The Lake Missoula Floods PDF

The Lake Missoula Floods and the Channeled Scabland Author(s): J. Harlen Bretz Source: The Journal of Geology , Sep., 1969, Vol. 77, No. 5 (Sep., 1969), pp. 505-543 Published by: The University of Chicago Press Stable URL: https://www.jstor.org/stable/30062250 JSTOR is a not-for-profit service tha...

The Lake Missoula Floods and the Channeled Scabland Author(s): J. Harlen Bretz Source: The Journal of Geology , Sep., 1969, Vol. 77, No. 5 (Sep., 1969), pp. 505-543 Published by: The University of Chicago Press Stable URL: https://www.jstor.org/stable/30062250 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]. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at https://about.jstor.org/terms The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  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 Pendgreatest 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 present16, 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 escaped the tremendous erosional attack Channeled Scabland (fig. 1). It flooded 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). [JOURNAL OF 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 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms 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 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  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 Bre 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 thedisputed 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, Oregon, and (5) various alternative hy- sense, that they had run essentially full dur- ing their erosion and were precisely what potheses. the name implied-river channels super- Pardee had earlier announced the posed on greatly altered preglacial stream existence of glacial Lake Missoula valleys (Bretz 1928b). Only extraordinary mountainous western Montana. He wrote: flooding could have crossed the violated "As long ago as 1885, Professor T. C. preglacial divides, and only extraordinary Chamberlin noted a curious phenomenon in velocity (born of huge volume) could the Flathead Lake region that he aptly scarify the bedrocks so tremendously. described as 'a series of parallel watermarks More than eyebrows were raised among of the nature of slight terraces sweeping geologists by this interpretation. Almost a around the sides of the valley and encircling storm of protest arose. Where did that un- the isolated hills within it, like gigantic heard-of quantity of water demanded by musical staves' " and that "Chamberlin the proposed explanation come from in so conceived the idea of a glacial dam and short a time? Why did not the great, rock- furthermore tentatively suggested that its walled Colorado Grand Canyon, with vastly location was in the Pend Oreille region with more time and only normal stream vol- outflow by way of Spokane" (Pardee 1910, ume for its development, supply the correct p. 376-386). explanation? Allison (1935) proposed a pos- By 1930 the writer had become con- This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms 508 J HARLEN BRETZ vinced from a field study that the topo- interpreted as giant current ripples (fig. 3). graphic setting of the lake's ice dam and theTheir heights ranged from 15 to 50 feet altitude of the lake surface had contained Clearly these discoveries were part of the the elements for a catastrophic emptying record of a collapsing dam at the north end when that dam should weaken (Bretz of the Bitterroot Range. This strongly sug- 1930a). Then in 1942 Pardee found, on three gested to the writer that other such giant low gaps in the crest of a partially sub- current ripples must exist along the scab- merged, mountainous, peninsular projec- land routes. Their discovery, however, did tion of the former lake's irregular not comeoutline, until 1952. that an abrupt lowering had produced great The building of the Grand Coulee Dam currents headed westward toward the dam across the Columbia near the Coulee's head 100 miles distant. He estimated the peak and the development of the Columbia Basin discharge at 10 cubic miles per hour and Irrigation Project provided a superb op- found a descending series of arcuate, asym- portunity to secure new data on the prob- metrical ridges of flood-transported debris lem. What made the final solution possible lying transverse to the slope beyond the was a cooperative field study of these new downstream end of each narrows. These he sources of information by H. T. U. Smith, 0 20 40 60 80 Scale - miles FIG. 2.