Glacial Lake Missoula

by

Obie Pepper

ES 767 Quaternary Geology, Emporia State University

Artist's Rendition of Lake Missoula

by seascape artist Byron Pickering

image acquired from: http://www.pickeringstudio.com/lkms.html.

Lake Missoula

During the Fraser (Late Wisconsin) Glaciation the Cordilleran ice sheet advanced southward from its origins in British Columbia. The Cordilleran ice sheet terminated in the United States between the Pacific Ocean and the Great Divide. Several composite lobes of this ice sheet were formed in between highlands and mountain ranges. One such lobe was the Purcell Trench lobe, which invaded the Idaho panhandle and the upper Clark Fork Valley. The Purcell Trench lobe acted as an ice dam and inhibited the flow at the mouth of the Clark Fork River. It was due to this blockage of the Clark Fork River that created the gigantic Glacial Lake Missoula. The ice dam that created Glacial Lake Missoula was presumed to be 2,000 feet tall. As the waters rose behind the ice dam they flooded the valleys of western Montana. When Glacial Lake Missoula was at full capacity it extended 200 miles eastward and created a vast inland sea. When this great glacial lake was at its maximum height and extent it was estimated to have contained more than 500 cubic miles of water. This awesome amount of water is more than Lake Erie and Lake Ontario combined. During Glacial Lake Missoula’s history it experienced periodic ice dam failures. Most of these failures would result in massive and catastrophic floods that would scour the land. The floodwaters, which were composed of ice and dirt, would rush down the Columbia River drainage, coursing through northern Idaho and eastern and central Washington, through the Columbia River Gorge, back up into Oregon's Willamette Valley, and finally dumping its remaining contents into the Pacific Ocean at the mouth of the Columbia River. Over an estimated period of 2,500 years the Purcell Trench lobe blocked the Clark Fork River dozens of times. Each sequence recreated Glacial Lake Missoula.

When Glacial Lake Missoula was at its maximum capacity a failure in the ice dam would result in a catastrophic event. When the ice dam was breached water would have burst out of Glacial Lake Missoula with 10 times the combined force of all the current rivers in the world. An ominous tower of water, almost incomprehensible in size, would have jolted the landscape like that of a great earthquake. The waters would have roared there way to the Pacific Ocean laying waste to the land, stripping off 200 feet of soil and gouging deep canyons, called coulees, in the underlying bedrock. The waters are estimated to have ripped there way across that land at a staggering 65 miles per hour. At that rate of speed the estimated 500 cubic miles of water would have taken only a matter of two days to drain.

Return to the Lake Missoula.

Channeled Scablands

During the late Miocene and early Pliocene the Pacific Northwest was the site of the largest basaltic lava flows to ever occur on the Earth’s surface. These lava flows covered a 63,000 square mile expanse of land. Over an estimated period of 10 to 15 million years lava flows continually inundated this region until the lava achieved a final thickness of 6,000 feet. The Earth’s crust gradually subsided into the space once occupied by magma. When the crust subsided it produced a large, partially depressed lava plain that is now known as the Columbian Basin or Plateau. The northwesterly advance of lava forced the Columbian River into its present course. When the igneous activity in this region halted forces within the Earth began to deform the Columbian Plateau in numerous places. The entire plateau was tilted slightly as the mountains in the north were uplifted. When Glacial Lake Missoula released its once captive waters over this area, time and time again, it sculpted a breathtaking landscape. The catastrophic floodwaters roared across the southward dipping Columbia Plateau and scoured out the land forming the coulees that characterize the landscape now known as the Channeled Scablands. Missoula’s floodwaters flowed southward to form two major cascades. The larger cataract, which is a waterfall, usually comprised of large volumes of water, falling over a single, sheer drop, was that of the Upper Coulee. The drop here was an astonishing 800 feet. The colossal power of this cataract plucked pieces of basalt away and caused a retreat in the falls of 20 miles. This event cut through the Columbian River valley near the present Grand Coulee Dam, and thus caused the self-termination of the falls. The smaller of the two major cataracts originated near Soap Lake. The less resistant basalt layers in this region gave way to the power of the great flood and formed waterfalls. These falls also retreated and formed Dry Falls, which is the dry remnant of one of the greatest waterfalls in geologic history. Dry Falls boasts dimensions on a grand scale that promptly puts Niagara Falls in its place. Dry Falls is three and one half miles wide with a drop of 400 feet. Niagara Falls, on the other hand, is only one mile wide with a drop of 165 feet.

