Editing Snake River (section)

==Geology==
[[Image:Hells Canyon Dam, Snake River - DPLA - 8521af84128ad4ffab75e7e3814da93f.jpg|thumb|upright|Hells Canyon, the connection between the Snake River Plain and the lower Snake River drainage systems, formed about 2.5&nbsp;million years ago from the overflow of Lake Idaho.|alt=View down a river entering a rocky canyon framed by high cliffs]]
{{See also|Geology of the Pacific Northwest}}
The present-day course of the Snake River was pieced together over millions of years from several formerly disconnected drainage systems. Much of what would become the Pacific Northwest lay under shallow seas until it was uplifted starting about 60&nbsp;million years ago (Ma). The outlet of the ancestral Columbia River to the Pacific was established about 40&nbsp;Ma.<ref>{{cite web |title = The Geologic Story of the Columbia Basin |url = https://www.bpa.gov/power/pl/columbia/4-geology.htm |publisher = Bonneville Power Administration |access-date = June 20, 2017 |archive-url = https://web.archive.org/web/20170708070906/https://www.bpa.gov/Power/pl/columbia/4-geology.htm |archive-date = July 8, 2017 |url-status = live }}</ref><ref>{{cite book|author=Moclock, Leslie |author2=Selander, Jacob|title=Rocks, Minerals, and Geology of the Pacific Northwest|year=2021|publisher=Timber Press |isbn=9781604699159|url=https://books.google.com/books?id=Eb3dDwAAQBAJ}}</ref>{{rp|288}} By about 17&nbsp;Ma, the "Salmon-Clearwater River", or the modern day lower Snake River, flowed west into the Columbia and on to the Pacific. Another ancient river system drained what is now the western Snake River Plain. Some geologists propose that this flowed to the Columbia on a course south of the present-day Blue Mountains, while others propose it drained towards [[Northern California]].<ref name="Reidel"/>{{rp|208–210}}<ref>{{cite book|author=Bingham, Richard T.|title=Plants of the Seven Devils Mountains of Idaho|publisher=U.S. Department of Agriculture|year=1987|url=https://books.google.com/books?id=nAFq1UCnxlcC}}</ref>{{rp|11}} The [[Columbia River Basalt Group|Columbia River basalts]], a series of massive [[flood basalt]] events that engulfed the Columbia Basin and surrounding lands, reshaped the landscape and erased most evidence of the pre-volcanic river channels starting about 17&nbsp;Ma. Erupting from fissures in the southern Columbia Basin, the first basalt flows pushed the ancient Salmon-Clearwater much further north than its present course.<ref name="Reidel">{{cite book|editor=Reidel, Stephen P.|title=The Columbia River Flood Basalt Province|publisher=The Geological Society of America|isbn=9780813724973|year=2013|url=https://books.google.com/books?id=xWpIDgAAQBAJ}}</ref>{{rp|201–208}}

About 12–10&nbsp;Ma, the Blue Mountains region began to experience uplift, raising the basalt layers to form a plateau.<ref>{{cite web|url=https://www.dnr.wa.gov/programs-and-services/geology/explore-popular-geology/geologic-provinces-washington/blue-mountains|title=Blue Mountains|publisher=Washington State Department of Natural Resources|date=|accessdate=January 20, 2024}}</ref> From about 11–9&nbsp;Ma, crustal deformation related to the [[Yellowstone hotspot]] caused the western half of the Snake River Plain to sink, creating a [[graben]]-type valley between parallel fault zones to the northeast and southwest.<ref name="Lifton"/> The outlet of the ancient Snake River was blocked, and water accumulated to form the vast [[Lake Idaho]] starting about 10 Ma.<ref name="Lifton">{{cite web|url=https://www.usgs.gov/observatories/yvo/news/snake-river-plain-a-tale-two-basins|title=The Snake River Plain: A Tale of Two Basins|publisher=U.S. Geological Survey|author=Lifton, Zach|date=November 21, 2022|accessdate=December 18, 2023}}</ref> The eastern half of the Snake River Plain formed as the [[North American Plate]] moved westward over the Yellowstone hotspot. Upwelling [[magma]] caused the continental crust to rise, forming highlands in a similar fashion to the modern Yellowstone plateau and leaving behind enormous basalt flows in its wake. As the hotspot migrated east relative to the North American Plate, the land behind it collapsed and sank, creating the geographic depression of the eastern Snake River Plain.<ref>{{cite web|url=https://earthobservatory.nasa.gov/images/35549/snake-river-plain-idaho|title=Snake River Plain, Idaho|publisher=NASA Earth Observatory|date=August 29, 2008|accessdate=December 18, 2023}}</ref><ref>{{cite web|url=https://digitalgeology.aws.cose.isu.edu/Digital_Geology_Idaho/Module11/mod11.htm|title=Neogene Snake River Plain-Yellowstone Volcanic Province|publisher=Idaho State University|author=Link, Paul|accessdate=December 18, 2023}}</ref>

