A Geologic Catastrophe
Schematic drawing of areas affected by floods. The teal color are glacial lakes; yellow are flooded areas
Throughout the earth's history the climate has fluctuated and at times the temperatures have been cooler than they are now. This change in temperature can last between 2-10 million years and is referred to as an ice age. During an ice age, the snow does not melt completely in the summer. Snow and ice continue to accumulate, compacting under its own pressure to become glaciers that flow like slow moving rivers of ice down the valleys and into the plains, spreading out across the continent. There have been at least five major ice ages in the past one billion years. The most recent, the Pleistocene Ice Age, began about 2 million years ago. Glaciers did not continually cover the earth during this time; there have been interglacial periods where temperatures warm slightly causing glaciers to melt and retreat.
During the last advance about 15,000 years ago, a finger from the glacial ice sheet moved south through the Purcell Trench in northern Idaho, part of what is now Lake Pend Oreille, creating a 2500 foot high ice dam across the the Clark Fork drainage. The water backed up behind the dam and filled the valleys to the east with water. The result was a glacial lake the size of Lake Erie and Lake Ontario combined. As the water rose to an elevation of 4250 feet, the pressure against the ice dam increased. This ultimately caused the dam to "float" and fail catastrophically by water seeping underneath into cracks and fissures or for other reasons still being studied. The torrent of water released by the ice dam failure slammed into Glacial Lake Columbia, which had formed along the edge of the melting ice sheet on the west side. David Alt described this as "like an elephant jumping into a mud puddle" in his book, Glacial Lake Missoula and its Humongous Flood.
It is estimated that the maximum rate of flow was equal to 386 million cubic feet per second. At that rate, the lake probably drained in a few days. Water moving at speeds between 30 and 50 miles per hour raced across eastern Washington on a 430-mile journey through the Columbia River Gorge to the Pacific Ocean, forever changing the landscape by stripping away the rich loess topsoil deposited by winds across the ice sheet, and by picking boulders out of the already fractured basaltic bedrock. The floodwater carved an immense network of channels as it raced across the area now called "scablands" and created Grand Coulee, Dry Falls, and Palouse Falls. The flood was bottle-necked at Wallula Gap near the Tri-Cities in Washington, causing tributaries to back up into Idaho, and again at Kalama Gap near Portland, Oregon, flooding the Willamette Valley. There it laid down some of its burden of soil and rocks and large ice-rafted boulders before reaching the Pacific Ocean.
During the last advance about 15,000 years ago, a finger from the glacial ice sheet moved south through the Purcell Trench in northern Idaho, part of what is now Lake Pend Oreille, creating a 2500 foot high ice dam across the the Clark Fork drainage. The water backed up behind the dam and filled the valleys to the east with water. The result was a glacial lake the size of Lake Erie and Lake Ontario combined. As the water rose to an elevation of 4250 feet, the pressure against the ice dam increased. This ultimately caused the dam to "float" and fail catastrophically by water seeping underneath into cracks and fissures or for other reasons still being studied. The torrent of water released by the ice dam failure slammed into Glacial Lake Columbia, which had formed along the edge of the melting ice sheet on the west side. David Alt described this as "like an elephant jumping into a mud puddle" in his book, Glacial Lake Missoula and its Humongous Flood.
It is estimated that the maximum rate of flow was equal to 386 million cubic feet per second. At that rate, the lake probably drained in a few days. Water moving at speeds between 30 and 50 miles per hour raced across eastern Washington on a 430-mile journey through the Columbia River Gorge to the Pacific Ocean, forever changing the landscape by stripping away the rich loess topsoil deposited by winds across the ice sheet, and by picking boulders out of the already fractured basaltic bedrock. The floodwater carved an immense network of channels as it raced across the area now called "scablands" and created Grand Coulee, Dry Falls, and Palouse Falls. The flood was bottle-necked at Wallula Gap near the Tri-Cities in Washington, causing tributaries to back up into Idaho, and again at Kalama Gap near Portland, Oregon, flooding the Willamette Valley. There it laid down some of its burden of soil and rocks and large ice-rafted boulders before reaching the Pacific Ocean.
The Cause
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Two geologists, J Harlen Bretz and Joseph T. Pardee, were instrumental in finding the solution to this geologic mystery. Bretz spent a great deal of his life studying the geologic landscape of eastern Washington. He proposed the features he observed must have been formed by large scale flooding of catastrophic proportion.
At that time, most geologists abided by the principles of Uniformitarianism, the idea that past geological events could be explained by forces observable today. Since a flood of that proportion had never been seen, Bretz's idea was dismissed. To make matters worse, Bretz could not identify the source of this flood. Joseph T. Pardee, a Montanan who worked for the United States Geologic Survey (USGS) near Missoula, Montana, surmised as early as 1910 that the lake had formed as glaciers moved south, blocking the Clark Fork River. In 1942, Pardee published a paper suggesting that "giant ripple marks" with heights of 15-30 feet were evidence that deep and swift flowing currents had passed through the area near Paradise, Montana. Pardee attributed this phenomenon to the sudden failure of the ice dam that impounded Glacial Lake Missoula. With this new evidence, Bretz finally had his source. Later Conclusions
It took many more years until the geologic community accepted Bretz's interpretation of the Channeled Scablands and presented him with an award in 1972. Today we know that this event occurred not just once but 40 or more times. We also have learned it is not unique to the Pacific Northwest and that catastrophic floods have occurred other places in the world. |
Glacial Lake Missoula at highest level of about 4200 ft. White area is ice, including Flathead Lake.
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Video Link
The Great Ice Age Floods by Washington State University, 10 minute overview.