-Outline map of Lake Missoula. Adapted from Pardee (1910, fig. 4) PLATE 1.-The Channeled Scabland as "seen" by Nimbus I from an altitude of 500 miles. About forty named places can be identified in this depiction. (Nimbus AVCS photograph, Goddard Space Center, September 5, 1964.) This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms JOURNAL 0! GEOLOGY~ VOLUME 77 BRETZ, PLATE I. This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms JOURNAL o~ GEOLOGY, VoLtm~ 77 BRETZ, PLAT~ 2 A B This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  LAKE MISSOULA FLOODS AND CHANNELED SCABLAND 509 then of the University of Kansas, G. E.giantNeffcurrent ripples on gravel bars, difficult of the U.S. Bureau of Reclamation, and to identify at ground level under a cover of the writer. Their report (1956) finally sage es- brush, were indubitably present (Bretz tablished the Missoula flood theory and et al. 1956, pi. 4, fig. 1). announced that Lake Missoula floods had Since 1957, Richmond has published occurred several times during the Pleisto- nearly a dozen papers on the glacial his- cene. Aerial photographs by the U.S. De- tory which he and collaborators have de- partment of Agriculture and the U.S. ciphered east of the Lake Missoula region Bureau of Reclamation, supplemented by but within the northern Rocky Mountain obliques taken by Smith, disclosed a dozen province. That history involves three differ- localities on the plateau scabland where ent glaciations separated by long intervals FIG. 3.-Map of giant current ripples north of Plains, Montana. From Pardee (1942) PLATE 2.-A: Wallula Gap through the Horse Heaven Hills anticline. Seen from Pasco Basin (photo- graph by Bretz). B: Giant current ripples on summit of shoulder bar, junction of Palouse and Snake canyons. Aerial photo CCH-16-197 from Production and Marketing Administration. This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms 510 J HARLEN BRETZ of nonglacial climate when erosion of glacial glaciation and three during the Pinedale drift deposits (table 1), accumulation (one of them of in-consisting of two episodes). terglacial sediments, and weathering of soils With the three pre-Wisconsin glaciations, occurred. he thus has found six "strong" soil-making Buried soils of varying development are a intervals when glacial ice had retreated vital part of these correlations. The latest from the region and three more times of ice paper in the Richmond series (Richmond withdrawal in the later record. et al. 1965) traces these old soil markers From his data, Richmond has deduced westward into the Columbia Plateau and the occurrence of three Lake Missoula identifies the occurrence of three Missoula floods. The earliest occurred at the close of TABLE 1 GLACIAL STRATIGRAPHY OF THE WIND RIVER MOUNTAINS, WYOMING* Terraces at Glaciations and Stades and Bull Lake Soils Interglaciations Interstades (Feet above River) Neoglaciation Gannett Peak Stade Interstade. Weak azonal Temple Lake Stade Altithermal interval. Immature zonal Pinedale. Late stade. Interstade Weak azonal 40 Middle stade Interstade Weak azonal Early stade. Nonglacial interval. Mature zonal Bull Lake Late stade 50 Nonglacial interval Mature zonal 100 Early stade 200 Interglaciation. Very strong zonal Sacagawea Ridge 380 500 Interglaciation Very strong zonal Cedar Ridge. 660 Interglaciation Very strong zonal Washakie Point 760 * G. M. Richmond (1965) after Wright and Frey (1965). the pre-Wisconsin glaciations, followed by floods, their times separated by soil-making intervals and by deposition of loess. Two one at the close of the late stade of the Bull ash layers (Powers and Wilcox 1964) and Lake glaciation and another at the end of (Fryxell 1965) have been identified as fromthe early stade of the Pinedale glaciation. Mount Mazama (Crater Lake), Oregon, and If the Pend Oreille ice front reached the from Glacier Peak, Washington, their ages site of the Lake Missoula dam in each of 6,500 years ± B. P. and 12,000 years ± B.P., these nine advances, as many burstings