The Lake Missoula Floods and the Channeled Scabland Author(s): J. Harlen Bretz Source: The Journal of Geology, Vol. 77, No. 5 (Sep., 1969), pp. 505-543 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/30062250 . Accessed: 15/01/2015 10:22 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp . JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. http://www.jstor.org This content downloaded from 128.193.164.203 on Thu, 15 Jan 2015 10:22:29 AM All use subject to JSTOR Terms and Conditions THE LAKE MISSOULA FLOODS AND THE CHANNELED SCABLAND1 J HARLEN BRETZ Department of the Geophysical Sciences, University of Chicago ABSTRACT This paper reviews the outstanding evidence for (1) repeated catastrophic outbursts of Montana's glacially dammed Lake Missoula, (2) consequent overwhelming in many places of the preglacial divide along the northern margin of the Columbia Plateau in Washington, (3) remaking of the plateau's preglacial drainage pattern into an anastomosing complex of floodwater channels (Channeled Scabland) locally eroded hundreds of feet into underlying basalt, (4) convergence of these flood-born rivers into the Columbia Valley at least as far as Portland, Oregon, and (5) deposition of a huge delta at Portland. Evidence that the major scabland rivers and the flooded Columbia were hundreds of feet deep exists in (1) gravel and boulder bars more than 100 feet high in mid-channels, (2) subfluvial cataract cliffs, alcoves, and plunge pools hundreds of feet in vertical dimension, (3) back-flooded silts high on slopes of preglacial valleys tributary to the scabland complex, and (4) the delta at Portland. Climatic oscillations of the Cordilleran ice sheet produced a succession of Lake Missoulas. Following studies by the writer, later investigators have correlated the Montana glacial record with recurrent scabland floods by soil profiles and a glacial and loessial stratigraphy, and have approximately dated some events by volcanic ash layers, peat deposits, and an archaeological site. Several unsolved problems are outlined in this paper. GENERAL STATEMENT bounding valley bottoms. Closed basins as Although paleo-Indians probably were deep as 135 feet were bitten out of the already in North America, no human ear underlying basalt. Dozens of short-lived heard the crashing tumult when the Lake cataracts and cascades were born, the Missoula glacial dam (the front of the Pend greatest of which left a recessional gorge, Oreille lobe of the Cordilleran ice sheet) Upper Grand Coulee, 25 miles long. The burst and the nearly 2,000-foot head of im- greatest cascade was 9 miles wide. The flood pounded water was free to escape from the rolled boulders many feet in diameter for Clark Fork River valley system of western miles and, subsiding, left river bars now Montana and across northern Idaho. It standing as mid-channel hills more than 100 catastrophically invaded the loess-covered feet high. Current ripples 10 feet and more Columbia Plateau in southeastern Washing- in height diversify some bar surfaces. A ton (Bretz et al. 1956, fig. 23) and reached gravel delta 200 square miles in area was Pacific Ocean levels via the Columbia built at the junction of Willamette and River, 430 miles or more from the glacial Columbia river valleys (Bretz 1925, figs. dam. So great a flood is unknown at present 16, 17). Portland, Oregon, and Vancouver, elsewhere in the world. It has been esti- Washington, now cover some of it. Almost mated to have run for 2 weeks. It was 800 2,000 square miles of the plateau's basaltic feet deep through the Wallula Gap on the bedrock lost its preflood loessial cover which Oregon-Washington line. otherwise is on record in the 100 or more On the Columbia Plateau in Washington, "islands" 40 acres to 40 square miles in it transformed a dendritic preglacial drain- area-tracts that stood high enough above, age pattern into the amazing plexus of the or far enough from, main spillways to have Channeled Scabland (fig. 1). It flooded escaped the tremendous erosional attack across stream divides of the plateau, some meted out to the plateau's preglacial valleys. of which stood 300-400 feet above today's Prevailingly, these "islands" have steep marginal slopes and prow points at their up- 1 Manuscript received February 6, 1969. stream ends (Bretz et al. 1956, fig. 22). [JOURNALOF GEOLOGY,1969, Vol. 77, p. 505-543] Back-flooding left deposits for dozens of © 1969. The University of Chicago. All rights reserved. miles in nonglacial valleys which entered the 505 This content downloaded from 128.193.164.203 on Thu, 15 Jan 2015 10:22:29 AM All use subject to JSTOR Terms and Conditions 