Field Guides

Downloaded from fieldguides.gsapubs.org on January 17, 2011 Field Guides Cataclysms and controversy−−Aspects of the geomorphology of the Columbia River Gorge Jim E. O'Connor and Scott F. Burns Field Guides 2009;15;237-251 doi: 10.1130/2009.fld015(12) Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Field Guides Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes © 2009 Geological Society of America Downloaded from fieldguides.gsapubs.org on January 17, 2011 The Geological Society of America Field Guide 15 2009 Cataclysms and controversy—Aspects of the geomorphology of the Columbia River Gorge Jim E. O’Connor U.S. Geological Survey, 2130 SW 5th Avenue, Portland, Oregon 97201, USA Scott F. Burns Department of Geology, Portland State University Portland, Oregon 97207, USA ABSTRACT Landslides and fl oods of lava and water tremendously affected the Columbia River during its long history of transecting the Cascade Volcanic Arc. This fi eld trip touches on aspects of the resulting geology of the scenic Columbia River Gorge, including the river-blocking Bonneville landslide of ~550 years ago and the great late- Pleistocene Missoula fl oods. Not only did these events create great landscapes, but they inspired great geologists. Mid-nineteenth century observations of the Colum- bia River and Pacifi c Northwest by James Dwight Dana and John Strong Newberry helped germinate the “school of fl uvial” erosion later expanded upon by the south- western United States topographic and geologic surveys. Later work on features related to the Missoula fl oods framed the career of J Harlen Bretz in one of the great geologic controversies of the twentieth century. INTRODUCTION landslides, including the river-blocking Bonneville landslide of ~550 years ago; and (2) the much drier east end of the gorge, This guide accompanies the “Kirk Bryan” fi eld trip for the where marks of the Missoula Floods have inspired inquiry and October 2009 Annual Meeting of the Geological Society of debate since the early twentieth century. America convened in Portland, Oregon. This one-day fi eld trip Some of the material in this guide derives from our own examines geomorphic features related to the remarkable Neogene published work, including O’Connor and Waitt (1995), Benito and Quaternary history of lava fl ows, landslides, and cataclysmic (1997), Benito and O’Connor (2003), Butler and O’Connor fl ooding in the Columbia River Gorge of Oregon and Washing- (2004), and O’Connor (2004); but much of it relies on a long list ton (Fig. 1). Along the way, the trip passes through landscapes of naturalists and geologists who have traversed this landscape endowed with a rich history of geologic inquiry and controversy, since the 1805–1806 exploration of Lewis and Clark. The guide some of which nourished the very foundations of the “American is structured to provide a general background of the geography School” of geomorphology, exemplifi ed by Kirk Bryan himself. and geology of the river corridor, spiced with a bit of local “roots This trip focuses on two areas: (1) the rainy heart of the of geomorphology.” This introductory section is then followed Columbia River Gorge, where Cascade Range uplift together by brief overviews of landslides in the Columbia River Gorge with Columbia River incision have conspired to create great and the Missoula Floods—the two primary topics of the trip. O’Connor, J.E., and Burns, S.F., 2009, Cataclysms and controversy—Aspects of the geomorphology of the Columbia River Gorge, in O’Connor, J.E., Dorsey, R.J., and Madin, I.P., eds., Volcanoes to Vineyards: Geologic Field Trips through the Dynamic Landscape of the Pacifi c Northwest: Geological Society of America Field Guide 15, p. 237–251, doi: 10.1130/2009.fl d015(12). For permission to copy, contact [email protected]. ©2009 The Geological Society of America. All rights reserved. 