[[Image:Columbia_River_Flood-Basalt_Province.jpg|thumb|right|The Columbia River Basalt Province covers a vast area of the inland Pacific Northwest.|alt=Map showing the extent of the Columbia River Basalt Province, a volcanic geologic province of the inland Pacific Northwest]]
The gradual eastward migration of this topographic high had the effect of pushing the Continental Divide to the east. Prior to the formation of the eastern Snake River Plain, the drainage area east of about [[Arco, Idaho]] – the modern headwaters and upper course of the Snake River – flowed towards the [[Atlantic Ocean]] via the [[Mississippi River]] system.<ref name="Wood and Clemens"/> The migrating Continental Divide tilted the regional slope such that drainage flowed west into Lake Idaho, whose water levels saw a significant increase about 4.5&nbsp;Ma.<ref name="Lifton"/><ref name="Wood and Clemens"/> The Snake River Plain drainage system continued to expand east, towards what is now Yellowstone National Park. During this expansion, the Snake also captured the [[Bear River (Great Salt Lake)|Bear River]], which was only rerouted towards its modern outlet in the Great Salt Lake Basin about 50,000 or 60,000 years ago by lava flows in southeast Idaho.<ref name="ISU"/><ref>{{cite journal|title=The Bear River's History and Diversion: Constraints, Unsolved Problems, and Implications for the Lake Bonneville Record|journal=Developments in Earth Surface Processes|volume=20|year=2016|pages=28–59|author1=Pederson, J.L.|author2=Janecke, S.U.|author3=Reheis, M.C.|author4=Kaufman, D.S.|author5=Oaks, R.Q. Jr.|doi=10.1016/B978-0-444-63590-7.00002-0|url=https://www.sciencedirect.com/science/article/abs/pii/B9780444635907000020|accessdate=December 18, 2023}}</ref>

In the Columbia Basin about 10.5&nbsp;Ma, the Elephant Mountain basalt eruption forced the Salmon-Clearwater River into roughly its present course through southeast Washington.<ref name="Reidel"/>{{rp|221–222}}<ref>{{cite book|title=Floods, Faults, and Fire: Geological Field Trips in Washington State and Southwest British Columbia|publisher=Geological Society of America|editor=Tucker, David Samuel |editor2=Stelling, Peter L.|year=2007|isbn=978-0-8137-0009-0 |url=https://books.google.com/books?id=F1Sx3i2CZyUC}}</ref>{{rp|237}} By 8.5&nbsp;Ma the Salmon-Clearwater was established in the Columbia River's modern path through [[Wallula Gap]], although the Columbia itself still flowed somewhere to the west. The last of the Columbia basalt flows occurred around 6&nbsp;Ma; by then, the present-day confluence of the Columbia River and Salmon-Clearwater had been established, with the combined flow draining through Wallula Gap.<ref name="Reidel"/>{{rp|222–223}} About 2.5&nbsp;Ma, Lake Idaho reached a maximum elevation of {{convert|3600|ft|m}} above modern sea level, and overflowed northward into the Salmon-Clearwater drainage near present-day [[Huntington, Oregon]]. Over a period of about two million years, the outflow carved Hells Canyon, emptying Lake Idaho and integrating the upper Snake and Salmon-Clearwater into a single river system.<ref name="Lifton"/><ref name="Wood and Clemens">{{cite web|url=https://idwr.idaho.gov/wp-content/uploads/sites/2/projects/north-ada-county/199807-WOODCLEM-2002.pdf|title=Geologic and Tectonic History of the Western Snake River Plain, Idaho and Oregon|author=Wood, Spencer H. |author2=Clemens, Drew M.|publisher=Idaho Department of Water Resources|year=2002|accessdate=December 18, 2023}}</ref>