respectively. From these data, the earliest might be expected in the record. Bretz et al. glaciation (which Richmond has revised to (1956) believe that the scabland topography constitute three separate events) is pre- records seven such floods, and Neff (unpub- Wisconsin and the following two (Bull Lake lished) believes that at least one flood oc- and Pinedale) are of Wisconsin age. curred before a heavy caliche, whose frag- Richmond has identified two stades of the ments are so prominent in flood gravels, had Cordilleran ice sheet during the Bull Lake accumulated. If correct, there have been at This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  LAKE MISSOULA FLOODS AND CHANNELED SCABLAND 511 least eight floods corresponding with the preglacial stream valleys they entered on nine glacial retreats in northern Montana, the plateau. Small streams with propor- tionately large frictional contact with bot- Idaho, and eastern Washington (see table 2). toms and sides had found this gradient ap- ESTIMATES ON DURATION, VOLUME, VELOC- propriate, but the great rivers had a far ITIES, AND THE MECHANICS OF EROSION smaller fraction of this retarding factor to AND TRANSPORTATION OF FLOOD-DERIVED deal with, and hence a far greater rate of WASTE flow. The early estimate that a Missoula flood Estimation of velocities attained range would require 2 weeks to pass through from Flint's "leisurely" rivers to the high Wallula Gap (fig. 1 and pi. 2A) was based rates implied by the erosion transportation on Chezy's formula, where two wetted consummated. The use of Chezy's formula perimeters and a surface gradient could be by Higgins indicates a velocity through obtained (estimate by D. F. Higgins in Wallula Gap of between 17.5 and 20.6 feet Bretz 1925, p. 258). The time estimate also per second; 20 feet per second is 13.6 miles considered, incorrectly, that the full 500 per hour. The total distance involved was cubic miles would be available. The error is 15 miles, and the gorge thus was refilled al- obvious when the altitude of the Clark most hourly. Fork valley floor at the dam site, about These figures are, of course, speculations, 2,000 feet A.T., is compared with the aver-unchecked for several unknown variables age altitude of scabland channel heads (ex- such as preflood depth of the Wallula water- cept for Grand Coulee's later deepening) gap, original width, loss of energy from the of 2,500 feet A.T. Only when no Okanogan erosion that unquestionably resulted and lobe (see fig. 1) blocked the Columbia west from transportation of debris, possible later of the head of Grand Coulee, and thus no uplift in the Horse Heaven Hills anticline, detour was probable across the plateau, etc. New maps of Wallula (1964) and Uma- could the entire lake discharge have had tilla (1962) quandrangles with smaller con- free flow down the Columbia Valley. tour intervals and larger horizontal scales Pardee's estimate of discharge rates along show that the highest flood probably one of the main arms of Lake Missoula reached almost 1,200 feet A.T. much exceeds Higgin's estimate at Wallula Some critics have disliked the idea of so Gap. much floodwater erosion in so brief a time Nevertheless, the writer is convinced that as the writer has been allowing. This ob- the narrow Wallula Gap was a bottleneck at jection was answered by citing the close some maximum flood or floods, because vertical jointing of the basalt lava flows upper altitudes of back-flooding in the and the argument that not abrasion but valleys of the Snake, Walla Walla, and plucking of large fragments was inevitable Yakima rivers correspond closely with the under the high velocity of the huge rivers. Wallula's upper limit. Capacious Yakima Abrasion of large fragments to cobble and Valley in particular carries an impressive pebble sizes occurred largely during trans- record of a back-flooding of very dirty, veryportation. Many boulder deposits lie but turbulent water that deposited a cover of short distances downstream from channel- silt (mostly rehandled loess) and fine angu- bottom outcrops of large-columned basalt, lar basalt granules with plenty of iceberg- and many such boulders still possess transported foreign boulders-surely not columnar outlines in part. the deposit of a static