Editing Columbia River (section)

== Geology ==
{{See also|Geology of the Pacific Northwest|Missoula Floods}}

[[File:Drum-Heller-Channels.jpg|thumb|alt=Reddish-brown terrain and many small green bushes surround a lake. Truncated ridges of dark rock run across the terrain parallel to the horizon and to each other beneath a blue sky.|[[Drumheller Channels]], part of the [[Channeled Scablands]] formed by the [[Missoula Floods]]]]

When the [[rift]]ing of [[Pangaea]], due to the process of [[plate tectonics]], pushed North America away from Europe and Africa and into the [[Panthalassic Ocean]] (ancestor to the modern Pacific Ocean), the Pacific Northwest was not part of the continent. As the North American continent moved westward, the [[Farallon Plate]] [[subduction|subducted]] under its western margin. As the plate subducted, it carried along [[island arc]]s which were accreted to the North American continent, resulting in the creation of the Pacific Northwest between 150 and 90&nbsp;million years ago.{{sfn|Bishop|2003|pp=13–14}} The general outline of the Columbia Basin was not complete until between 60 and 40&nbsp;million years ago, but it lay under a large inland sea later subject to uplift.<ref name="bonneville geology">{{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> Between 50 and 20&nbsp;million years ago, from the [[Eocene]] through the [[Miocene]] eras, tremendous volcanic eruptions frequently modified much of the landscape traversed by the Columbia.{{sfn|Bishop|2003|pp=86, 128}} The lower reaches of the ancestral river passed through a valley near where [[Mount Hood]] later arose. Carrying sediments from erosion and erupting volcanoes, it built a {{convert|2|mi|km|adj=on}} thick delta that underlies the foothills on the east side of the [[Northern Oregon Coast Range|Coast Range]] near [[Vernonia, Oregon|Vernonia]] in northwestern Oregon.{{sfn|Bishop|2003|p=98}} Between 17&nbsp;million and 6&nbsp;million years ago, huge outpourings of [[flood basalt]] lava covered the [[Columbia River Plateau]] and forced the lower Columbia into its present course.{{sfn|Bishop|2003|pp=132, 150}} The modern Cascade Range began to uplift 5 to 4&nbsp;million years ago.<ref name="Burke Museum">{{cite web |title = The Cascade Episode |url = https://www.bpa.gov/power/pl/columbia/4-geology.htm |publisher = Burke Museum |year = 2016 |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> Cutting through the uplifting mountains, the Columbia River significantly deepened the Columbia River Gorge.{{sfn|Bishop|2003|p=195}}

The river and [[Columbia River Drainage Basin|its drainage basin]] experienced some of the world's greatest known catastrophic floods toward the end of the last [[Quaternary glaciation|ice age]]. The periodic rupturing of ice dams at [[Glacial Lake Missoula]] resulted in the Missoula Floods, with discharges exceeding the combined flow of all the other rivers in the world, dozens of times over thousands of years.<ref name="Burke Museum" /> The exact number of floods is unknown, but geologists have documented at least 40; evidence suggests that they occurred between about 19,000 and 13,000&nbsp;years ago.{{sfn|Bishop|2003|pp=226–29}}

[[File:ColumbiaRGorgePano.jpg|thumb|left|alt=A wide river curves gently at the base of a mountain range. A meadow in the foreground gives way to an evergreen forest and then to the river. In the background, a layer of thin clouds veils a blue sky.|Panoramic view of Columbia River Gorge from [[Dog Mountain]] in Washington]]

The floodwaters rushed across eastern Washington, creating the [[channeled scablands]], which are a complex network of dry canyon-like channels, or coulees that are often [[braided river|braided]] and sharply gouged into the basalt rock underlying the region's deep topsoil. Numerous flat-topped [[butte]]s with rich soil stand high above the chaotic scablands.{{sfn|Stelling|Tucker|2007|pp=213–14, 230}} Constrictions at several places caused the floodwaters to pool into large temporary lakes, such as [[Lake Lewis]], in which sediments were deposited. Water depths have been estimated at {{convert|1000|ft|m}} at [[Wallula Gap]]{{sfn|Bishop|2003|p=227}} and {{convert|400|ft|m}} over modern Portland, Oregon.{{sfn|Houck|Cody|2000|p=19}} Sediments were also deposited when the floodwaters slowed in the broad flats of the Quincy, Othello, and Pasco Basins.{{sfn|Stelling|Tucker|2007|pp=213–14, 230}} The floods' periodic inundation of the lower Columbia River Plateau deposited rich sediments; 21st-century farmers in the Willamette Valley "plow fields of fertile Montana soil and clays from Washington's Palouse".{{sfn|Bishop|2003|p=227}}

