Missoula floods: Difference between revisions

{{Infobox floodweather event

| caption = [[Glacial Lake Columbia]] (west) and [[Glacial Lake Missoula]] (east) are shown south of the [[Cordilleran ice sheet]]. The areas inundated in the Columbia and Missoula floods are shown in red.

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]] hydrologist Jim O'Connor and [[Spain's]] [[Museo Nacional de Ciencias Naturales]] 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 m³/s, 13 times that of the [[Amazon River]]).<ref name="hill">{{cite news|url=http://www.oregonlive.com/O/relationships/index.ssf?/base/living/1212706534232180.xml&coll=7 |archive-url=https://web.archive.org/web/20080804005511/http://www.oregonlive.com/O/relationships/index.ssf?%2Fbase%2Fliving%2F1212706534232180.xml&coll=7 |archive-date=2008-08-04 |title=Science writer Richard Hill gives a brief geologic history of the Columbia River Gorge |access-date=2008-06-15 |work=The Oregonian }}</ref><ref name=Lehnigk>{{cite journal|url-statuslast1=deadLehnigk|first1=KE|last2=Larsen|first2=IJ|year=2022|title=Pleistocene megaflood discharge in Grand Coulee, Channeled Scabland, USA|journal=Journal of Geophysical Research: Earth Surface|volume=127|issue=1 |page=e2021JF006135|doi=10.1029/2021JF006135|bibcode=2022JGRF..12706135L |s2cid=245545657 |doi-access=free}}</ref> Alternate estimates for the peak flow rate of the largest flood includerange up to 17 cubic kilometers per hour.<ref name=Bjornstad>{{cite book | last =Bjornstad | first =Bruce N. | title =On the trail of the Ice Age floods : a geological field guide to the mid-Columbia basin / Bruce Bjornstad. | publisher =Keokee Books | date =c. 2006 | location =Sandpoint, Idaho | page =2 | isbn = 978-1-879628-27-4}}</ref> andThe rangemaximum upflow tospeed 60approached cubic36 kilometersmeters/second per(130&nbsp;km/h houror 80&nbsp;mph).<ref name = "Allen"Bjornstad/>{{cite book | last =Allen | first =John Eliot |author2=Burns, Marjorie |author3=Sargent, Sam C. | title =Cataclysms on the Columbia : a layman's guide to the features produced by the catastrophic Bretz floods in the Pacific Northwest | publisher =Timber Press | date =c. 1986 | location =Portland, OR | page =104 | isbn =978-0-88192-067-3}}

[[File:Giant ripple marks.jpg|thumb|upright=1.3|Giant ripple marks at Markle Pass near [[Camas Hot Springs]], Montana, U.S. View towards the northwest.]]

Bretz's view, which was seen as arguing for a [[Catastrophism|catastrophic]] explanation of the geology, ran against the prevailing view of [[Uniformitarianism (science)|uniformitarianism]], and Bretz's views were initially disregarded. The [[Geological Society of America|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, [[Joseph Pardee|J.T. Pardee]], had worked with Bretz and had evidence of an ancient [[Proglacial lake|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]] as the source of the Spokane flood and creator of the channeled scablands.<ref name=Bretz1923>{{cite journal|first=J Harlen|last=Bretz|year=1923|title=The Channeled Scabland of the Columbia Plateau|journal=Journal of Geology|volume=31|pages=617–649|doi=10.1086/623053|issue=8|bibcode = 1923JG.....31..617B |s2cid=129657556}}</ref><ref name=Bretz1925>{{cite journal|first=J Harlen|last=Bretz|year=1925|title=The Spokane flood beyond the Channeled Scablands|journal=Journal of Geology|volume=33|pages=97–115, 236–259|doi=10.1086/623179|issue=2|bibcode = 1925JG.....33...97B |s2cid=140554172}}</ref>

After Pardee studied the canyon of the [[Flathead River]], he estimated that flood waters in excess of {{convert|45|mph}} would be required to roll the largest of the boulders moved by the flood. He estimated the water flow was {{convert|9|mi3/h|km3/h}}, more than the combined flow of every river in the world.<ref name = "Alt">{{cite book | first = David | last = Alt |authorlink=David Alt|author2=Hundman, Donald W. | title = Northwest Exposures: A Geologic History of the Northwest | publisher = Mountain Press | pages=381–390 | year = 1995 | isbn = 978-0-87842-323-1}}</ref> More recent estimates place the flow rate at ''ten times'' the flow of all current rivers combined.<ref name=Bjornstad/>

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.{{Citation requiredneeded|date=May 2021}} 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.{{Citation requiredneeded|date=May 2021}} 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.<ref name= Clague>{{Cite journal | last1 = Clague | first1 = John J. | last2 =Barendregt |first2=Rene | last3=Enkin |first3=Randolph J. | first4=Franklin F., Jr. | last4=Foit | title = Paleomagnetic and tephra evidence for tens of Missoula floods in southern Washington | journal = Geology | volume = 31 | issue = 3| pages = 247–250 | publisher = The Geological Society of America | date = March 2003 | doi = 10.1130/0091-7613(2003)031<0247:PATEFT>2.0.CO;2 |bibcode = 2003Geo....31..247C }}</ref> This process is known as a [[glacial lake outburst flood]], and there is evidence that many such events occurred in the distant past.

