The Missoula floods (also known as the Spokane floods or the Bretz floods or Bretz's floods) were cataclysmic
glacial lake outburst flood
A glacial lake outburst flood (GLOF) is a type of outburst flood caused by the failure of a dam containing a glacial lake. An event similar to a GLOF, where a body of water contained by a glacier melts or overflows the glacier, is called a jĆ ...
s that swept periodically across eastern
Washington
Washington commonly refers to:
* Washington (state), United States
* Washington, D.C., the capital of the United States
** A metonym for the federal government of the United States
** Washington metropolitan area, the metropolitan area centered o ...
and down the
Columbia River Gorge
The Columbia River Gorge is a canyon of the Columbia River in the Pacific Northwest of the United States. Up to deep, the canyon stretches for over as the river winds westward through the Cascade Range, forming the boundary between the st ...
at the end of the last
ice age
An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gre ...
. These floods were the result of periodic sudden ruptures of the ice dam on the
Clark Fork River that created
Glacial Lake Missoula
Lake Missoula was a prehistoric proglacial lake in western Montana that existed periodically at the end of the last ice age between 15,000 and 13,000 years ago. The lake measured about and contained about of water, half the volume of Lake Mic ...
. After each ice dam rupture, the waters of the lake would rush down the Clark Fork and the
Columbia River, flooding much of eastern Washington and the
Willamette Valley
The Willamette Valley ( ) is a long valley in Oregon, in the Pacific Northwest region of the United States. The Willamette River flows the entire length of the valley and is surrounded by mountains on three sides: the Cascade Range to the eas ...
in western
Oregon
Oregon () is a state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington, while the Snake River delineates much of its eastern boundary with Idaho. T ...
. After the lake drained, the ice would reform, creating Glacial Lake Missoula again.
These floods have been researched since the 1920s. During the last
deglaciation that followed the end of the
Last Glacial Maximum, geologists estimate that a cycle of flooding and reformation of the lake lasted an average of 55 years and that the floods occurred several times over the 2,000-year period between 15,000 and 13,000 years ago.
U.S. Geological Survey
The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, and ...
hydrologist Jim O'Connor and
Museo Nacional de Ciencias Naturales
The National Museum of Natural Sciences ( es, Museo Nacional de Ciencias Naturales) is the national museum of natural history of Spain. It is situated in the center of Madrid, by the Paseo de la Castellana. It is managed by the Spanish National Re ...
scientist Gerardo Benito have found evidence of at least twenty-five massive floods, the largest discharging about 10 cubic kilometers per hour (2.7 million mĀ³/s, 13 times that of the
Amazon River).
Alternate estimates for the peak flow rate of the largest flood include 17 cubic kilometers per hour
and range up to 27 cubic kilometers per second.
The maximum flow speed approached 36 meters/second (130 km/h or 80 mph).
[
Within the ]Columbia River drainage basin
The Columbia River drainage basin is the drainage basin of the Columbia River in the Pacific Northwest region of North America. It covers . In common usage, the term often refers to a smaller area, generally the portion of the drainage basin that ...
, detailed investigation of the Missoula floods' glaciofluvial deposits
Glaciofluvial deposits or Glacio-fluvial sediments consist of boulders, gravel, sand, silt and clay from ice sheets or glaciers.
They are transported, sorted and deposited by streams of water.
The deposits are formed beside, below or downstream ...
, informally known as the ''Hanford formation'', has documented the presence of Middle and Early Pleistocene Missoula flood deposits within the Othello Channels, Columbia River Gorge, Channeled Scabland, Quincy Basin, Pasco Basin, and the Walla Walla Valley
The Walla Walla River is a tributary of the Columbia River, joining the Columbia just above Wallula Gap in southeastern Washington in the United States. The river flows through Umatilla County, Oregon, and Walla Walla County, Washington. Its drai ...
. Based on the presence of multiple interglacial calcretes interbedded with flood deposits, magnetostratigraphy
Magnetostratigraphy is a geophysical correlation technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout the section. The samples are analyzed to determine their '' ...
, optically stimulated luminescence
In physics, optically stimulated luminescence (OSL) is a method for measuring doses from ionizing radiation. It is used in at least two applications:
* Luminescence dating of ancient materials: mainly geological sediments and sometimes fired pott ...
dating, and unconformity
An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...
truncated clastic dike
A clastic dike is a seam of sedimentary material that fills an open fracture in and cuts across sedimentary rock strata or layering in other rock types. Clastic dikes form rapidly by fluidized injection (mobilization of pressurized pore fluids) ...
s, it has been estimated that the oldest of the Pleistocene
The Pleistocene ( , often referred to as the ''Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed in ...
