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Nicholas Misner
GEOL-309: Geomorphology
6-May-2016
Glacial Outburst Flooding as a Geomorphological Force in the Pacific Northwest
I. Introduction
I. a. The Story of Bretz
In 1909, during the ages of early formal geology, Joseph Pardee mapped the extent of the glacial
lake, and identified the ancient location of a glacial ice dam in the southern trench. But even then, the true
story had yet to be totally reconciled. It was not until J. Harlen Bretz-- a local geomorphologist--
theorized the origin of the Channeled Scablands that the pieces began coming together. The Scablands
feature islands of fertile, loessic soils interweaved with ancient stream channels scoured through the land
and down to the basaltic bedrock beneath with features of rock basins, giant ripple marks, and ancient
waterfall ledges interspersed throughout. As he studied them at length, he was the first to recognize that
the features formed due to a flood of truly gigantic proportions that occurred about 18,000 to 20,000 years
ago (Thomsen, 1974). However, in these times of the field, catastrophism was heresy, and Bretz was all
but laughed out of the Geological Study of America when he presented his paper in 1923. Even after
Joseph Pardee, who mapped the extent of Lake Missoula back in 1909, published another paper in 1942,
the scale of these floods was unbelievable. Only in 1962, during a field trip by the Quaternary Research
Congress, did senior members recognize the formations as indeed being the result of enormous floods
(Waltham, 2010). An apologetic telegram was sent shortly after.
Since then, it has been accepted that the Glacial Outburst Flooding of Glacial Lake Missoula had
drastic effects on the geomorphology of the Pacific Northwest, specifically in the areas surrounding the

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Columbia River valley. But this extraordinary claim of cyclic lake-draining took extraordinary claims;
what evidence has been gathered to address these claims to make them so well-respected in modern
geomorphology?
I. b. Formation of Lake Missoula
The sheer scale of this event is difficult to imagine, and how it came to be took many decades to
fully understand. During the Last Glacial Maximum, in the Pacific Northwest of the United States, a
second major glacier extended into the region of Washington state, Idaho, and Oregon from the
present-day Canadian Province of British Columbia. Aside from the Laurentide Ice Sheet, which spread
out of eastern Canada, the Cordilleran Ice Sheet spread from western Canada. During its advances in the
Quaternary period, a section of the Cordilleran called the Purcell Ice dammed upstream and downstream
along a major tributary of the Columbia River contained by the Bitterfoot Range to the west, the

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Continental Divide to the south, and the Rocky Mountains to the east. Once the river was backed up,
water began to gather in the river valley, finding no passage through the topography surrounding the
valley. Some sections of the growing lake reached 600m deep, and the whole of the backed-up region
reached about 2170km​3​
in volume, a little more than the combined volumes of modern Lakes Erie and
Ontario; this was Glacial Lake Missoula.
Overlooking the town of
Missoula is a hillside scored by multiple
terraces, the ones first recognized by
Professor T.C. Chamberlain back in
1886. Terraces are lake or river
formations created by erosion of
lakeshores. To find these features here
was intriguing. These terraces had to
have been formed by successive
shoreline formations during its glacial lake formation (Waltham, 2010). Varves-- seasonal bed formations
seen in lakes that produce high-resolution, annual-scale layering --formed in a unit of laminated sediment
located in Ninemile Creek, which formed in the basin of the ancient lake, show a gradual
upwards-trending decrease in thickness, indicating a progressive increase in the lake’s depth (Hanson, et
al., 2015).
Lakes formed by ice-damming, however, are far from permanent. Water eats away at the ice over
time, as it also begins to undercut the formation. The ice, then, is lifted up from its location and
continually eroded as the water simultaneously carves into it from above. And, after enough time, the
entirety of the dam can fail simultaneously.

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II. Glacial Outburst Flooding
At once, the entirety of Glacial Lake Missoula began to drain through the Eddy Narrows.
Examining the size of transported boulders here yields incredible results for the velocity: portions of the
flood reached speeds up to 80km/hr. By looking at the amount of soil stripped off the sides of the valleys,
the cross-section can be determined. Together, these values estimate a flood discharge of 38km​3​
/hr
(9.12mi​3​
/hr), which exceeds the modern Amazon River’s discharge 600 times over. The flood pulses
resulted in some incredible formations, such as in modern Camas Prairie, which is located nearby a
northern arm of Lake Missoula. Enormous standing waves were generated here, forming huge ripples in
the bedload of gravel- and cobble-size sediments 30m high and 100m apart (Waltham, 2010).
Examining paleomagnetic properties of the sediment in the ancient lakebed reveals its
depositional environment. At Ninemile Creek, there is a roadcut that features 583 varved sedimentary
layers, altogether documenting an approximately 1500yr record. Two of the properties examined in
paleomagnetism are the natural remanent magnetization,
meaning how much of the magnetic effects of a previous
environment remain, and the induced magnetization.
These are weighed against one another in the
Königsberger ratio (Q), as remanence against the induced
magnetization. This ratio represents the efficiency of the
sample to acquire magnetic properties, and if Q>1, the
sediment was deposited in a low-energy environment.
What was found in Ninemile Creek is that all samples
taken indicate a low-energy depositional environment,