As the water emerged from the Columbia River gorge, it backed up again at the {{Convert|1|mi|km|1}} wide ''narrows'' near [[Kalama, Washington]]. Some temporary lakes rose to an elevation of more than {{Convert|400|ft|m|-1|abbr=on}}, flooding the Willamette Valley to [[Eugene, Oregon]], and beyond. Iceberg-rafted [[glacial erratic]]s and erosion features are evidence of these events. Lake-bottom [[sediment]]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 40 to 6027 cubic kilometers per hour (96.5 to 15 cubic miles per hour).<ref name=Bjornstad/><ref name="Allen"Lehnigk/> The maximum flow speed approached 36 meters/second (130&nbsp;km/h or 80&nbsp;mph).<ref name=Bjornstad /> Up to 1.9×10¹⁹ joules of potential energy were released by each flood (the equivalent of 4,500 [[TNT equivalent|megatons of TNT]]). For comparison, this is 90 times more powerful than the most powerful nuclear weapon ever detonated, the 50 -megaton "[[Tsar Bomba]]".<ref>{{Cite web|url=https://www.ctbto.org/specials/testing-times/30-october-1961-the-tsar-bomba|title=30 October 1961 - the Tsar Bomba: CTBTO Preparatory Commission}}</ref><ref name="Allen"/> The cumulative effect of the floods was to excavate {{convert|210|km3}} of loess, sediment and basalt from the Channeled Scablands of eastern Washington and to transport it downstream.<ref name="Allen">{{cite book|last=Allen|first=John Eliot|author2=Marjorie Burns |author2-link=Marjorie Burns |author3=Scott Burns |title=Cataclysms on the Columbia: The Great Missoula Floods|publisher=Ooligan Press|location=Portland, Or.|year=2009|edition=Rev. 2nd|isbn=978-1-932010-31-2}}</ref>{{update inline|date=October 2022}}

The multiple flood hypothesis was first proposed by R.B. Waitt, Jr. in 1980. Waitt argued for a sequence of 40 or more floods.<ref name=Waitt80>{{cite journal|last=Waitt|first=R.B., Jr|year=1980|title=About 40 last-glacial Lake Missoula jökulhlaups through southern Washington|journal=Journal of Geology|volume=88|pages=653–679|doi=10.1086/628553|issue=6|bibcode = 1980JG.....88..653W |s2cid=129144773}}</ref><ref name=Waitt84>{{cite journal|last=Waitt|first=R.B., Jr|year=1984|title=Periodic jökulhlaups from Pleistocene Glacial Lake Missoula—New evidence from varved sediment in northern Idaho and Washington|journal=Quaternary Research|volume=22|issue=1|pages=46–58|doi=10.1016/0033-5894(84)90005-X|bibcode = 1984QuRes..22...46W |s2cid=128700879 }}</ref><ref name=Waitt85>{{cite journal|last=Waitt|first=R.B., Jr|year=1985|title=Case for periodic, colossal jökulhlaups from Pleistocene glacial Lake Missoula|journal=Geological Society of America Bulletin|volume=96|pages=1271–1286|bibcode=1985GSAB...96.1271W|doi=10.1130/0016-7606(1985)96<1271:CFPCJF>2.0.CO;2|issue=10}}</ref> 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 Formation|Touchet bed]] deposits from two successive floods were found to be separated by two layers of volcanic ash ([[tephra]]) with the ash separated by a fine layer of windblown dust deposits, located in a thin layer between sediment layers ten [[rhythmite]]s below the top of the Touchet beds. The two layers of volcanic ash are separated by {{convert|1|-|10|cm|in|1}} of airborne nonvolcanic silt. The tephra is [[Mount St. Helens]] 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.<ref name=Waitt85/>

In comment on the Komatsu analysis, Brian Atwater's teamand colleagues 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]] and through the Columbia Gorge would be expected to produce higher flow resistance and correspondingly higher floods.<ref>{{cite journal|first1=Brian F.|last1=Atwater|first2=Gary A.|last2=Smith|first3=Richard B.|last3=Waitt|title=The Channeled Scabland: Back to Bretz?: Comment and Reply: COMMENT|journal=Geology|date=June 2000|volume=28|page=574|doi=10.1130/0091-7613(2000)28<576:TCSBTB>2.0.CO;2|issue=6|bibcode = 2000Geo....28..574A }}</ref>

The dating for Waitt's proposed separation of layers into sequential floods has been supported by subsequent [[paleomagnetism]] studies, which supportssupport a 30–40 year interval between depositions of Mount St. Helens’Helens' ash, and hence flood events, but do not preclude an up to 60 year interval.<ref name="Clague"/> 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.<ref name=Brunner>{{Cite journal | last = Brunner | first = Charlotte A. |author2=Normark, William R. |author3=Zuffa, Gian G. |author4=Serra, Francesca | title = Deep-sea sedimentary record of the late Wisconsin cataclysmic floods from the Columbia River | journal = Geology | volume = 27 | issue = 5| pages = 463–466 | year = 1999 | doi = 10.1130/0091-7613(1999)027<0463:DSSROT>2.3.CO;2 |bibcode = 1999Geo....27..463B }}</ref>

|last=DietrichZenter

|first=WilliamNick

|title=TrailingDating anthe ApocalypseIce Age Floods

|date=20072023-0901-3005

* [httphttps://purlbooks.galileogoogle.usg.edu/ugafaxcom/J84xNASx1x18xCx36books?id=2do_AAAAIAAJ&dq=channeled+scabland+nummedal&pg=PA1 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 River Drainage Basin|Columbia Basin]], June 5–8, 1978 / sponsored by Planetary Geology Program, Office of Space Science, National Aeronautics and Space Administration ; edited by Victor R. Baker and Dag Nummedal.