Shorelines of glacial Lake Missoula along the eastern hills of the Little Bitterroot Valley, 75 miles northwest of Missoula, Montana.

The planned Lake Missoula flood interpretive pathway

The Lake Missoula flood occurred at the peak of the Ice Age when a volume of water about 2,220 cubic kilometers (540 cubic miles), three times the volume of Lake Erie or one-half the volume of Lake Michigan, burst through its ice dam in northern Idaho and swept through eastern Washington into northern Oregon and out into the Pacific Ocean.1 

The water at the ice dam in northern Idaho was 600 m (2000 ft) high and upstream covered what is now the city of Missoula, Montana, to a depth of 300 m (1000 ft).  Shorelines from this ice age lake are commonly seen in the valleys of western Montana.  It is believed this giant lake emptied in two days, rushing 120 m (400 ft) deep over the present locations of Spokane, Washington, and Portland, Oregon.  It sped at up to 100 kph (60 mph) through tight spots in eastern Washington and 80 mph through the Columbia Gorge between Washington and Oregon.  It rapidly carved out the distinctive landscape known as the Channel Scablands, including the majestic canyon known as Grand Coulee.

Geologists believe that it is about time that a marked trail commemorating the flood’s path and explaining its significance needs to be set up from western Montana into northwest Oregon.2  Just like with the path of the expedition of famous US explorers Lewis and Clark, and that of pioneers who followed the Oregon Trail, scientists plan to set up signs and interpretive centers highlighting important features along the 600-mile flood path.  With the proposed help of the National Park Service, these memorials will be placed mostly along major highways, which would draw large crowds. 

The interpretive signs likely will tell the usual story of how J Harlan Bretz first noticed the strange landforms of eastern Washington and how he postulated a flood of enormous magnitude.  While pointing out the abundance of features supporting the Lake Missoula flood, the story will continue with the fact that geologists of the day did not believe Bretz, who was finally vindicated with field research.  The signs are sure to mention that geologists now believe there were well over 50 floods over a period greater than 2,500 years at the peak of the last ice age, some 14,000 years ago….

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