Editing Missouri River (section)

==Geology==
[[File:Missouri-Mississippi confluence.jpg|thumb|upright|High [[silt]] content makes the Missouri River (left) noticeably lighter than the [[Mississippi River]] (right) at their confluence north of [[St. Louis, Missouri|St. Louis]].|alt=Top down view of two rivers merging, one dark and clear and the other light with clouds of sediment]]

The Rocky Mountains of southwestern Montana at the [[headwaters]] of the Missouri River first rose in the [[Laramide Orogeny]], a [[Mountain formation|mountain-building]] episode that occurred from around 70 to 45&nbsp;million years ago (the end of the [[Mesozoic]] through the early [[Cenozoic]]).<ref name="cenozoic map">{{cite web |last=Jones |first=Craig H. |title=Photo map of the western United States: Cenozoic |url=http://www.colorado.edu/GeolSci/Resources/WUSTectonics/PhotoMap.html |website=Western US Tectonics |publisher=University of Colorado |access-date=February 12, 2011 |archive-url=https://web.archive.org/web/20160831064247/http://www.colorado.edu/geolsci/Resources/WUSTectonics/PhotoMap.html |archive-date=August 31, 2016 |url-status=dead}}</ref> This [[orogeny]] [[Tectonic uplift|uplifted]] [[Cretaceous]] rocks along the western side of the [[Western Interior Seaway]], a vast shallow sea that stretched from the Arctic Ocean to the Gulf of Mexico, and deposited the sediments that now underlie much of the drainage basin of the Missouri River.<ref>
{{cite web
 |url          = http://jan.ucc.nau.edu/~rcb7/crepaleo.html
 |title        = Cretaceous Paleogeography, Southwestern US
 |publisher    = Northern Arizona University
 |access-date  = October 1, 2010
 |archive-url  = https://web.archive.org/web/20100820080046/http://jan.ucc.nau.edu/~rcb7/crepaleo.html
 |archive-date = August 20, 2010
 |url-status   = dead
 |df           = mdy-all
}}
</ref><ref>
{{cite report
 |last1=Nicholls |first1=Elizabeth L.
 |last2=Russell |first2=Anthony P.
 |title=Paleobiogeography of the Cretaceous Western Interior Seaway: the vertebrate evidence
 |publisher=University of Calgary
 |series=Department of Biological Sciences
 |date=September 18, 1989  <!--|access-date=February 12, 2011-->
}}
</ref><ref>{{harvp|King|1971|pp=27–28}}</ref>
This Laramide uplift caused the sea to retreat and laid the framework for a vast drainage system of rivers flowing from the Rocky and [[Appalachian Mountains]], the predecessor of the modern-day Mississippi watershed.<ref>{{harvp|King|1971|pp=130–131}}</ref><ref>{{harvp|Baldridge|2004|pp=190–204}}</ref><ref>{{harvp|Roberts|Hodsdon|2001|pp=113–116}}</ref> The Laramide Orogeny is essential to modern Missouri River [[hydrology]], as snow and ice melt from the Rockies provide the majority of the flow in the Missouri and its tributaries.<ref>{{harvp|Benke|Cushing|2005|p=434}}</ref>

