Glacial Lake Missoula in northwestern Montana was created when a lobe of the Cordilleran Ice Sheet blocked the Clark Fork River. With the river’s outlet blocked by a huge ice dam a glacial lake formed behind that dam and became known as glacial Lake Missoula. After many decades the ice dam failed creating huge floods that washed across northern Idaho, Washington and Oregon severely eroding the landscape. Evidence of floodwater erosion is present today. Early in the 20th Century J Harlen Bretz and Joseph T. Pardee spent years of their careers researching this vast area and discovering causes and effects of the land features in these states.
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Glacial Lake Missoula was created when a tongue of the Cordilleran Ice Sheet extended through the Purcell Trench into Northern Idaho. It blocked the Clark Fork River in Idaho with an ice dam near present day Lake Pend Oreille (United States Geological Survey [USGS]). A lake the size of Lake Ontario and Lake Erie combined was created. This lake emptied when the ice dam failed, releasing a flood (USGS). This great flood created a distinctive landscape across Washington and Oregon that has been studied by many geologists.
J Harlen Bretz started a great controversy in 1923 when he suggested that a catastrophic flood created the Channeled Scablands of Washington State (Baker, 1995). The Channeled Scabland was just one feature Bretz investigated. Another geologist, Joseph T. Pardee, contributed to the understanding evidence of great flooding and the glacial Lake Missoula through his research in the Montana basins.
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J Harlen Bretz and Joseph T. Pardee did early research in Washington and Montana. Bretz’ primary research was in the Channeled Scablands of Washington. He hypothesized that the scablands and other distinct features could only have been created by a catastrophic flood (Breckenridge, 1989). Bretz was criticized by prominent geologists of his day for this outrageous hypothesis. His ideas challenged the uniformitarian principles of geology, but eventually his hypothesis was accepted by geologist (Breckenridge, 1989.) Pardee studied in the Spokane area of Washington and the basins of Montana. During his studies he found evidence of a large glacial lake that formed in Montana.
A literature survey and search of the World Wide Web revealed previous studies supporting the theory that catastrophic floods created the distinctive landscapes in Washington and Oregon.
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Glaciation of Northern Washington, Idaho and Montana
The Cordilleran Ice Sheet extended from the Pacific Ocean to the western margin of the Laurentide ice sheet. At times, the two ice sheets formed a continuous sheet of ice over 4000 km wide (Booth et al, 2003). It reached from coastal south and southeast Alaska, down along the Coastal Mountains of British Columbia into northwest Montana and northern Washington (Booth, 2003). Several lobes were associated with the Cordilleran Ice Sheet. The Purcell Trench lobe moved southward (to 48° 10’N) into Northern Idaho through the Purcell Trench and blocked the Clark Fork of the Columbia River with an ice dam (Booth, 2003). (Fig 01) The ancient lake created by the ice dam across the Clark Fork River is known as Glacial Lake Missoula. Several other large glacially dammed lakes also formed along the margins of the Cordilleran Ice Sheet (Breckenridge, 1989).
Other glacial lakes were created as the lobes off the Cordilleran Ice Sheet moved southward into Washington. The Okanogan Lobe dammed the Columbia River and formed glacial Lake Columbia. This glacial lake was a huge version of the present lake impounded by Grand Coulee Dam (USGS). Another lobe of the ice sheet, the Columbia River Lobe, dammed the Spokane Valley creating shallow Lake Spokane (USGS).
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Glacial Lake Missoula
An ice dam was formed when the Purcell Trench Lobe blocked the Clark Fork River in Idaho close to present day Hope, Idaho. The 610 m ice dam created by the glacier impounded water behind it stretching 322 km southward. This inland sea is now known as glacial Lake Missoula. Breckenridge (1989) states that Lake Missoula may have existed between 17,000 and 11, 000 years B.P. The lake covered almost 10,000 km² of land (Carrara, Kiver, and Stradling, 1996) and was equal to the volume of both Lakes Ontario and Erie together. Present day Missoula, Montana, would have been under several hundred meters of water.
The lobes of the Cordilleran Ice Sheet would have been close to maximum extension during the Late Wisconsin glaciation 12,700 and 15,300 years B.P. (Carrara, Kiver, and Stradling, 1996). During its existence, Lake Missoula drained many times when the ice dam failed. Evidence in the Priest Lake area indicates this happened at least 14 times, causing catastrophic floods. Every two to six decades, the ice dam failed and flooding would occur. (Carrara, Kiver, and Stradling, 1996). The glacier would again advance and Clark Fork would be dammed and Lake Missoula would fill again.
