Editing Willamette River (section)

==Geology==
The Willamette River basin was created primarily by [[plate tectonics]] and [[volcanism]] and was altered by erosion and sedimentation, including deposits from enormous glacial floods as recent as 13,000&nbsp;years ago.<ref name="Wallick">Wallick, ''et al.'', pp. 495–97</ref> The oldest rocks beneath the Willamette Valley are the [[Siletz River Volcanics]].<ref name=orr2012>Orr (2012), pp. 188–89</ref> About 35&nbsp;million years ago, these rocks were [[Subduction|subducted]] by the [[Farallon Plate]] beneath the [[North American Plate]], creating the [[forearc]] basin that would later become the Willamette Valley.<ref name = "Wallick"/><ref name=orr2012/><ref name = "Benke 616">Benke, ''et al.'', p. 616</ref> The valley was initially part of the [[continental shelf]], rather than a separate inland sea.<ref>Orr (1999), p. 203</ref> Many layers of marine deposits formed in the forearc basin and cover the older Siletz River Volcanics.<ref name=orr2012/> About 20 to 16&nbsp;million years ago, [[Tectonic uplift|uplift]] formed the Coast Range and separated the basin from the Pacific Ocean.<ref name = "Wallick"/>

[[Basalt]]s of the [[Columbia River Basalt Group]], from eruptions primarily in [[eastern Oregon]], flowed across large parts of the northern half of the basin about 15&nbsp;million years ago.<ref name = "Wallick"/> They covered the [[Tualatin Mountains]] (West Hills), most of the [[Tualatin Valley]], and the slopes of hills farther south, with up to {{convert|1000|ft|m}} of [[lava]].<ref>Orr (1999), p. 206</ref> Later deposits covered the basalt with up to {{convert|1000|ft|m}} of silt in the Portland and Tualatin basins.<ref>Orr (1999), p. 208</ref> During the [[Pleistocene]], beginning roughly 2.5&nbsp;million years ago, volcanic activity in the [[Cascade Range|Cascades]] combined with a cool, moist climate to produce further heavy sedimentation across the basin, and [[braided river]]s created [[alluvial fan]]s spreading down from the east.<ref name = "Wallick"/>

[[File:Erratic rock Oregon.JPG|thumb|left|alt=Erratic Rock State Natural Site|The glacial Bellevue Erratic at [[Erratic Rock State Natural Site]]. The rock was transported to the Willamette Valley by the [[Missoula Floods]].<ref name="Erratic Rock">{{cite news |last=Hill |first=Richard L. |title=Project takes on story of region's ice-age cataclysms |newspaper=The Oregonian |date=November 29, 2000 |page=A19}}</ref>]]

Between about 15,500 and 13,000&nbsp;years ago, the [[Missoula Floods]]—a series of large outpourings originating at [[Glacial Lake Missoula]] in Montana—swept down the Columbia River and backfilled the Willamette watershed.<ref name = "Wallick" /> Each flood produced "discharges that exceeded the annual discharge of all the present-day rivers of the world combined".<ref name="Benke 616"/> Filling the Willamette basin to depths of {{convert|400|ft|m}} in the Portland region, each flood created a temporary lake, [[Lake Allison]], that stretched from Lake Oswego to near Eugene.<ref name = "Orr 1999 212-13">Orr (1999), pp. 212–13</ref> The ancestral Tualatin Valley, part of the Willamette basin, flooded as well; water depths ranged from {{convert|200|ft|m}} at Lake Oswego to {{convert|100|ft|m}} as far upstream (west) as [[Forest Grove, Oregon|Forest Grove]].<ref name = "Orr 1999 212-13"/> Flood deposits of silt and clay, ranging in thickness from {{convert|115|ft|m}} in the north to about {{convert|15|ft|m}} in the south, settled from this muddy water to form today's valley floor.<ref name = "Wallick"/> The floods carried Montana icebergs well into the basin, where they melted and dropped [[glacial erratic]]s onto the land surface. These rocks, composed of granite and other materials common to central Montana but not to the Willamette Valley, include more than 40&nbsp;boulders, each at least {{convert|3|ft|m|1}} in diameter.<ref name = "Orr 1999 214">Orr (1999), p. 214</ref> Before being partly chipped away and removed, the largest of these originally weighed about {{convert|160|ST|MT}}.<ref name = "Orr 1999 214"/>

The northern part of the watershed is underlain by a network of [[Fault (geology)|faults]] capable of producing earthquakes at any time, and many small quakes have been recorded in the basin since the mid-19th&nbsp;century.<ref name = "Orr 1999 215-219">Orr (1999), pp. 215–19</ref> In 1993, the [[1993 Scotts Mills earthquake|Scotts Mills earthquake]]—the largest recent earthquake in the valley, measuring 5.6 on the [[Richter magnitude scale|Richter scale]]—was centered near [[Scotts Mills, Oregon|Scotts Mills]], about {{convert|34|mi|km}} south of Portland.<ref>{{cite web |last=Ludwin |first=R. S. |author2=Qamar, A. I. |author3=Malone, S. D. |author4=Crosson, R. S. |author5=Moran, S. |author6=Thomas, G. C. |author7=Steele, W. P. |title=Scotts Mills Earthquake Summary |url=http://assets.pnsn.org/notable/ScottsMills/scottsmills.html |publisher=University of Washington Department of Earth and Space Sciences |date=October 11, 1996 |access-date=September 5, 2014 |archive-url=https://web.archive.org/web/20160628205405/http://assets.pnsn.org/notable/ScottsMills/scottsmills.html |archive-date=June 28, 2016 |url-status=live}}</ref> It caused $30&nbsp;million in damage, including harm to the [[Oregon State Capitol]] in Salem.<ref>{{cite web |title=Earthquake Education: Historic Earthquakes in the Pacific Northwest |url=http://www.oregongeology.org/earthquakes/HistoricEQs.htm |publisher=Oregon Department of Geology and Mineral Industries |archive-url=https://web.archive.org/web/20180529051931/http://www.oregongeology.org/earthquakes/HistoricEQs.htm |archive-date=May 29, 2018 |url-status=dead}}</ref> Evidence suggests that massive quakes of 8 or more on the Richter scale have occurred historically in the [[Cascadia subduction zone]] off the Oregon coast, most recently in [[1700 Cascadia earthquake|1700&nbsp;CE]], and that others as strong as 9 on the Richter scale occur every 500 to 800&nbsp;years.<ref>{{cite web |title=Hazard Mitigation Plan |publisher=City of Troutdale |url=http://www.ci.troutdale.or.us/documents/Hazard_Mitigation_Plan_Final.pdf |archive-url=https://web.archive.org/web/20140724064319/http://www.ci.troutdale.or.us/documents/Hazard_Mitigation_Plan_Final.pdf |archive-date=July 24, 2014 |pages=10–1–10–27 |date=October 25, 2010}}</ref> The basin's high population density, its nearness to this subduction zone, and its loose soils, which tend to amplify shaking, make the Willamette Valley especially vulnerable to damage from strong earthquakes.<ref name = "Orr 1999 215-219"/>