The Missouri and many of its tributaries cross the Great Plains, flowing over or cutting into the [[Ogallala Aquifer|Ogallala Group]] and older mid-Cenozoic sedimentary rocks. The lowest major Cenozoic unit, the [[White River Formation]], was deposited between roughly 35 and 29&nbsp;million years ago<ref name=laramide_cliamte>
{{cite journal
|author=Chapin, Charles E.
|title=Interplay of oceanographic and paleoclimate events with tectonism during middle to late Miocene sedimentation across the southwestern USA
|journal=Geosphere
|year=2008
|volume=4 |issue=6 |page=976
|doi=10.1130/GES00171.1
|bibcode=2008Geosp...4..976C
|doi-access=free
}}</ref><ref name=wyoming_white_river>{{cite web |author1=Love, J.D. |author2=Christiansen, Ann Coe |title=White River Formation |url=https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=WYTwr;0 |website=Mineral Resources On-Line |series=Spatial Data |publisher=U.S. Geological Survey |access-date=February 12, 2011 |archive-url=https://web.archive.org/web/20140808113037/http://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=WYTwr%3b0|archive-date=August 8, 2014 |url-status=live}}</ref> and consists of [[claystone]], [[sandstone]], [[limestone]], and [[Conglomerate (geology)|conglomerate]].<ref name=wyoming_white_river/><ref>{{cite web |author1=Denson, N.M. |author2=Gill, J.R. |author3=Roberts, A.E. |title=White River Formation |url=https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=MTTwr;0 |website=Mineral Resources On-Line |series=Spatial Data |publisher=U.S. Geological Survey|access-date=February 12, 2011 |archive-url=https://web.archive.org/web/20120312015307/http://tin.er.usgs.gov/geology/state/sgmc-unit.php?unit=MTTwr;0 |archive-date=March 12, 2012 |url-status=dead}}</ref> Channel sandstones and finer-grained [[overbank deposit]]s of the [[fluvial]]<ref name="gs_arikaree">{{cite web |author1=Martin, J.E. |author2=Sawyer, J.F. |author3=Fahrenbach, M.D. |author4=Tomhave, D.W. |author5=Schulz, L.D. |title=Arikaree Group |url=https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=SDTa;0 |website=Mineral Resources On-Line |series=Spatial Data |publisher=U.S. Geological Survey |access-date=February 12, 2011 |archive-url=https://web.archive.org/web/20140808094838/http://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=SDTa%3b0 |archive-date=August 8, 2014 |url-status=live }}</ref> [[Arikaree Group]] were deposited between 29 and 19&nbsp;million years ago.<ref name=laramide_cliamte/> The [[Miocene]]-age Ogallala and the slightly younger [[Pliocene]]-age Broadwater Formation deposited atop the Arikaree Group, and are formed from material eroded off of the Rocky Mountains during a time of increased generation of topographic relief;<ref name=laramide_cliamte/><ref>
{{cite journal
 |author=McMillan, Margaret E.
 |year=2002
 |title=Postdepositional tilt of the Miocene-Pliocene Ogallala Group on the western Great Plains: Evidence of late Cenozoic uplift of the Rocky Mountains
 |journal=Geology 
 |volume=30  |issue=1  |page=63
 |doi=10.1130/0091-7613(2002)030<0063:PTOTMP>2.0.CO;2
}}
</ref> these formations stretch from the Rocky Mountains nearly to the Iowa border and give the Great Plains much of their gentle but persistent eastward tilt, and also constitute a major aquifer.<ref>{{harvp|King|1971|pp=128–130}}</ref>

Immediately before the [[Quaternary]] [[Quaternary glaciation|Ice Age]], the Missouri River was likely split into three segments: an upper portion that drained northwards into Hudson Bay,<ref>{{cite report
|last=Moak |first=William
|url=http://maps.unomaha.edu/maher/2003mbfieldtrip/moakreport/pleistgeo.html
|title=Pleistocene Glaciation and Diversion of the Missouri River in Northern Montana
|publisher=University of Nebraska, Omaha
|series=Department of Geography and Geology
|access-date=October 1, 2010
|archive-url=https://web.archive.org/web/20120415090611/http://maps.unomaha.edu/maher/2003mbfieldtrip/moakreport/pleistgeo.html
|archive-date=April 15, 2012
|url-status=dead
}}
</ref><ref>
{{cite web
|url=http://www3.northern.edu/natsource/HABITATS/Missio1.htm
|title=Missouri River
|publisher=Northern State University
|access-date=October 12, 2010
|archive-url=https://web.archive.org/web/20130523094749/http://www3.northern.edu/natsource/HABITATS/Missio1.htm
|archive-date=May 23, 2013
|url-status=dead
}}
</ref>
and middle and lower sections that flowed eastward down the regional slope.<ref>{{harvp|Thornbury|1965|pp=248–249, 295–296}}</ref>
As the Earth plunged into the Ice Age, a [[pre-Illinoian]] (or possibly the [[Illinoian]]) [[Glacial period|glaciation]] diverted the Missouri River southeastward toward its present confluence with the Mississippi and caused it to integrate into a single river system that cuts across the regional slope.<ref>{{harvp|Thornbury|1965|pp=248–249}} and references cited there</ref> In western Montana, the Missouri River is thought to have once flowed north then east around the [[Bear Paw Mountains]]. [[Sapphire]]s are found in some spots along the river in western Montana.<ref name=gemgallerymtloc>{{cite web |title=Montana Sapphires |series=Gemology |url=http://www.gemgallery.com/#montana_sapphire_gemology |website=Gem Gallery |access-date=October 29, 2011 |archive-url=https://web.archive.org/web/20111028044536/http://www.gemgallery.com/#montana_sapphire_gemology |archive-date=October 28, 2011 |url-status=live}} Note: Includes map of major Montana sapphire mines</ref><ref>{{cite book |last=Voynick |first=Stephen M. |title=Yogo The Great American Sapphire |url=https://books.google.com/books?id=MHHMAAAACAAJ |publisher=Mountain Press Publishing |orig-year=1985 |year=1987 |edition=March 1995 print |place=Missoula, MT |isbn=978-0-87842-217-3 |page=193}}</ref> Advances of the continental ice sheets diverted the river and its tributaries, causing them to pool up into large temporary lakes such as Glacial Lakes [[Glacial Lake Great Falls|Great Falls]], Musselshell and others. As the lakes rose, the water in them often spilled across adjacent local drainage divides, creating now-abandoned channels and [[coulee]]s including the [[Shonkin Sag]], {{convert|100|mi|km}} long. When the glaciers retreated, the Missouri flowed in a new course along the south side of the Bearpaws, and the lower part of the Milk River tributary took over the original main channel.<ref>
{{cite web
|author=Moak, William
|url=http://maps.unomaha.edu/maher/2003mbfieldtrip/moakreport/pleistgeo.html
|title=Pleistocene Glaciation and Diversion of the Missouri River in Northern Montana
|publisher=University of Nebraska, Omaha
|series=Department of Geography and Geology
|access-date=June 28, 2011
|archive-url=https://web.archive.org/web/20120415090611/http://maps.unomaha.edu/maher/2003mbfieldtrip/moakreport/pleistgeo.html
|archive-date=April 15, 2012
|url-status=dead
}}
</ref>

