Lake Missoula: Spokane to Missoula

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay

Acknowledgement

I need to make it crystal clear that none of this is my own research. Thepictures on this and associated pages were taken on a GSA field trip in 2003 ledby Norm Smyers of the U. S. Forest Service and Roy Breckenridge of the Idaho Geological Survey, and the interpretationspresented here are largely those of Dr. Smyers and Dr. Breckenridge as presented on the field trip andits guidebook. I thank Dr. Smyers and Dr. Breckenridge for an outstanding fieldexperience.

Below is a shaded relief map showing Glacial Lake Missoula (Dark blue) assuming a shoreline of 1280 meters, the most commonly used elevation. Also shown are Interstate highways and other highways traveled on this trip. Large modern lakes are in black: clockwise from lower left they are Lake Coeur d'Alene, Lake Pend Oreille, and Flathead Lake. Although there can be no doubt a huge and deep lake was impounded here, specifics are tantalizingly hard to come by.

Paucity of organic detritus makes dating of features impossible for the most part.
The convoluted outline of the lake means there were few areas where long wave fetch could cut prominent shorelines. As a result, shorelines are frequently subtle and specific shorelines cannot be correlated over long distances.
Even the maximum water elevation is up for debate, with some researchers going as high as 1340 meters.
The lack of datable shorelines means we cannot say much about shoreline deformation and glacio-isostasy. Unlike Lake Bonneville, Lake Missoula did not have a single large basin but a number of smaller, though deep, interconnected basins. Also, the edge of the Cordilleran Ice Sheet in this area was largely made up of valley glaciers. Since the loads were mostly smaller than the typical 100-km radius of crustal stiffness, the isostatic effect was probably small.

I traveled much of this route, in different directions, in June, 2003,looking for Lake Missoula features as I went. I was surprised at not being able to see them most of the time, and those I did see were faint. Having taken this trip, I feel better about my inability to see features. In many places, nobody can see them. Unlike the Scablands, Lake Missoula features are subtle.

My route in June 2003 took me past Lake Coeur d'Alene and Flathead Lake. I assumed, just passing by, that like many large Western lakes they were artificial. In fact Lake Coeur d'Alene, Lake Pend Oreille, and Flathead Lake are all natural. Lake Coeur d'Alene is dammed by Missoula flood deposits while Lake Pend Oreille and Flathead Lake are dammed by moraines.

Glacial Lake Missoula

The locations of glaciers are somewhat generalized, and except for the Laurentide Ice Sheet on the high plains, no attempt has been made to show glaciers that did not affect Lake Missoula. The gap between the mountains and the Laurentide Ice Sheet is the southern end of the so-called Ice-Free Corridor,the most likely route for humans to enter southern North America. Below is an index map identifying major features of the area.

Glacial Lake Missoula

Spokane

Views of the Spokane River and 1974 World's Fair site.




	On I-90 just east of Spokane. The water tower is on a bedrock hill and the ridge left of the hill is a pendant bar. At their peak the flood waters would have been about to the top of the picture.

1.1 Ross Point and Rathdrum Prairie

Ross Point is the only significant hill north of I-90 between the east sideof Spokane and Coeur d'Alene. It offers an overview of the floods outburst area.

Above is a map of the outburst area. The flood gravel fan is in yellow and anaverage Lake Columbia in light blue. Of course large amounts of gravel pouredinto the lake as well. Almost all the lakes were dammed by flood gravels,including Lake Coeur d'Alene.

	Looking northeast from I-90 toward the flood outburst area, just about at the Idaho state line. The low hill just visible at the extreme right is the stop location.



	Left and below: our group.

1.2 Lake Coeur d'Alene

The gravel dam at Lake Coeur d'Alene is not very dramatic. A rest area just east of the knob in the middle of the north shore offers views of the lake.

The map above is a southward continuation of the map for stop1. Scablandscarved by the overflow of Lake Columbia are shown in light brown. Flood waterspouring into the Coeur d'Alene Valley found a passage over the drainage divideat 780 meters and poured down into Latah Creek (light green).

Wallace, Idaho

1.3 Taft Interchange

	The water here would have been very deep but shoreline features are all but impossible to see, not surprising considering this would have been a long, narrow, protected arm. Shoreline features do show up on aerial photos of burned-over areas. Lake Missoula would be a good prospect for LIDAR imaging. The yellow trees are western larch and perfectly healthy. They shed their needles in the fall.

1.4 Tarkio Flood Gravels

1.5 Lake Missoula Rhythmites

	Views northwest with I-90 crossing the river. Horizontal stripes on the distant bare summit are probably shorelines.


	Views of the rhythmites at successively smaller scales. These are not varves but probably reflect some sort of regular, maybe annual, sediment input. Total absence of datable material in this silt hampers chronology severely.