lake, surely that of a Gerard Matthes, eminent authority on strong up-valley current (Bretz 1932). river dynamics, has described (1948) the Besides great volume, the scabland rivers "kolk" in river-bed erosion and has agreed owed much of their velocities to the gradi- that its mechanics are fully applicable to ents, averaging 20-25 feet per mile, of thethe scabland rivers. The kolk is a marked This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  gravelfi; Max.dep- enig Drumhel destroy Chanelsi Wardenfls Cityquadrngles ParkLendCoul Maximuflodcrs sonad TrailLke fals InitalHud-Brbg CeHartlinBscoQuyRk CrekRocyt LowerCabBs-ind Bevrlyba tBowers-LindRckypblquaOM,EFgm(x). TABLE2 +12,0 Uper B.P dam, mouthf Crabek sonCrek riples inpeat Volcanish DryCoule LowestWil- gren-Bab coksab- land CouleHad Bevrlyba LaterEv- ColumbiaVeyUr TENAIVCORLFSHGDUYM* Great andshel, SoapLke Castlef; Blade Dryand Volcanish Columbia draine Steambo Rockgla- ciated; Withrow moraine; Northup canyo formatin falsrech Valey; lumbia LakeCo- LobeCulrk OkanogUperGdLwClumbiVy ~Verticalspngdoufv. Dam;nofldNesp Dam;flodSteb Dam;flodIste Nodam;fl Nodam;nfl Nodam;fl This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  Rockyt intao ofplateu ansto- mosi Functioal Bowers-Lind gravelfi; Warden fals fromC- lumbia Valeyinto Quincy basin(erly erncat- racts; Drumhel intaed fromC- lumbia Valeyinto Quincy basin(Bul Pinedal) Lake) Discharge Growing Threwst- Discharge preflod fil Excavtionf HartlineBscoQuy Crek loChane, and Kontz Coule LowerCab InitalOhe- Crek Wilson Crek riples sonCrek riples UperCab TABLE2-Contiued Intermdia HighestWl- gren-Bab coksab- land,is- charge fromCa- ternoch intoQucy basin(erly erncat- racts fromC- lumbia Valeyinto Quincy basin(Bul belowGr. Pinedal) Lake) CouleHad Earlyve- Threwst- Discharge ColumbiaVey alongCu- lemon- cline Initalcourse ofSteam-ncli boatflscny cadeovr Coule monclie LobeCulHa OkanogUperGdLw Dam;flodEryetpnig Nodam;fl Dam;flodInitcs- Nodam;fl This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms 514 J HARLEN BRETZ turbulence with upward lift and negative Gap to Portland, nearly 100 miles (Bretz pressures which develop at the foot of 1925), were checked by Allison (1933), who steep, downstream-facing declivities in the agreed in general with the field facts but bottoms of stream beds. In effect, it is a differed in interpretation. Allison asked for subfluvial tornado, an analogy which Mat- a dam 900-1,100 feet high to account for thes approved. He noted that the kolk is the high water downstream from Wallula. "probably the most important macro- Flint (1938) also wanted a dam to cause his turbulence phenomenon in natural streams" prescabland aggradation in the plateau and "the most powerful form of concen- valleys. A later study by Lowry and Bald- trated energy at work on stream beds." win (1952), dealing with that part of this Plucking action thus was greatly augmented stretch across the Cascade Range known as wherever submerged basalt ledges with the the Columbia Gorge, reported no evidence proper form appeared during flooded regi- that damming of any kind (volcanic, mens. Many low-cataract cliffs and their tectonic, glacial, landslide) as advocated plunge pools therefore were only "sub- by Allison and by Flint occurred at any fluvial cataracts." The comment by an time when the Columbia was carrying engineer that "basalt is a rock hard to drill glacial meltwater from Canada, Montana, but easy to blast" is apropos. Idaho, or Washington. An unusual problem of transportation A detailed study of the Portland region exists in the gritty silts with mingled (Trimble 1963) accepted the writer's in- weathered and unweathered angular basalt terpretation of the extensive gravel deposit granules and erratic boulders that were in the broad valley at the junction of Colum- carried back up nonglacial valleys which bia and Willamette rivers as flood-made. enter the plateau scabland. Only a surface The "Portland Delta" (Bretz 1925, figs. gradient could have produced the up-valley 16, 17) is composed dominantly of gravel currents. At the bottom, the flow was all with foreset bedding that dips westward in uphill! Adequate turbulence for the full most exposures and was originally described affected length of each valley is obviously as a "subfluvial delta," a "subaqueous required, unless some means of flotation deposit" made under perhaps 100 feet of existed. For the boulders high on valley floodwater. It covers at least 200 square slopes, floating masses of ice are indicated. miles. (There are no conventional deltas in For the fines which blanket gentler slopes, a any scabland gravel deposit. All show that slurry of excessively muddy water from their surfaces were swept by strong, deep erosion of loess, perhaps charged with currents during accumulation.) Trimble minutely crushed ice, may be envisaged. has mapped even larger areas of contempo- With waning of a flood, most such fine raneously deposited sand and silt beyond debris would settle to the bottom of any the limits reached by the gravel. They lie back-flood valley, where the bulk of the southward up the Willamette and Tualatin record is found. Here, also, the excessive valleys and northward down the Columbia turbulence had diminished to a minimum, Valley. His inclusive term is "lacustrine for such valley-bottom deposits are pre- deposits." He wrote of "flood waters of al- vailingly sorted and stratified. In places most unbelievable proportions" and was there is evidence of repetition of the epi- fully aware of the source. He obtained the sodes. Yet these back-flood deposits re- ponding, up to 375 feet A.T., by means of a main, to the writer, the least understood of hydraulic damming at a constricted place all the phenomena that record the Missoula in the Columbia Valley about 30 miles floods. north of Portland-the same mechanism The writer's descriptions of massive which the writer used for the backup north of Wallula Gap. erosional and depositional flood effects along the Columbia River Valley from Wallula Earlier students of the geology of the This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  LAKE MISSOULA FLOODS AND CHANNELED SCABLAND 515 broad lowland between the Cascade and field evidence to support this view is at Coast ranges, now containing valleys of the "The Narrows" near the junction of John Columbia, Willamette, Tualatin, and other Day and Columbia canyons. rivers, noted evidence there of widespread Here the John Day flows parallel to the ponding with ice-rafted erratic bouldersColumbia up for its last 9 miles, separated to 400 feet A.T. (Condon 1871; Diller 1896; from it by an asymmetrical, peninsular up- Washburne 1915; Allison 1935). The writer land about 2 miles wide and 1,200 feet A.T. earlier (1919) espoused Condon's idea of a Most of this upland drains to the John Day, "Willamette Sound" at sea level but later whose canyon walls are dendritically dis- abandoned it for the glacial-flood hypothe- sected and nowhere possess cliffs. sis. Lowry and Baldwin (1952) have re- In marked contrast, the Columbia can- turned to the concept of a eustatic rise in sea yon wall along the peninsular divide is a level as the cause of the ponding. continuous cliff for the entire distance and A situation closely comparable to that of ranges from 660 to 800 feet high. No ra- the Portland Delta appears to have existed vines dissect it. The Columbia's side of the at the downstream end of Wallula Gap divide has obviously been cut back marked- where the broad Umatilla structural basin ly by widening of the master valley at some was entered by floods from the scabland. geologically recent time. Here a rather featureless gravel deposit Both valleys had present depths before covers many townships thence for about 30 any glacial floods arrived. Both had been miles down valley, beyond which the river eroded well down into basalt. again encounters a rock-walled course near At the east end of the peninsular upland Arlington, Oregon. A dozen or more scab- was a low place where a flood in the Colum- land-type basalt buttes project through the bia Valley spilled a portion of its volume gravel, and several broad abandoned chan- across and made the sharply defined but nels incise it. The topography of the gravel very limited basalt scabland called The is not that of a deltaic deposit in a body of Narrows (fig. 4). standing water. It consists of broad, low Only a brief life for The Narrows spill- bars distributed through an altitude range over could have been possible before back- of 300 feet or more and