Missoula floods happened before 1.5 million years ago. Because of the fragmentary nature of older glaciofluvial deposits, which have been largely removed by subsequent Missoula floods, within the Hanford formation, the exact number of older Missoula floods, which are known as ''ancient cataclysmic floods'', that occurred during the Pleistocene cannot be estimated with any confidence.[Medley, E. (2012) ''Ancient Cataclysmic Floods in the Pacific Northwest: Ancestors to the Missoula Floods.'' Unpublished Masters thesis, Portland State University, Portland, Oregon. 174 pp.][Spencer, P. K., and M. A. Jaffee (2002) ''Pre-Late Wisconsinan Glacial Outburst Floods in Southeastern Washingtonā€”The Indirect Record.'' Washington Geology. vol. 30, no. 1/2, pp. 9ā€“16.]
Flood hypothesis proposed
Geologist
A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, althoug ...
J Harlen Bretz
J Harlen Bretz (2 September 1882 ā€“ 3 February 1981) was an American geologist, best known for his research that led to the acceptance of the Missoula Floods and for his work on caves.
Early life and education
Bretz was born on 2 September 188 ...
first recognized evidence of the catastrophic floods, which he called the ''Spokane floods'', in the 1920s. He was researching the Channeled Scablands in Eastern Washington
Eastern Washington is the region of the U.S. state of Washington located east of the Cascade Range. It contains the city of Spokane (the second largest city in the state), the Tri-Cities, the Columbia River and the Grand Coulee Dam, the Hanf ...
, the Columbia Gorge
The Columbia River Gorge is a canyon of the Columbia River in the Pacific Northwest of the United States. Up to deep, the canyon stretches for over as the river winds westward through the Cascade Range, forming the boundary between the sta ...
, and the Willamette Valley
The Willamette Valley ( ) is a long valley in Oregon, in the Pacific Northwest region of the United States. The Willamette River flows the entire length of the valley and is surrounded by mountains on three sides: the Cascade Range to the eas ...
of Oregon
Oregon () is a state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington, while the Snake River delineates much of its eastern boundary with Idaho. T ...
. In the summer of 1922, and for the next seven years, Bretz conducted field research of the Columbia River Plateau
The Columbia Plateau is a geologic and geographic region that lies across parts of the U.S. states of Washington, Oregon, and Idaho. It is a wide flood basalt plateau between the Cascade Range and the Rocky Mountains, cut through by the Columbi ...
. He had been interested in unusual erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is dis ...
features in the area since 1910 after seeing a newly published topographic
Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps.
Topography is a field of geoscience and planetary scien ...
map of the Potholes Cataract. Bretz coined the term Channeled Scablands in 1923 to refer to the area near the Grand Coulee
Grand Coulee is an ancient river bed in the U.S. state of Washington. This National Natural Landmark stretches for about 60 miles (100 km) southwest from Grand Coulee Dam to Soap Lake, being bisected by Dry Falls into the Upper and Lower ...
, where massive erosion had cut through basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
deposits. Bretz published a paper in 1923, arguing that the Channeled Scablands in Eastern Washington were caused by massive flooding in the distant past.
Bretz's view, which was seen as arguing for a catastrophic explanation of the geology, ran against the prevailing view of uniformitarianism
Uniformitarianism, also known as the Doctrine of Uniformity or the Uniformitarian Principle, is the assumption that the same natural laws and processes that operate in our present-day scientific observations have always operated in the universe in ...
, and Bretz's views were initially disregarded. The Geological Society of Washington, D.C, invited the young Bretz to present his previously published research at a January 12, 1927, meeting where several other geologists presented competing theories. Another geologist at the meeting, J.T. Pardee, had worked with Bretz and had evidence of an ancient glacial lake that lent credence to Bretz's theories. Bretz defended his theories, and this kicked off an acrimonious 40-year debate over the origin of the Scablands. Both Pardee and Bretz continued their research over the next 30 years, collecting and analyzing evidence that led them to identify Lake Missoula
Lake Missoula was a prehistoric proglacial lake in western Montana that existed periodically at the end of the last ice age between 15,000 and 13,000 years ago. The lake measured about and contained about of water, half the volume of Lake Mic ...
as the source of the Spokane flood and creator of the channeled scablands.
After Pardee studied the canyon of the Flathead River
The Flathead River ( fla, label= Salish, ĨlqĢ“etkŹ· ntxĢ£Ź·etkŹ·, , kut, kananmituk), in the northwestern part of the U.S. state of Montana, originates in the Canadian Rockies to the north of Glacier National Park and flows southwest into Fla ...
, he estimated that flood waters in excess of would be required to roll the largest of the boulders moved by the flood. He estimated the water flow was , more than the combined flow of every river in the world. More recent estimates place the flow rate at ''ten times'' the flow of all current rivers combined.[
The Missoula floods have also been referred to as the ''Bretz floods'' in honor of Bretz.]