Ice dams are unstable structures and there are four possible causes for ice dam failure (Breckenridge, 1989). Those causes are: 1) Flotation of the ice dam causing water to flow subglacially. 2) Water pressure causing ice deformation. 3) The subglacial tunnels enlarging by water. 4) Icebergs enlarging tunnels. Lake Missoula emptied when the water behind the dam reached ~600 m or 90% of the thickness of the ice causing the ice to become buoyant and dam failure (Breckenridge, 1989). This sudden outburst of flood water is called a jökulhlaup.
Pardee calculated that 2167 km³ of water would be stored when the water was at the elevation of 1265 m against the ice dam. Craig and Hanson (1985) later estimated the volume of water to be 2514 km³ at that elevation (Breckenridge, 1989). The lake would drain rapidly and Pardee estimated the drainage speed at 15.3 km³/hr. In 1973, Baker estimated drainage at maximum rate would be 62.5 km³/hr (Breckenridge, 1989).
As Pardee studied the geomorphological evidence it became apparent a huge lake had been present in the intermountain basins in western Montana. At Missoula, Montana, strandlines (Fig. 02) are visible on the hillsides above the town. Pardee gave credit to T.C. Chamberlin for finding the strandlines and suggesting that a glacial lake had been impounded in the area with the ice dam located in northern Idaho (Baker, 1995). The highest strandlines are at 1280 m and this would place the floodwaters at 635 m deep at the dam site in northern Idaho.
In 1940, Pardee presented a paper before the American Association for the Advancement of Science in Seattle, Washington. His topic was titled Ripple Marks (?) in Glacial Lake Missoula. He described the intermontane basin known as Camas Prairie in northwestern Montana. This basin has huge ripple marks across its surface 15 m high and 150 m apart or more (Fig. 03) (Baker, 1995).
These ripple marks are composed of foreset-bedded gravels. The rapid drainage of the lake created these land features. This was the evidence that proved J Harlen Bretz’ catastrophic flood hypothesis of creation of the Channeled Scablands and many other flood features in Washington and Oregon (Breckenridge, 1989).
The giant current ripples were not the only evidence presented of failure of glacial Lake Missoula’s dam. The rapid flow of water during the drainage of the lake created huge bars of debris transported by the currents (Baker, 1995) (Fig. 04). Pardee had described the source of flood water that carved the Channeled Scablands in Washington, but did not make a connection between glacial Lake Missoula and the Channeled Scablands during this presentation (Baker, 1995).
Fig. 04 At the mouth of a small tributary to the Flathead River valley an eddy bar was formed near Penna, Montana, when glacial Lake Missoula suddenly drained. Note lacustrine silt terrace in the foreground created when glacial Lake Missoula reformed. (Photo courtesy of GSA Today, used by permission.)
Glacial Lake Missoula appears to be a self-dumping lake. As water was backed up in the lake, the level would rise up the side of the ice dam. When the water reached ~600 m on the dam, the ice would become buoyant and the dam would begin to float. Waitt (1985) suggested the ice dam of Lake Missoula failed when hydrostatic pressure wedged the water between the ice and the rock (Breckenridge (1989). This continued until the ice would become buoyant and the water would flow through the subglacial tunnels. When the roof of the ice tunnels collapsed the dam would fail and suddenly Lake Missoula would empty.
After the dam failed and the lake dumped, a period of time passed until the glacial lobe moved enough to block the Clark Fork River again. Water to fill the lake came from many sources. Most of the meltwater was from the Cordilleran Ice Sheet to the north of the lake. Other sources of water included the alpine glaciers lying to the east and south of the lake. The southern half of the drainage basin contributed a large amount of water as natural run off from nonglaciated areas of the drainage basin (Waitt, 1985). Dam failure and refilling of the lake followed the same cycle over thousands of years. The flood followed two routes when the ice dam failed. The major path was from the end of Pend Oreille Lake, across the Rathdrum Prairies and into the Spokane Valley. The other major flood route would follow the Pend Oreille River and spill over the divides into the Little Spokane River. There were other ice-dammed lakes along the Cordilleran Ice Sheet and these lakes would also cause flooding when their dams failed (Breckenridge, 1989).