Over the last several thousand years a series of large landslides have occurred on the north side of the Columbia River Gorge, sending massive amounts of debris south from [[Table Mountain (Skamania County, Washington)|Table Mountain]] and [[Greenleaf Peak]] into the gorge near the present site of Bonneville Dam. The most recent and significant is known as the [[Bonneville Slide]], which formed a massive earthen dam, filling {{convert|3.5|mi|km}} of the river's length.<ref name=Hill-1999>{{cite news |first = Richard L. |last = Hill |title = Radiocarbon Dates Indicate the Bonneville Landslide May Be Far Younger Than Thought |work = [[The Oregonian]] |date = September 9, 1999 |url = http://landslides.usgs.gov/recent/archives/1999bonneville.php |access-date = September 7, 2009 |url-status = dead |archive-url = https://web.archive.org/web/20100923023338/http://landslides.usgs.gov/recent/archives/1999bonneville.php |archive-date = September 23, 2010 |df = mdy }}</ref><ref name=reynolds>{{cite journal |last = Reynolds |first = Nathaniel D. |date = December 2001 |title = Dating the Bonneville Landslide with Lichenometry |journal = Washington Geology |volume = 29 |issue = 3/4 |pages = 11–16 |url = http://www.dnr.wa.gov/Publications/ger_washington_geology_2001_v29_no3-4.pdf |access-date = September 7, 2009 |archive-url = https://web.archive.org/web/20150630032045/http://www.dnr.wa.gov/Publications/ger_washington_geology_2001_v29_no3-4.pdf |archive-date = June 30, 2015 |url-status = live }}</ref> Various studies have placed the date of the Bonneville Slide anywhere between 1060 and 1760 AD; the idea that the landslide debris present today was formed by more than one slide is relatively recent and may explain the large range of estimates.<ref name=reynolds /> It has been suggested that if the later dates are accurate there may be a link with the [[1700 Cascadia earthquake]].<ref name=reynolds /><ref name=Hill-2002>{{cite news |first = Richard L. |last = Hill |title = Science&nbsp;– Landslide Sleuths |work = The Oregonian |date = May 15, 2002 |url = https://earthquake.usgs.gov/regional/pacnw/paleo/greateq/20020515.html |access-date = September 7, 2009 |archive-url = https://web.archive.org/web/20080906210151/http://earthquake.usgs.gov/regional/pacnw/paleo/greateq/20020515.html |archive-date = September 6, 2008 |url-status = dead }}</ref> The pile of debris resulting from the Bonneville Slide blocked the river until rising water finally washed away the sediment. It is not known how long it took the river to break through the barrier; estimates range from several months to several years.<ref name=oconnor>{{cite journal |last = O'Connor |first = Jim E. |date = September 2004 |title = The Evolving Landscape of the Columbia River Gorge: Lewis and Clark and Cataclysms on the Columbia |journal = [[Oregon Historical Quarterly]] |volume = 105 |issue = 3 |pages = 390–421 |doi = 10.1353/ohq.2004.0043 |jstor = 20615448  |s2cid = 131976728 }}</ref> Much of the landslide's debris remained, forcing the river about {{convert|1.5|mi|km}} south of its previous channel and forming the [[Cascade Rapids]].<ref name=norman>{{cite web |last1 = Norman |first1 = David K. |last2 = Roloff |first2 = Jaretta M. |title = A Self-Guided Tour of the Geology of the Columbia River Gorge—Portland Airport to Skamania Lodge, Stevenson, Washington |publisher = [[Washington Department of Natural Resources]], Division of Geology and Earth Resources |date = March 2004 |url = http://www.dnr.wa.gov/Publications/ger_ofr2004-7_geol_tour_columbia_river_gorge.pdf |access-date = September 8, 2009 |archive-url = https://web.archive.org/web/20170131051201/http://file.dnr.wa.gov/publications/ger_ofr2004-7_geol_tour_columbia_river_gorge.pdf |archive-date = January 31, 2017 |url-status = live }}</ref> In 1938, the construction of Bonneville Dam inundated the rapids as well as the remaining trees that could be used to refine the estimated date of the landslide.<ref name=norman />{{sfn|Rybář|Stemberk|Wagner|2002|p=695}}

In 1980, the [[1980 eruption of Mount St. Helens|eruption of Mount St.&nbsp;Helens]] deposited large amounts of sediment in the lower Columbia, temporarily reducing the depth of the shipping channel by {{convert|26|ft|m}}.<ref>{{cite web |url = http://pubs.usgs.gov/fs/2000/fs036-00/ |title = Mount St. Helens |publisher = United States Geological Survey |access-date = September 10, 2008 |archive-url = https://web.archive.org/web/20130512162409/http://pubs.usgs.gov/fs/2000/fs036-00/ |archive-date = May 12, 2013 |url-status = live }}</ref>