The Missouri's nickname, the "Big Muddy", was inspired by its enormous loads of [[sediment]] or silt – some of the largest of any North American river.<ref name="Spotlight"/><ref>{{harvp|Benke|Cushing|2005|pp=432–434}}</ref> In its pre-development state, the river [[Sediment transport|transported]] some {{convert|175|to|320|e6ST|e6t|abbr=off|sp=us}} per year.<ref>{{cite web
|url=http://www.mostreamteam.org/Documents/Research/BigRiver/MissouriRiverSediment.pdf
|title=Missouri River Sediment
|website=Missouri River Stream Team Website
|series=Missouri River Recovery Plan Fact Sheet, U.S. Army Corps of Engineers
|access-date=October 6, 2010
|archive-url=https://web.archive.org/web/20120407071907/http://www.mostreamteam.org/Documents/Research/BigRiver/MissouriRiverSediment.pdf
|archive-date=April 7, 2012
|url-status=live
}}</ref> The construction of dams and levees has drastically reduced this to {{convert|20|to|25|e6ST|e6t|abbr=off|sp=us}} in the present day.<ref name="sediment-management"/> Much of this sediment is derived from the river's [[floodplain]], also called the meander belt; every time the river changed course, it would erode tons of soil and rocks from its banks. However, damming and channeling the river has kept it from reaching its natural sediment sources along most of its course. Reservoirs along the Missouri trap roughly {{convert|36.4|e6ST|e6t|abbr=off|sp=us}} of sediment each year.<ref name="MainstemSystem"/> Despite this, the river still transports more than half the total silt that empties into the Gulf of Mexico; the [[Mississippi River Delta]], formed by sediment deposits at the mouth of the Mississippi, constitutes a majority of sediments carried by the Missouri.<ref name="sediment-management">
{{cite web
|url=http://dels.nas.edu/Report/Missouri-River-Planning-Recognizing-Incorporating/13019
|title=Missouri River Planning: Recognizing and Incorporating Sediment Management (2010)
|publisher=The National Academies
|series=Division on Earth and Life Studies
|access-date=October 7, 2010
|archive-url=https://web.archive.org/web/20120602143710/http://dels.nas.edu/Report/Missouri-River-Planning-Recognizing-Incorporating/13019
|archive-date=June 2, 2012
|url-status=live
}}
</ref><ref>
{{cite news
|last=Schliefstein |first=Mark
|url=http://www.nola.com/politics/index.ssf/2010/09/missouri_river_helped_build_lo.html
|title=Missouri River helped build Louisiana coast, but it won't help restore it
|website=New Orleans Net
|date=September 29, 2010
|access-date=October 6, 2010
|archive-url=https://web.archive.org/web/20101002013903/http://www.nola.com/politics/index.ssf/2010/09/missouri_river_helped_build_lo.html
|archive-date=October 2, 2010
|url-status=dead
}}
</ref>