506 J HARLEN BRETZ plateau from Idaho on the east (Bretz 1929, another lobe (Okanogan) of the Cordilleran fig. 1) and central Washington on the west. ice sheet. Two floods found no Okanogan The above statements are generalized, lobe in the way, and most of their water es- for there probably were at least seven caped around the north and west sides of the successive burstings of the dam, each (ex- plateau via the preglacial Columbia valley. cept the last) repaired by later glacial ad- The earliest flood outlined the anastomo- vances to again impound the valley system sis of the Channeled Scabland. Later ones of the Clark Fork River in western Mon- deepened and widened its main spillways tana and make another glacial Lake Mis- and thus left earlier high-lying and less soula. Five of these floods could only cross deeply eroded channels unrefreshed, to be- the plateau, because the westward-leading come obscured by interflood weathering, capacious valley of the Columbia River rain wash, and wind-made deposits. north of the plateau was then blocked by The satellite Nimbus I has confirmed FIG. 1.--Glaciated and flooded areas on the Columbia Plateau in southeastern Washington. Abbrevia- tions: BB, Bacon basin; B-WC, Bowers-Weber Coulee; CC, Crater cataract; CN, Chandler Narrows; DC, Devils Canyon; DHC, Drumheller Channels; EB, Evergreen-Babcock ridge; FH, Frenchman Hills; FSC, Frenchman Springs cataract; KC, Koontz Coulee; LC, Lind Coulee; OC, Othello Channels; PC, Palouse Canyon; PHC, Potholes cataract; PR, Palouse River; RC, Rocky Coulee; RP, Ringold plain; SG, Sentinel Gap; SM, Saddle Mountain; WC, Washtucna Coulee; WCR, Wilson Creek; WM, Withrow moraine; *, known occurrences of giant ripples. This content downloaded from 128.193.164.203 on Thu, 15 Jan 2015 10:22:29 AM All use subject to JSTOR Terms and Conditions LAKE MISSOULA FLOODS AND CHANNELED SCABLAND 507 this judgment. One of its pictures, made sible compromise by having numerous ice- from an altitude of about 500 miles (pl. 1), berg jams along the channels-thus lo- encompasses almost the whole plateau in cal pondings and divide crossings-with Washington and, despite some atmospheric subsequent bursting of such dams to pro- obscuration, has made possible a compari- duce repeated local flood conditions. But son with the scabland map. The satellite, no one has found field evidence for such which "saw" most of the bedrock floors dams of floating ice. Flint (1938) was far of the anastomosis clearly, failed to record more conservative. He asked for invading more than 200 miles of high-lying channel- glacial meltwater streams "no larger than ways already known as such from their berg- the Snake today" and for aggradation of carried erratic boulders, a few gravel de- spillways until the violated divides could be posits, and some scabby channel-bottom crossed. He argued that the scabland was basalt outcrops. made by "leisurely" streams during re- moval of the fills. The terms "channels," HISTORY OF INVESTIGATIONS "bars," and "floods" were repugnant to him. The writer's first study of the Washing- Other adverse comments made largely by ton portion of the Columbia Plateau (Bretz uniformitarian-minded geologists need not 1923a, 1923b) dealt in part with Grand be noted here (see Bretz 1928c and Bretz et Coulee and its origin. But the concept of al. 1956). flood origin of the coulee or the scabland In the meantime, the writer continued his complex waited for another year and was field studies and produced nearly a dozen then termed the "Spokane Flood," with no more papers that dealt with (1) specific suggestion for the source of the water. A tracts in the scabland, (2) local scabland preliminary map of the complex (Bretz making by the Columbia River at The 1923b, pi. 3) was entitled "Channeled Dalles (Bretz 1924), Oregon-Washington, Scabland," the adjective added to the local (3) back-flooding in valleys entering the term for the bare or almost bare rock of the disputed region from nonglacial drainages, drainageways, which Nimbus I "saw" from (4) the remarkable erosional and deposi- 500 miles above them. From that time on, tional features of the Columbia River valley the writer has insisted that these spillways from the plateau westward across the Cas- were not stream valleys in the ordinary cade Range to the huge delta at Portland, sense, that they had run essentially full dur- Oregon, and (5) various alternative hy- ing their erosion and were precisely what potheses. the name implied-river channels super- Pardee had earlier announced the former posed on greatly altered preglacial stream existence of glacial Lake Missoula (fig.