At the end of the last Ice Age (the Wisconsonian Ice Age), a branch of the Cordilleran ice sheet moved out of Canada into the Idaho panhandle region. There it formed a 2,000 feet (610 m)-high ice dam that blocked the mouth of the Clark Fork River, creating glacial Lake Missoula, which impounded greater than 2000 cubic kilometers (500 cubic miles) of water. The lake extended up the valleys eastward for over 200 miles (322 km). The periodic rupturing of the ice dam resulted in the Missoula Floods – cataclysmic floods that swept across Idaho and Eastern Washington, and then down the Columbia River Gorge approximately 40 times during a 2,000 year period. The flood front swept in a wave across Idaho and Washington at speeds approaching 100 kilometers per hour (65 miles per hour), and Glacial Lake Missoula drained in periods as short as 2 days.[2]

The Columbia River channel downstream was blocked by the Okanogan lobe of the Cordilleran, impounding water in Glacial Lake Columbia. As a result the floods could not continue down the Columbia River, being forced instead to flood over the highlands of Eastern Washington, vastly transforming the landscape by forming the Grand Coulee, Moses Coulee, the Channeled Scablands, Dry Falls, Palouse Falls and many similar features. The cumulative effect of the floods was to excavate 210 cubic kilometres (50 cu mi) of loess, sediment and basalt from the channeled scablands of eastern Washington and to transport it downstream.[2] Over a period of 2,000 to 2,500 years, the repition of ice dam failure and flood was repeated 40-60 times, leaving a lasting mark on the landscape.

There are a number of characteristic features that illustrate the effect of these ice dams and power of the resulting floods:

  • Large portions of the scabland region of Eastern Washington were stripped bare of topsoil and left as the base basalt rock.
  • The landscape is extensively eroded, with deep sheer-walled canyons in the Grand Coulee, Moses Coulee, the Wallula Gap, and the Columbia River Gorge.
  • There are giant ripple marks 30 feet (9.1 m) high and gravel bars over 4 miles (6 km) long and 300 feet (91 m) high, which would have required extreme currents to form.
  • There are 200-ton erratic boulders which originated in Canada and Montana, found well up valleys such as the Willamette River valley far south of any evidence of glacial activity; these boulders were ice rafted in and depositied as the ice was stranded & melted in backwaters of temporary lakes.
  • The scablands formed include deep kolks formed by local turbulence, which plucked basalt boulders out and excavated pits in the middle of relatively smooth plains.
  • Backwaters of temporary lakes formed during the floods show extensive & periodic deposits of silt and sand
 
 
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