	Dropstone in fine silt. The quarter is resting on a thin gravel interbed. Gravel interbeds have been interpreted as possible results of subaerial exposure and stream deposition during low level post-drainage intervals
	Two thin gravel interbeds are visible just below center as thin darker layers.
	Convoluted silt in cross-bedded fine gravel. There appears to be an intact thin silt layer between two gravel units. Are the silt masses in the gravel rip-up clasts?

1.6 Lake Missoula Shorelines

	hillsnorthwest of Missoula with a number of faint shorelines.
	Mount Jumbo from Interstate 90. Numerous shorelines are apparent. This hill is also shown below.
Below: views of the shorelines from the University campus. I-90 passes just north of the campus and through a narrow pass (Hell Gate Pass). The hill with the L and the large castle-like marking (the result of weed eradication, not any intentional design) is Mount Jumbo north of I-90, and the hill with the M is Mount Sentinel, south of I-90. The shorelines are quite obvious. The very straight gullies going down the slopes have been described as "mysterious." They have been explained as the possible result of debris flows triggered when sudden drainage of the lake left water-saturated hillslopes unsupported. I personally don't see anything more mysterious here than water flowing downhill - these are rather planar slopes and gullies would be expected to be straight.

References

HARLEN BRETZ, J, THE LAKE MISSOULA FLOODS AND THE CHANNELED SCABLAND Journal of Gology, Vol. 77, NO 5, P 505-543, SEPT 1969.

Gary A. Smith, Missoula flood dynamics and magnitudes inferred from sedimentology of slack-water deposits on the Columbia Plateau, Washington; Geological Society of America Bulletin; January 1993; v. 105; no. 1; p. 77-100

Richard B. Waitt Case for periodic, colossal joekulhlaups from Pleistocene glacial Lake Missoula Geological Society of America Bulletin; October 1985; v. 96; no. 10; p. 1271-1286

Brian F. Atwater, Periodic floods from glacial Lake Missoula into the Sanpoil Arm of glacial Lake Columbia, northeastern Washington; Geology; August 1984; v. 12; no. 8; p. 464-467

John Shaw, Mandy Munro-Stasiuk, Brian Sawyer, Claire Beaney, Jerome-Etienne Lesemann, Alberto Musacchio, Bruce Rains, and Robert R. Young, The Channeled Scabland; back to Bretz? Geology; July 1999; v. 27; no. 7; p. 605-608

John J. Clague, Rene Barendregt, Randolph J. Enkin and Franklin F. Foit, Jr., Paleomagnetic and tephra evidence for tens of Missoula floods in southern Washington; Geology; March 2003; v. 31; no. 3; p. 247-250; DOI: 10.1130/0091-7613(2003)031<0247:PATEFT>2.0.CO;2 � 2003 Geological Society of America (Geological Society of America)

J Harlen Bretz, Harold Theodore Uhr Smith, and George E. Neff, Channeled scabland of Washington; new data and interpretations; Geological Society of America Bulletin; August 1956; v. 67; no. 8; p. 957-1049

Gerardo Benito, and Jim E. O'Connor, Number and size of last-glacial Missoula floods in the Columbia River valley between the Pasco Basin, Washington, and Portland, Oregon; Geological Society of America Bulletin; May 2003; v. 115; no. 5; p. 624-638; DOI: 10.1130/0016-7606(2003)115<0624:NASOLM>2.0.CO;2

Jim E. O'Connor, and Victor R. Baker, Magnitudes and implications of peak discharges from glacial Lake Missoula; Geological Society of America Bulletin; March 1992; v. 104; no. 3; p. 267-279

Ira Shimmin Allison, New version of the Spokane flood, Geological Society of America Bulletin; August 1933; v. 44; no. 4; p. 675-722

Montana Joseph Thomas Pardee, Unusual currents in glacial Lake Missoula; Geological Society of America Bulletin; November 1942; v. 53; no. 11; p. 1569-1599

Bruce N. Bjornstad, Karl R. Fecht, and Christopher J. Pluhar, Long History of Pre-Wisconsin, Ice Age Cataclysmic Floods: Evidence from Southeastern Washington State; The Journal of Geology, volume 109 (2001), pages 695�713 DOI: 10.1086/323190

Eric V. McDonald, and Alan J. Busacca, Record of pre-late Wisconsin giant floods in the channeled scabland interpreted from loess deposits; Geology August 1988; v. 16; no. 8; p. 728-731

Waitt, R.B., 1994, Scores of gigantic, successively smaller Lake Missoula floods through channeled scabland and Columbia Valley, Chapter 1K in Swanson, D.A. and Haugerud, R.A., eds., Geologic field trips in the Pacific Northwest: (1994 Geological Society of America Annual Meeting), Seattle, University of Washington, 88 p.

Norman B. Smyers1, Roy M. Breckenridge, 2003; Glacial Lake Missoula, Clark Fork ice dam, and the floods outburst area: Northern Idaho and western Montana; Geological Society of America Field Guide 4: Western Cordillera and Adjacent Areas

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Created 9 April 2003, Last Update 07 June 2020