declining in altitude flooding up the John Day brought it to an westward. The irregular gorge beyond the end. Yet in that time the distributary had a Umatilla basin is in places nearly 800 feet cascade down into John Day Canyon that deep, and the upland above its walls carries began as a drop of about 500 feet. This cas- flood channels with floors of 700-800 feet cade eroded the basalt floor of The Narrows A.T. Here many stranded granite boulders to depths as great as 200 feet, making cliffs lie above 1,000 feet. Only a few miles 100 feet high, a maze of rock basins, and farther downstream is a spectacular spill- vertically walled castellated buttes, and over, southward out of the Columbia gorge leaving a huge mid-channel butte whose and into the closely adjacent tributary summit, half a mile long, quarter of a mile John Day Canyon, its original floor here wide, and 956 feet A.T., records the preflood 950 feet A.T. (Bretz 1928a). surface (Bretz 1928a, pi. 23, figs. 1, 2, 15). CHARACTER OF A FLOOD FRONT Debris from this erosion built a bar on the bottom of John Day Canyon. By the time Both Snake River Valley, upstream from the entrance of the easternmost scabland the back-flooding in the John Day had been rivers, and Yakima Valley, southwest of consummated, this deposit had spread the plateau scabland, carry evidence of across the canyon to a depth of 360 feet high-energy back-flooding currents that over an area of nearly half a square mile. strongly suggest a steep borelike front of the was completed as a great bar, and hence It entering back-flood. But probably the best the John Day back-flood had topped it This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  FIG.4-MapofTheNrws,OgndvijAltWcqumUSy benroughtamdlcs20-fi.MTNwAqS gravelbisonthWcqud.Amkpw,CHfSPj oftheriv,asumn20blydpcw dipngbasltfow.ThevyuIrc strikeofhplanuc.NgbwdvyT This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  LAKE MISSOULA FLOODS AND CHANNELED SCABLAND 517 before The Narrows spillover had ceased to Hills anticline that crosses the snake supply detritus. Yakima Valley mouth to become part of The figures are simply extraordinary the forlarge Horse Heaven Hills anticline. such a brief episode. The Columbia flood From average widths of 15 miles, the struc- front, after initiating The Narrows spill- tural valley narrows at Chandler to 3 miles over, had only 9 miles farther to go before and the stream-made notch is less than a it must have sent a portion back up the mile wide (Bretz 1930b, figs. 8, 9). The John Day to reach the foot of the cascade. Corral Canyon quadrangle map (fig. 3) Perhaps The Narrows has carried several shows the anticlinal crest to have a topog- spillovers, but its initiation required much raphy wholly unlike anything else in the more than 700 feet of floodwater in the upfold or on any other surfaces in the dis- Columbia Valley. trict, but is duplicated in thousands of The writer is convinced that no gradual places in the Channeled Scabland complex rise in the flooded Columbia can account of the plateau. It is butte-and-basin scab- for these features at The Narrows. The land (Bretz 1930b), as isolated as that at flood arrived catastrophically. It was a The Narrows on the Columbia-John Day great wall of water, its crested front con- divide, and, like the glacial silts and stantly outrunning and overrunning its boulders for 50 miles up the Yakima, basal portion. Because of huge volume, its whose upper limit is 1,100 feet, it also has current was essentially unimpeded by an upper limit, but here only 950 feet A.T. channel bottom and side friction. There are ten rock basins strung along the Because a flood front normally becomes crest of the little anticline below 950 feet attenuated downstream and no steep front A.T., and hills of tilted basalt 100 feet high as here interpreted at The Narrows could stand among them. Total relief is more have been maintained across the broad than 200 feet in a horizontal distance of 500 Umatilla basin farther upstream, an ac- feet. cessory mechanism for regenerating one is Although the Yakima River clearly has required. Such a mechanism probably lies in never flowed on this crest, stream gravel a hydraulic damming in the encanyoned occurs in several places. A coarse, rubbly ba- Columbia (800 feet deep and 1 miles wide) salt gravel with a few granites, quartzites, a few miles upstream from The Narrows etc., lies on the western (up-valley) slopes spillover (see Arlington quadrangle map, of bedrock eminences and is foreset up the U.S. Geological Survey). This would cause Yakima, away from the Columbia. This lo- considerable backup in the Umatilla basin, cal up-valley current tore loose much boul- raising flood levels there and reconstructing dery basalt, some of it fresh, some considera- a steep front in the canyon. Evidence of bly weathered, which came to rest in the hydraulic damming with accessory effects foresets. is to be expected in all marked narrowings The writer (Bretz 1930b, fig. 6) has in- of the preglacial Columbia. dicated that the Yakima Valley upstream Another demonstration of the character from Chandler Narrows and below 1,100 of an arriving flood's front is visible close tofeet A.T. has a capacity of 3.17 cubic miles the junction of the Yakima and Columbia and could be filled by the Columbia River valleys, which here lie in nearly parallel in maximum flood today up to 1,100 feet structural downwards, separated by the A.T. in about 4 days. However, only strongly expressed Rattlesnake Hills anti- tremendous velocity engendered by a steep cline. surface gradient can account for the The Yakima structural valley is notably Chandler Narrows' erosional features (fig. constricted some 10 miles upstream from 5). One thinks of repetitions of the cata- the junction of the two rivers by a minor strophic experience. The record of Missoula upfold on the southern slopes of the Rattle- floods will provide this! This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms 518 J HARLEN BRETZ The scabland torrents were excessively plateau and found en route very little loess turbid. Relict loessial hills near the north to carry off. Such floods were not only less end of the Cheney-Palouse tract are 100 turbid, but were far less impeded en route feet high, and loessial scarps overlooking to Wallula. They must have arrived in the Palouse-Snake scabland divide are 200 greater initial volume than floods forced to feet high. This denuded tract has lost nearly thread the complex of plateau channels. 1,000 square miles of its former loess cover Wallula's westernmost scabland channel to these torrents. Furthermore, the back- atop the 700-foot cliff summits (fig. 6) is floods in tributary valleys (Snake, Walla about 60 feet deep and much too wide to be Walla, Yakima, Palouse, etc.) left an almost ascribed to its preglacial predecessor. The universal deposit of the rehandled loess, gravel bar and gravel floor south of the col mixed with fresh and decayed basalt grains and the huge gravel spill thence down into and granules, up to the limits reached by Columbia Valley indicate considerable deep- the floodwaters. ening in the channel's headward part and In contrast, any flood that encountered an adequate spill across at the inception of no Okanogan dam followed the open, its use, perhaps of the order of 100 feet in capacious Columbia Valley around the depth. The upper flood limit may well have FIG. 5.-Map of Chandler Narrows, Washington. From Corral Canyon quadrangle topographic map, U.S. Geological Survey; contour interval, 20 feet. Although this map of 1951 departs in no significant way from the map of 1917 (Bretz 1930b, Fig. 9), it depicts an even more rugged topography of the gutted portions of the minor Chandler anticline, shows a sharper steepening of the anticlinal nose above 1,080-1,100 feet A.T., and reveals the existence of a few undrained depressions on the lee side of the devastated portion of the anticlinal crest. The structure could hardly have been more advantageously placed for receiving the record it carries. This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  FIG. 6.