Flood initiation
As the depth of the water in Lake Missoula increased, the pressure at the bottom of the ice dam increased enough to lower the freezing point of water below the temperature of the ice forming the dam. This allowed liquid water to seep into minuscule cracks present in the ice dam. Over a period of time, the friction from water flowing through these cracks generated enough heat to melt the ice walls and enlarge the cracks. This allowed more water to flow through the cracks, generating more heat, allowing even more water to flow through the cracks. This feedback cycle eventually weakened the ice dam so much that it could no longer support the pressure of the water behind it, and it failed catastrophically. This process is known as a glacial lake outburst flood
A glacial lake outburst flood (GLOF) is a type of outburst flood caused by the failure of a dam containing a glacial lake. An event similar to a GLOF, where a body of water contained by a glacier melts or overflows the glacier, is called a jĆ ...
, and there is evidence that many such events occurred in the distant past.
Flood events
As the water emerged from the Columbia River gorge, it backed up again at the wide narrows near Kalama, Washington
Kalama (kaw-law-maw) is a city in Cowlitz County, Washington, United States. It is part of the Longview, Washington Metropolitan Statistical Area. The population was 2,959 as of the 2020 census.
Etymology
James W. Phillips' ''Washington State P ...
. Some temporary lakes rose to an elevation of more than , flooding the Willamette Valley to Eugene, Oregon, and beyond. Iceberg-rafted glacial erratic
A glacial erratic is glacially deposited rock differing from the type of rock native to the area in which it rests. Erratics, which take their name from the Latin word ' ("to wander"), are carried by glacial ice, often over distances of hundre ...
s and erosion features are evidence of these events. Lake-bottom sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...
s deposited by the floods have contributed to the agricultural richness of the Willamette and Columbia Valleys. Glacial deposits overlaid with centuries of windblown sediments ( loess) have scattered steep, southerly sloping dunes throughout the Columbia Valley, ideal conditions for orchard and vineyard development at higher latitudes.
After analysis and controversy, geologists now believe that there were 40 or more separate floods, although the exact source of the water is still being debated. The peak flow of the floods is estimated to be 27 cubic kilometers per hour (6.5 cubic miles per hour).[ The maximum flow speed approached 36 meters/second (130 km/h or 80 mph).] Up to 1.9Ć—1019 joules of potential energy were released by each flood (the equivalent of 4,500 megatons of TNT). For comparison, this is 90 times more powerful than the most powerful nuclear weapon ever detonated, the 50-megaton "Tsar Bomba
The Tsar Bomba () ( code name: ''Ivan'' or ''Vanya''), also known by the alphanumerical designation "AN602", was a thermonuclear aerial bomb, and the most powerful nuclear weapon ever created and tested. Overall, the Soviet physicist Andrei ...
". The cumulative effect of the floods was to excavate of loess, sediment and basalt from the Channeled Scablands of eastern Washington and to transport it downstream.
Multiple flood hypothesis
The multiple flood hypothesis was first proposed by R.B. Waitt, Jr. in 1980. Waitt argued for a sequence of 40 or more floods. Waitt's proposal was based mainly on analysis from glacial lake bottom deposits in Ninemile Creek and the flood deposits in Burlingame Canyon. His most compelling argument for separate floods was that the Touchet bed deposits from two successive floods were found to be separated by two layers of volcanic ash (tephra
Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism.
Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they re ...
) with the ash separated by a fine layer of windblown dust deposits, located in a thin layer between sediment layers ten rhythmite
A rhythmite consists of layers of sediment or sedimentary rock which are laid down with an obvious periodicity and regularity. They may be created by annual processes such as seasonally varying deposits reflecting variations in the runoff cycle, b ...
s below the top of the Touchet beds. The two layers of volcanic ash are separated by of airborne nonvolcanic silt. The tephra is Mount St. Helens
Mount St. Helens (known as Lawetlat'la to the indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United St ...
ash that fell in Eastern Washington. By analogy, since there were 40 layers with comparable characteristics at Burlingame Canyon, Waitt argued they all could be considered to have similar separation in deposition time.[
]
Controversy over number and source of floods
The controversy whether the Channeled Scabland landforms were formed mainly by multiple periodic floods, or by a single grand-scale cataclysmic flood from late Pleistocene Glacial Lake Missoula or from an unidentified Canadian source, continued through 1999. Shaw's team of geologists reviewed the sedimentary sequences of the Touchet beds and concluded that the sequences do not automatically imply multiple floods separated by decades or centuries. Rather, they proposed that sedimentation in the Glacial Lake Missoula basin was the result of jƶkulhlaup
A jƶkulhlaup ( ) (literally "glacial run") is a type of glacial outburst flood. It is an Icelandic term that has been adopted in glaciological terminology in many languages.