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aside from samples taken from early-stage lake filling. Times with Q<1 indicate stages of turbidity
currents and rapid sedimentation (McDonald, et al., 2012).
Water roaring out the Eddy Narrows travelled west, down the Columbia River and into a second
ice-dammed lake: Glacial Lake Columbia. What can be seen here, pertaining to the outburst flooding
itself, is a sequence of varved lake silts and clays along Latah Creek, south of Spokane. This sequence is
interrupted by gravel horizons whenever there was another outburst flood from Lake Missoula, but there
are a total of sixteen horizons, indicating a cyclic filling-flooding process in the Lake (Waltham, 2010).
III. Glacial Lake Columbia and the Channeled Scablands
As the floodwaters escaped into Lake Columbia, they continued to find no easy escape. As the
flood continued filling Lake Columbia, nothing could exit the usual outlet since it, too, was blocked by
ice-damming from the Okanogan Lobe of the Cordilleran Ice Sheet. Instead, the waters found every other
possible outlet south, through what are now the Channeled Scablands, and west through two coulees,
which is a local name for a dry, rocky gorge that was carved out by large floods. It is likely that, at the
time, the Grand Coulee and Moses Coulee were shadows of their present selves, and the floodwaters
carved them out (Waltham, 2010).
Manila Creek is now located in the north-western arm of Glacial Lake Columbia and features a
roadcut with up to 46 flood beds. These formations are characterized by soft-sediment deformation
structures indicative of rapid deposition. Additional paleomagnetic studies here feature a lower
Königsberger ratio, which is expected in ripple/soft-sediment deformation, and is characteristic of very
rapid sedimentation. This is consistent with what is expected of a lake experiencing a very sudden, very
violent influx of sediment from a massive flood event. Additionally, sediments were studied by Hanson,
et al. for the S-ratio, which is a measure of low-coercivity ferromagnetic grains to high-coercivity grains.
Coercivity is the resistance of a sample to becoming demagnetized in the presence of another magnetic

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field. A consistent decreasing in the S-ratio during times corresponding to the tail-end of flooding is
attributable to the deposition of fine-grained hematite sourced from Glacial Lake Missoula (Hanson, et al.,
2015).
Waters from the flooding headed south of Lake Columbia as it overflowed, however. The
stratigraphy of the region at the time of the floods began with Miocene-era flood basalts between 30Ma
and 10Ma (Thomsen, 1974), topped with Pleistocene-era loess, approximately 30-80m thick. But when
the floodwaters reached this region, it stripped away all but the highest-elevation lousse down to the
basaltic bedrock, leaving only islands of loess behind (Waltham, 2010). McDonald, et al. studied three
roadcuts in the region of the Channeled Scablands, and two of the loessic layers present in the Columbia
River Valley region (informally, L1 and L2, the former being the most recently deposited) to better date
the local formations. They note that flood-cut unconformities related to flooding during Oxygen Isotope
Stage 2 (OIS-2, ~29ka Before Present, roughly correspondent to the Last Glacial Maximum) or before
preserved in the loessic islands would take three forms: truncation of features in the local Washtucna soil,
deposition of basaltic gravel lenses, and deposition of ice rafted debris. Hanson, et al. present three
roadcut sites that they studied: one near Connell, one near Ritzville, and one labelled the Washtucna-9
site.
Their Connell site features an unconformity correlative to OIS-4 (~71ka B.P.), specifically as
erosion and truncation of the underlying basalt, and the presence of basaltic cobble and gravel., Near
Ritzville, there is an unconformity in the Old Maid Coulee paleosol filled with basaltic lenses, indicating
OIS-4 flooding. Additionally, above the Washtucna Paleosol are additional basaltic cobbles, indicating
OIS-2 flooding as well. The Washtucna-9 site, however, features more than a dozen loessic units and/or
paleosols, along with several unconformities, making it a complex unit to study. It is characterized by
eolian deposition and by erosion in a lengthy historical frame. Flood-cut unconformities are present at
Stage 2 and Stage 4. Using locally-deposited tephra, or ejected volcanic sediments, to bracket the ages of