Dry Falls

All in all the torrential flood waters of Lake Missoula carved out and deposited 50 square miles of earth. Enormous boulders weighing in excess of 200 tons were scattered about the landscape like driftwood. The great flood deposited giant mountains of gravel the size of 30-story buildings. Ripple marks adorn the landscape of the Channeled Scablands leaving unmistakable evidence of the velocity of the floodwaters.

Return to the Channeled Scablands.

J. Harlen Bretz

In the 1920's J. Harlen Bretz was with the United States Geological Survey (U.S.G.S.) when he spent a summer in the field in northeastern Washington state. Bretz pondered the geologic origin of the strange landscape he encountered while in the field. His personal theory was that the formation of the Channeled Scablands was due to a catastrophic flood. He described the unique geology of the Channeled Scabland to the Scientific Community, but was hesitant to unveil his personal theory. This was due to the fact that most geologists would not accept a theory involving a flood of that magnitude. The principle of Uniformitarianism was the leading theory held by geologists at the time. The principle of Uniformitarianism was based on the assumption that all geologic processes that occur today are the same processes that occurred in the past. Since a flood with the magnitude of the Lake Missoula flood had not been witnessed in recorded history it was simply just not a rationale explanation in the eyes of most geologists at the time. Bretz tried to find evidence to disprove the theory of a great flood, but all of his findings pointed to his original theory. Geologist demanded his theory be discredited on the mere fact it did not meet the principle of Uniformitarianism. In 1927 the Geologic Society of Washington, D.C. invited Bretz to attend a meeting to readdress his theory. Little did Bretz realize that there was a hidden agenda involved in this meeting. The six "Challenging Elders", as Bretz referred to them, were bent on disproving his theory in a public debate. One speaker, W.C. Alden, remarked that the volume of water needed to cause such a flood was incomprehensible, and pointed to the collapse of lava caves as a possible explanation. Others said that rivers caused the erosional landscape over millions of years rather than a single catastrophic event. Bretz countered with evidence regarding all of their arguments, and was not deterred from his theory. One year later Bretz found the source for his great flood. He had found out about the existence of glacial Lake Missoula. Bretz made one more trip to the Scablands area in 1952 along with two geologists from the Bureau of Reclamation. They spent that summer amassing more evidence for existence of the great flood. In 1965 an international team of geologists studied the area. After there studies were concluded an ailing Bretz received a telegram from the geologists admitting, "We are now all Catastrophists."

Return to the J. Harlen Bretz.

References

The Great Floods of Glacial Lake Missoula, World Wide Web homepage URL:
http://192.211.16.13/curricular/energies/Aprojfolder/missoula/Three.htm. Retrieved on (11-22-02).

Glacial Lake Missoula, World Wide Web homepage URL:
http://www.pickeringstudio.com/lkms.html. Retrieved on (11-22-02).

Lake Missoula and Cataclysmic Floods, World Wide Web homepage URL:
http://www.fs.fed.us/r6/columbia/missoula.htm. Retrieved on (11-22-02).

The Missoula Floods Controversy, World Wide Web homepage URL:
http://www.opb.org/programs/ofg/1001/missoula/theory.htm. Retrieved on (11-22-02).

USGS/Cascades Volcano Observatory, Vancouver, Washington. Glacial Lake Missoula and the Missoula Floods , World Wide Web homepage URL:
http://vulcan.wr.usgs.gov/Glossary/Glaciers/IceSheets/description_lake_missoula.html. Retrieved on (11-22-02).

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This web page was created to fulfill the graduate program requirements for Quaternary Geology at Emporia State University.


This page was created on 11-24-02.

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