The Teton Range, a defining topographic feature of the modern Snake River headwaters, first began to rise about 10&nbsp;Ma as the Teton Fault began to move, displacing the mountain block upward as the surrounding land dropped.<ref>{{cite web|url=https://www.usgs.gov/geology-and-ecology-of-national-parks/geology-grand-teton-national-park|title=Geology of Grand Teton National Park
|publisher=U.S. Geological Survey|accessdate=December 18, 2023}}</ref> About 2&nbsp;Ma, the Hoback Fault formed east of the Tetons, and a graben valley developed between the Hoback and Teton fault zones, creating Jackson Hole.<ref name="Jackson Hole geology">{{cite web|url=https://www.nps.gov/parkhistory/online_books/grte/grte_geology/sec9.htm|title=Quaternary – Time of Ice, More Lakes, and Continued Crustal Disturbance|work=Creation of the Teton Landscape: The Geologic Story of Grand Teton National Park|publisher=U.S. National Park Service|author=Love, J.D. |author2=Reed, John C.|year=1971|accessdate=December 18, 2023}}</ref> As the valley dropped, water filled it to create Lake Teewinot, which drained east into the [[Green River (Colorado River tributary)|Green River]]–[[Colorado River]] system. About 1 Ma, the Snake River captured the Jackson Hole watershed, draining Lake Teewinot and finally connecting the modern-day Snake headwaters to the rest of the river.<ref name="ISU">{{cite web|url=https://www.isu.edu/media/libraries/college-of-science-and-engineering/geology/digital-geology-of-idaho/srptopo_development.pdf|title=Snake River Plain Topographic Development|publisher=Idaho State University|date=|accessdate=December 18, 2023}}</ref> This landscape around the Snake headwaters was sculpted by multiple Ice Age glaciations. Starting about 200,000 years ago, the Buffalo glaciation filled Jackson Hole to a depth of {{convert|2000|ft|m}}. Ice flowed down the Snake River Canyon all the way to Idaho.<ref name="Jackson Hole geology"/> The Bull Lake glaciation, about 80,000–35,000 years ago, and the Pinedale glaciation, ending about 15,000 years ago, were much smaller and did not fill the entire valley. These glaciations carved the distinctive peaks of the Tetons into their present form and scoured lake basins in the valley floor, including modern-day Jackson Lake.<ref name="Jackson Hole geology"/>

While the Snake River course beyond Jackson Hole was not directly impacted by glaciations, its landscape was dramatically changed by Ice Age flooding events. About 30,000 years ago, the climate of western North America was much wetter than today. The Great Salt Lake Basin filled with water to form the massive [[Lake Bonneville]], about the size of modern-day [[Lake Michigan]]. About 15,000 years ago the lip of [[Red Rock Pass (Idaho)|Red Rock Pass]] south of present-day [[Pocatello, Idaho]] abruptly collapsed, releasing a tremendous volume of water from Lake Bonneville into the Snake River Plain. The peak of the flood was about 500 times bigger than the largest recorded flood of the Snake at Idaho Falls in modern times.<ref name="isu bonneville">{{cite web|url=https://www.isu.edu/digitalgeologyidaho/bonneville/|title=Lake Bonneville flood|publisher=Idaho State University|work=Digital Geology of Idaho|date=|accessdate=December 18, 2023}}</ref> The flood completely altered the landscape of the Snake River Plain, creating the Snake River Canyon and its waterfalls, vast boulder fields, cliffs and [[coulee]]s.<ref name="isu bonneville"/><ref>{{cite web|url=https://digitalatlas.cose.isu.edu/hydr/lkbflood/lbf.htm|title=The Lake Bonneville Flood|publisher=Idaho State University|accessdate=December 18, 2023}}</ref> The floodwaters then emptied through Hells Canyon; however, most evidence of their effects on the lower Snake River was erased by the much larger [[Missoula Floods]] that engulfed the Columbia Basin during the same period.<ref name="isu bonneville"/> Caused by the repeated collapse of an ice dam in western Montana, dozens of floods overflowed into the lower Snake River from the north, backing water as far upstream as Lewiston. The formerly west-flowing Palouse River was rerouted to flow south into the Snake River, forming Palouse Falls, whose outsized [[plunge pool]] attests to the force of the floods.<ref>{{cite web|url=https://www.isu.edu/digitalgeologyidaho/palouse-lake-missoula/|title=Palouse and glacial Lake Missoula|publisher=Idaho State University|work=Digital Geology of Idaho|date=|accessdate=December 18, 2023}}</ref><ref>{{cite web|url=https://parks.wa.gov/sites/default/files/2023-07/Ice%20Age%20Floods%20Brochure.pdf|title=Ice Age Floods|publisher=Washington State Parks|date=April 13, 2022|accessdate=December 18, 2023}}</ref>