The freshwater floods from Lake Missoula probably are the largest floods of this type in geologic history (Chambers, 1984). How many times Lake Missoula filled, the ice dam failed and a catastrophic flood occurred is debatable. Rhythmites, sedimentary rocks or graded sequences of sediments forming a unit bed or lamina laid in rhythmically bedded deposits, are used as evidence for multiple floods. There are no carbon dates for Lake Missoula rhythmites because of the lack of dating material. Very small amounts of woody materials have been collected and the sample collected was contaminated at the site (Chambers, 1984). Rhythmites are present around the edge of the Lake Missoula site. Other areas where many rhythmites are found are in Washington and Oregon. Waitt (1985) states there is much evidence demonstrating the fact that glacial Lake Missoula flooded many times. Tributaries of the Columbia River where water back flooded when it could not go through narrow channels were settling basins for the sediment suspended in the flood waters. These basins contain rhythmic graded sand and silt beds and these flood sediment beds can be counted bed by bed (Waitt, 1985). In some of these rhythmic beds Waitt found evidence of subaerial environments between the graded beds. He believed during the last glaciation at least ~40 separate floods occurred from Lake Missoula. Other evidence found in the rhythmites include ash layers intercalated in the rhythmite sequence near Mabton (Waitt, 1985). Clasts are deposited between rhythmites at Latah Creek and Sanpoil Valley. Backwater areas in the Priest Valley, Idaho, the Willamette Valley, Oregon, and Ninemile Creek, Montana, have rhythmite layers of sand and silt attributed to the Lake Missoula flooding. The Columbia River Basin shows evidence of great flooding.
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Columbia River Basin
The Columbia River basin was covered by lava 10 to 15 million years B.P. During the Miocene and Pliocene times, the lava was extruded from low shield volcanoes and fissures across the surface of central Washington. The basalt extends across northeast Oregon, eastern Washington and central Idaho and in areas it is more than 3 km thick. The Columbia River basalt equaled more than 175,000 km³. As the basalt extruded, the earth’s crust gradually sank. This area of subsistence produced a depression today known as the Columbia Basin or Plateau (USGS). Over time, the basalt was covered with loess up to 76 m thick (National Park Service [NPS]).
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Flood Features of Idaho, Washington and Oregon
Through the work of J Harlen Bretz and Joseph T. Pardee, Glacial Lake Missoula was determined to have created the cataclysmic floods that carved the Channeled Scablands and other flood features across Washington and Oregon.This is the area J Harlen Bretz first researched in Washington, the scarred land surfaces of the Columbia River basin known as the Channeled Scablands. He started in 1922 when he took a small field party of advanced students on his first field research trip. He did field research for seven seasons. During those field seasons he covered the entire scablands on foot and then by car with groups of students or his family (Baker, p. 173). As Bretz studied the scablands, it became apparent to him they were not created by glaciation or common stream erosion. Bretz found mounded gravel bars that were subfluvial bars. The scablands across the area were braided courses coming from Long Lake Canyon, Dry Coulee, and Lenore Canyon. No canyons were present where these braided features occurred over the basalt surfaces. (Baker, p. 170). In 1923 he wrote two papers and made presentations to the Geological Society of America. His first paper gave detailed physiographic description of the scablands without suggestion of how they were created. The second paper gave his hypothesis that a catastrophic flood created the scablands or glacial meltwater. He thought floodwaters may have come from the Spokane area and used the name Spokane Flood (NPS). Geologists of his day rejected his theory of a catastrophic flood creating the scablands.
Joseph T. Pardee’s superior, W.C. Alden, chief of Pleistocene geology at the U.S. Geological Survey, sent him to the scablands near Spokane in 1921. Pardee wrote a report in 1922 suggesting the scablands were created by unusual glaciation. Bretz visited the same sites a couple of seasons later and determined Pardee’s glacial deposits were flood bars (Baker, p. 170). Pardee’s greatest contribution to identifying the source of the water creating the scablands was his research in the basins of Montana. His first paper about glacial Lake Missoula was published in 1910, one year after he started his research (Baker, p. 172). There are other areas in Idaho, Washington and Oregon showing the effects of the flooding of glacial Lake Missoula besides the Channeled Scablands. Fig. 05 illustrates giant current ripples at Spirit Lake in Idaho (Baker, 1995)
Fig. 05 Giant current ripples at Spirit Lake, Idaho. The tree cover makes the ripples stand out in this photo. (Photo courtesy GSA Today, used with permission.)
Dry Falls is the largest waterfall in North America. It was created when catastrophic flooding from Lake Missoula occurred. Water no longer flows over the Dry Falls. It is huge when compared to Niagara Falls, which is 1.6 km long and drops 50 meters. Dry Falls, in comparison, is 5.6 km long and water would have fallen 122 m (Washington State Department of Natural Resources).
Wallula Gap in Washington State slowed the surge of floodwaters toward the Pacific Ocean. As much as 834 km³ of water per day was being delivered to Wallula Gap. The gap was only capable of discharging 208 km³ per day. The water unable to escape through the gap pooled behind it and backed up into Pasco Basin, Yakima Valley and Touchet Valley forming a temporary lake known as Lake Lewis (NPS).