It originally referred to the well-known subglacial outburst flood ...
s draining into Lake Missoula from British Columbia to the north. Further, Shaw's team proposed the scabland flooding might have partially originated from an enormous subglacial reservoir that extended over much of central British Columbia, particularly including the Rocky Mountain Trench, which may have discharged by several paths, including one through Lake Missoula. This discharge, if occurring concurrently with the breach of the Lake Missoula ice dam, would have provided significantly larger volumes of water. Further, Shaw and team proposed that the rhythmic Touchet beds are the result of multiple pulses, or surges, within a single larger flood.[
]
In 2000, a team led by Komatsu simulated the floods numerically with a 3-dimensional hydraulic model. They based the Glacial Lake Missoula discharge rate on the rate predicted for the Spokane Valley
The Spokane Valley is a valley of the Spokane River through the southern Selkirk Mountains in the U.S. state of Washington.
The valley is home to the cities of Spokane and its suburbs Spokane Valley, Liberty Lake, and Millwood. The valley i ...
ā€“Rathdrum Prairie
The Rathdrum Prairie is a flat in the U.S. state of Idaho. The prairie contains the cities of Coeur d'Alene, Post Falls, Hayden, Rathdrum, State Line, and Huetter. The prairie also contains part of the Spokane Valleyā€“Rathdrum Prairie Aquif ...
immediately downstream of Glacial Lake Missoula, for which a number of previous estimates had placed the maximum discharge of 17 Ć— 106m3/s and total amount of water discharged equal to the maximum estimated volume of Lake Missoula (2184 km3). Neglecting erosion effects, their simulated water flow was based on modern-day topography. Their major findings were that the calculated depth of water in each flooded location except for the Spokane Valleyā€“Rathdrum Prairie was shallower than the field evidence showed. For example, their calculated water depth at the Pasco Basinā€“Wallula Gap transition zone is about 190 m, significantly less than the 280ā€“300 m flood depth indicated by high-water marks. They concluded that a flood of ~106m3/s could not have made the observed high-water marks.
In comment on the Komatsu analysis, Atwater's team observed that there is substantial evidence for multiple large floods, including evidence of mud cracks and animal burrows in lower layers which were filled by sediment from later floods. Further, evidence for multiple flood flows up side arms of Glacial Lake Columbia spread over many centuries have been found. They also pointed out that the discharge point from Lake Columbia varied with time, originally flowing across the Waterville Plateau into Moses Coulee but later, when the Okanagon lobe blocked that route, eroding the Grand Coulee to discharge there as a substantially lower outlet. The Komatsu analysis does not evaluate the impact of the considerable erosion observed in this basin during the flood or floods, although the assumption that the flood hydraulics can be modeled using modern-day topography is an area which warrants further consideration. Earlier narrower constrictions at places such as Wallula Gap
Wallula Gap () is a large water gap of the Columbia River in the northwest United States in southeast Washington. It cuts through the Horse Heaven Hills basalt anticlines in the Columbia River Basin, just south of the confluence of the Wall ...
and through the Columbia Gorge would be expected to produce higher flow resistance and correspondingly higher floods.
The current understanding
The dating for Waitt's proposed separation of layers into sequential floods has been supported by subsequent paleomagnetism studies, which supports a 30ā€“40 year interval between depositions of Mount St. Helensā€™ ash, and hence flood events, but do not preclude an up to 60 year interval. Offshore deposits on the bed of the Pacific at the mouth of the Columbia River include 120 meters of material deposited over a several thousand-year period that corresponds to the period of multiple scabland floods seen in the Touchet Beds. Based on Waitt's identification of 40 floods, this would give an average separation between floods of 50 years.
See also
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References
Further reading
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External links
* USGS Circular 125
The World's Largest Floods, Past and Present: Their Causes and Magnitudes
* PBS
The Public Broadcasting Service (PBS) is an American public broadcaster and non-commercial, free-to-air television network based in Arlington, Virginia. PBS is a publicly funded nonprofit organization and the most prominent provider of educat ...
's NOVA (TV series)
Mystery of the Megaflood
Ice Age Floods Institute (IAFI)
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* ttp://purl.galileo.usg.edu/ugafax/J84xNASx1x18xCx36 The channeled scabland a guide to the geomorphology of the Columbia Basin, Washington : prepared for the Comparative Planetary Geology Field Conference held in the Columbia Basin, June 5ā€“8, 1978 / sponsored by Planetary Geology Program, Office of Space Science, National Aeronautics and Space Administration ; edited by Victor R. Baker and Dag Nummedal.
National Park Service: Ice Age Floods
Sculpted by Floods: The Northwest's Ice Age Legacy (KSPS Documentaries)
{{Columbia River
Floods in the United States
Geology of Montana
Geology of Oregon
Geology of Washington (state)
Columbia River Gorge
Megafloods
Glacial landforms