237 Downloaded from fieldguides.gsapubs.org on January 17, 2011 238 O’Connor and Burns We then describe the trip itself, with brief information on some many dynamic interactions along the lower river corridor. Volu- of the sights en route and more detailed discussion of the stops minous lava fl ows of the Columbia River Basalt Group (CRBG) themselves. No actual road log is presented, but stop locations issued from vents in northeastern Oregon and adjacent Washing- should be clear from the provided map fi gures, coordinates, and ton and Idaho, mostly between 17 and 12 Ma, blanketing vast the route description. plains of eastern Oregon and Washington, and displacing the Columbia River west against the Washington Cascade Range. Physiography and Geology of the Columbia River Gorge Several of the basalt fl ows found their way to the Pacifi c Ocean through paleovalleys cut through the Cascade Range (Beeson The Columbia River Gorge, extending from the Deschutes et al., 1989). Within the Columbia River Gorge and to the east, River confl uence west to the Portland basin, is Columbia River’s valley walls and, in places, the channel itself, are formed in as long-held path through the active volcanic arc of the Cascade much as 1200 m of this lava. The CRBG is locally overlain by Range. The gorge exists because of the great erosive power of Miocene to Quaternary paleo-Columbia River gravel (Troutdale the Columbia River, which drains 660,480 km2 of western North and Alkali Canyon formations), volcaniclastic deposits shed off America, fl owing 2000 km from its headwaters at Columbia Lake the Cascade Range (Dalles Formation), and young calc-alkaline in southeastern British Columbia, Canada, to its confl uence with lavas and volcaniclastic detritus associated with the modern vol- the northeast Pacifi c Ocean near Astoria, Oregon. In terms of canic arc. Quaternary loess caps most uplands, except where drainage area, the Columbia River is the 39th largest river basin removed by gullies, landslides, the Missoula fl oods, or covered in the world, but ranks higher with respect to mean fl ow, mean by their deposits. Quaternary arc volcanism has emplaced lava elevation, and slope (Vörösmarty et al., 2000). The basin drains fl ows (and cones) along the river corridor as well as directly into several physiographic regions, including the Middle and North- the valley bottom. ern Rocky Mountains, Columbia Plateau, Cascade Range, and The central axis of the Cascade Range has risen 700– Pacifi c Border (Fenneman, 1931). Politically, the basin includes 1000 m during the past 3 m.y. (Tolan and Beeson, 1984; Bee- parts of British Columbia, Canada, most of Idaho, large parts of son and Tolan, 1990), approximately matched by incision of Oregon and Washington, and small areas of Montana, Wyoming, the Columbia River. At the eastern end of the gorge, the CRBG Utah, and Nevada. has been uplifted less (Fig. 2) but is nevertheless deformed by a At low fl ow, the river is tidally affected 235 km upstream to series of east-west trending folds and high-angle reverse faults Bonneville Dam near our fi rst stop (at the historical head of tide). that defi ne many of the broad basins and narrow constrictions the Upstream, the river has been transformed into a series of almost river now fl ows through. continuous lakes by U.S. Army Corps of Engineers dams. On today’s trip, we will pass Bonneville Dam (completed 1937), The Early Geologists and Geomorphologists Dalles Dam (1957), and John Day Dam (1971), in stepwise man- ner pooling the river from near sea level to 84 m (276 ft) above First motivated by a mix of territorial consolidation, curi- sea level (asl). With a mean annual river fl ow of ~5400 m3/s, as osity and new commercial opportunities, European exploration measured since 1879 at The Dalles, these dams provide hydro- began early in the nineteenth century. The fi rst written records power generating capacity totaling 5120 megawatts. Prior to pertaining to the geology and geomorphology were the astute fl ow regulation, annual peak fl ows almost always resulted from observations in the journals of Lewis and Clark, followed by snowmelt in the interior subbasin, attaining the highest known James Dwight Dana (1841), John Strong Newberry (1855), and discharge of 35,100 m3/s on 7 June 1894 at The Dalles. Thomas Condon (1850s). Further exploration included brief A major river draining the interior western North America forays in the late nineteenth century by Clarence King, Samuel has passed through the Cascade Range since the early Miocene, Emmons, Grove Karl Gilbert, Clarence Dutton, and Joseph Silas making this one of the very few (and certainly the largest) world Diller. Details of some early fi ndings and observations will be rivers draining toward an active convergent margin and bisect- described in conjunction with the stop descriptions, but Dana and ing a volcanic arc.

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