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L1 and L2, the McDonald study used luminescence ages, and geochemical ages to determine that the
flood-cut unconformities occurred 46,000 to 77,000 years ago, which is consistent with the Glacial
Outburst Floods associated with the penultimate advancement of the Cordilleran Ice Sheet.
IV. Bottlenecked Temporary Lakes Lewis and Condon
During particularly large floods, a temporary lake was formed due to hydraulic damming south of
Lake Columbia: Lake Lewis. Bedded, fine-grained sediments called slackwater sediment and ice-rafted
debris can be found up to 365m above sea level, or ~250m of lake depth. The lake formed as water
struggled to make its way through the Wallula Gap, backing up along its tributaries far to the north,
despite emptying at 40mi​3​
/day (Waltham, 2010).
Even once the floodwater made it through the Wallulla Gap, it reached yet another bottleneck at
the Columbia Gorge, forming a second temporary lake: Lake Condon. This lake also reached depths up to
250m deep before draining through the Portland Basin. Pulses there were up to 100m deep, which
overflowed into the Willamette Valley to form one, final temporary lake, appropriately named Lake
Willamette. At this point, the suspended load of the floodwaters was quite fine, and once deposited here,
the sediment contributed to the excellent soils in the region. Also dropped into this valley, however, are
isolated boulders that feature more angular profiles. These boulders were actually dropped off of icebergs
that floated the full 800km downstream to Lake Willamette (Waltham, 2010).

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V. Pacific Ocean Outflow
After escaping Lake Willamette, the floodwaters traversed the last 100km to reach the Pacific
Ocean, at the time being an approximately 10km-longer journey than present-day. Studies of marine
sediment core tops off the Washington state coast display an abundance of a particular variety of
freshwater algae, called diatoms. Additionally, these cores were analyzed for paleosalinity, to find
evidence for the emptying of the freshwater floods into the region. Paleosalinity levels were revealed to
be astoundingly low during
certain periods: 17.5, 20, 23, 27,
and 30.5ka before present. A
relationship was drawn for this
region between the abundance of
freshwater diatom density and
salinity level reductions. Using
diatom densities in the marine
sediment cores revealed a
reduction in local paleosalinity
up to 6 Practical Salinity Units
(psu) relative to the background
values of approximately 32.5psu
(Lopes and Mix, 2009).

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VI. Conclusion
Stratigraphic evidence clearly delineates the formation of a very large, very deep lake in the
relatively recent past (Waltham, 2010; Thomsen, 1974). Dating of the loess formations in the Pacific
Northwest confirm their deposition during the Quaternary period using several dating methods in
conjunction with the study of several roadcuts in the region. These same methods connect the
rejuvenation of the windwblown sediment with the violent Lake Missoula floods (McDonald, et al.,
2012). Additionally, studying the paleomagnetism of the deposited sediments in the Columbia River
valley corroborate their provenance in the ancient glacial lake (Hanson, et al., 2015). Paleosalinity records
also complete the story, displaying great decreases in seawater salinity due to an influx of freshwater off
the Washington state coast in locations consistent with paleoceanographic models of the local water flow
(Lopes and Mix, 2009). Evidence all along the path of the Lake Missoula floodwaters all lines up to
support J. Harlen Bretz’s original claims and beyond.

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References
Hanson, M.A., Enkin, R.J., Barendregt, R.W., and Clague, J.J., 2015, Provenance and deposition of
glacial Lake Missoula lacustrine and flood sediments determined from rock magnetic properties:
Quaternary Research, v. 83, p. 166-177, doi: 10.1016/j.yqres.2014.09.005.
Lopes, C., and Mix, A.C., 2009, Pleistocene megafloods in the Northeast Pacific: Geology [Boulder], v.
37, p. 79-82, doi: 10.1130/G25025A.1.
McDonald, E.V., Sweeney, M.R., and Busacca, A.J., 2012, Glacial outburst floods and loess
sedimentation documented during Oxygen Isotope Stage 4 on the Columbia Plateau, Washington
State: Quaternary Science Reviews, v. 45, p. 18-30, doi: 10.1016/j.quascirev.2012.03.016.
Thomsen, D.E., 1974, The Day the Dam Burst: Science News, v. 106, p. 250-251.
Waltham, T., 2010, Lake Missoula and the Scablands, Washington, USA: Geology Today, v. 26, p.
152-158, doi: 10.1111/j.1365-2451.2010.00763.x.