Two areas in Oregon shaped by the jökulhlaups from Lake Missoula are at Crown Point and the Willamette Flood Plain. At Crown Point it is possible to see the transition of the Columbia River Gorge from steep rugged terrain to a more gradual stream valley. The plain here is rolling and cultivated. The Willamette Flood Plain in Oregon is the largest remaining native bottomland interior grassland that remains unplowed (NPS).
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Defining the cause of the Channeled Scablands in Washington State started with J Harlen Bretz and Joseph T. Pardee, but it has not ended. Geologists continue to investigate the flood features and glacial Lake Missoula in northwestern United States today. Glacial Lake Missoula and the jökulhlaups from the lake continue to effect people in our modern world.
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Baker, Vic. Surprise Endings to Catastrophism and Controversy on the Columbia. GSA Today. Vol. 5, No. 9, September1995, pp. 169-173.
Booth, Derek B., Troost, Kathy Goetz, Clague, John J., and Waitt, Richard B. The Cordilleran Ice Sheet. August 2003. pp. 1-47. World Wide Web URL: http://geomapnw.ess.washington.edu/services/publications/manuscript/pub51.5-INQUA%20Chapter%202.pdf Accessed 26 November 2004.
Breckenridge, Roy M. Lower Glacial Lakes Missoula and Clark Fork Ice Dams. In Breckenridge, Roy M. (ed.), Glacial Lake Missoula and the Channeled Scabland: Missoula, Montana to Portland, Oregon, July 20-26, 1989, American Geophysical Union, 28th International Geological Congress, Field Trip Guidebook T310, pp. 13-21.
Breckenridge, Roy M. Pleistocene Ice Dams and Glacial Lake Missoula Floods in Northern Idaho and Adjacent Areas. In Chamberlin, V.E., Breckenridge, R.M., and Bonnichsen, Bill, eds. Guidebook to the Geology of Northern and Western Idaho and Surrounding area: Idaho Geological Survey Bulletin 28, 1989. pp. 5-16.
Carrara, Paul E., Kiver, Eugene P., and Stradling, Dale F. The Southern Limit of Cordilleran Ice in the Colville and Pend Oreille Valleys of Northeastern Washington during the Late Wisconsin Glaciation. Canadian Journal of Earth Sciences. V. 33, 1996. p. 769-778.
Chambers, Richard L. Sedimentary Evidence for Multiple Glacial Lakes Missoula. Montana Geological Society, 1984 Field Conference Northwestern Montana, pp. 189-199.
Geological Society of America. World Wide Web URL:http://www.geomapnw.ess.washington.edu/services/ publications/manuscript/pub51.5-INQUA%20Chapter%202.pdf Accessed 26 November 2004.
Glazer, Robert. New Information on the Great Spokane Flood. The Work of Eugene Kiver and Dale Stradling. Eastern Washington University Explorer. v. 2, no. 1, p. 8-13.
National Park Service. World Wide Web URL: http://www.nps.gov Accessed 28 November 2004.
National Park Service. Ice Age Floods Study of Alternatives and Environmental Assessment. February 2001. Jones and Jones, Architects and Landscape Architects, Seattle, Washington.
National Park Service. Ice Age Floods Study of Alternatives and Environmental Assessment. February 2001. World Wide Web URL: http://www.nps.gov/iceagefloods/d.htm Accessed 28 November 2004
Taylor, Lawrence Dow. The Outrageous Hypothesis of Dr. J Harlen Bretz, '05 A Perspective on the Life of a World Renowned Geologist & Teacher. World Wide Web URL:http://www.albion.edu/library/specialcollections/Schleg2003.asp Accessed 26 November 2004.
United States Geological Survey. Glacial Lake Missoula and the Missoula Floods. World Wide Web URL: http://www.albion.edu/library/specialcollections/Schleg2003.asp Accessed 15 November 2004.
Waitt, Jr., Richard B. Case for periodic, colossal jökulhlaups from Pleistocene glacial Lake Missoula. Geological Society of America Bulletin, Vol. 96, October 1985. pp. 1271-1286.
Waitt, Jr., Richard B. About Forty Last---Glacial Lake Missoula Jökulhlaups through Southern Washington. Journal of Geology, 1980, Vol. 88, pp. 653-679.
Washington State Department of Natural Resources. World Wide Web URL: http://www.dnr.wa.gov/geology/esweek/webtrips.htm Accessed 8 December 2004.
ES 767 Quaternary Geology
Earth Science Department
Emporia State University
Created 12 December 2004, to meet the requirements of ES 767 Quaternary Geology.
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