods—and the ones with the greatest impact to the interior Columbia Basin—came from glacial Lake Missoula in northwestern Montana, where ice dams across the Clark Fork River formed and failed many times, each time releasing a wall of water that surged southwest through the Columbia Basin, inundating most of the Monument several hundred feet deep. However, smaller floods may have escaped down-valley from glacial lakes Clark and Columbia along the northern margin of the Columbia Basin or down the Snake River from glacial Lake Bonneville.

The Ice Age Floods began as early as one million years ago, with the most recent occurring around 13,000-15,000 years ago. The Ice Age Floods inundated the Monument dozens—if not hundreds—of times. The floods affected the landscape in different ways. As the water moved across eastern Washington, it eroded the basalt, forming channels of barren rocky land referred to as the channeled scabland. At other localities, such as away from the main flood channels, the water deposited massive bars of sand and gravel in only a few days. Where the water ponded behind obstacles such as Wallula Gap, it left behind deposits of sand and silt known as the Touchet Beds.

When the floodwaters entered the Pasco Basin, they quickly became impounded behind solid basalt in the Wallula Gap, which was too restrictive for the volume of water involved. This temporary, reoccurring hydraulically dammed lake is known as Lake Lewis. Lake Lewis is estimated to have had a surface area of approximately 4,500 square miles and to have reached a depth of about 900 feet. In the Monument, the elevation of the lake level at times rose to 1,200 feet above sea level, which corresponds to the 1200 Foot Road near the toe of Rattlesnake Mountain. The reoccurring lake is estimated to have lasted only a week or less.

The floods are of such national interest that in 2009 Congress created the Ice Age Floods National Geologic Trail. The National Park Service is currently in the process of 'creating' the trail, which will link together existing local, state and federally sites, facilities and attractions into a "partnership" park that can be driven from point to point to tell the story of the floods and their impact on the Pacific Northwest. To find out more, please visit www.nps.gov/iceagefloods/.

Since the end of the Missoula Floods, winds have reworked the deposits of sand and silt left behind, shifting them into dune sands in the lower elevations and loess (windblown silt) around the margins of the Pasco Basin. Anchoring vegetation has stabilized many sand dunes. Where human activity has disturbed this vegetation, dunes have been reactivated. More recently, dunes have been reactivated by the removal of vegetation resulting from fires.

Last Updated: May 20, 2013
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