-Scabland on summit, west side of Wallula Gap. From Wallula quadrangle topographic map, U.S. Geological Survey; contour interval 20 feet. Largely modified preflood topography. Most significa place is in sec. 31, between R and S in HORSE, where a gravel bar just south of the channel entering the north has detoured the preglacial ravine entering from the southwest which appears to have once tinued north along the channeled route. The bar originally dammed local drainage on the east side o channel, making a depression 25 feet deep, now drained by postflood erosion. Basalt surfaces at this hi record of floodwater are of scabland character. For a mile, this southward-leading channel descends 175 feet on a gravel floor and there pitches off for 500 feet in less than 2,000 feet of horizontal dista reach the bottom of the gorge just south of the narrowest place in the Gap. This pitch-off is essen delta which is all frontal slope; all of its detritus came from flood erosion in the channel and a tributary scabland-marked channel east of it. This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms 520 J HARLEN BRETZ been close to 1,200 feet A.T., 100flood-made feet higher Pardee's giant current ripples in than the upper limits of back-flooded the eastern sandypart of the lake or the pro- and pebbly silt known on the slopes of nounced scarifying of lower mountain walls Yakima and Walla Walla valleys. Possibly and the building of high eddy deposits back the relatively clear floods that went around in reentrants below the lake's level. the plateau left this high record. But they The two stades, one with two episodes, of should have carried berg-borne erratic the Bull Lake glaciation are recorded on the boulders, but no one has yet specifically Columbia Plateau in Washington by three noted any lying higher than the upper definite units in the loess cover, each limits of the pebbly silt. One thinks there- carrying a caliche record of long exposure fore of the alternative, that a slight post- before burial. This compound glaciation was flood uplift along the axis of the Horse followed by another flood at, according to Heaven Hills anticline may account for Richmond, about 32,000 years B.P. these aberrantly high channels on the west Excavation for the Wanapun dam on the side of Wallula Gap, and thus abandons the Columbia (Fryxell 1962) uncovered a speculation regarding floods around, versus coarse gravel in a flood-eroded rock channel. floods across, the plateau. The gravel was almost wholly of angular, poorly sorted basalt fragments, foreset- AGE ESTIMATES AND DETERMINATIONS bedded with cut-and-fill structures and the Richmond's latest correlation debris chart (1965) in part. This was clearly open-work of Quaternary events in the northern a deposit from the operation of nearby Rocky Mountains (table 1) names three glaciations Frenchman Springs cataract and thus a as pre-Wisconsin, two as Wisconsin, and a record of Grand Coulee's functioning. In neoglaciation (Little Ice Age) of valley the gravel were fragments of wood and glacier growth. Blackwelder's pre-Wisconsin dried peat which were C14-dated as 32,700 Buffalo Gap glaciation becomes the Wa- ± 900 years B.P. The organic material was shakie Point, the Cedar Ridge, and the clearly reworked from an interstadial bog Sacajawea glaciations, verifying Black- back on the plateau. The great coulee there- welder's suspicion that more detailed studies fore operated at least once to make the would subdivide Buffalo Gap. Richmond's depression for the bog (probably in the first Missoula flood is indicated as late in Quincy basin) and once again to tear up the the Sacajawea glaciation, possibly 100,000 bog accumulation at the time the cataract years ago. operated. Only the wood and peat are 32,700 Friedman (in Richmond 1965), from hy- ± 900 years old. dration rims on obsidian pebbles in till of the The identification of two volcanic ash early stade of Blackwelder's Bull Lake members in the Palouse loesses (Powers glaciation, suggested that the event oc- and Wilcox 1964; Fryxell 1965) provided curred 80,000 years B.P. Richmond's earli- two more dates in plateau history. The est flood is indicated as preceding the early Glacier Peak eruption occurred 12,000 stade of this glaciation and was therefore years B.P., the Mazama eruption 6,500 years pre-Wisconsin. B.P. The southern part of Grand Coulee is a Alden (1953) surmised that Lake Mis- closed basin containing alkaline Soap Lake soula had been formed and emptied two or in its deepest part and diatomaceous lake three times and suggested that "lowering of silts on higher slopes (Richmond 1965). In the ice dam might result in floods of great these silts is the dated Glacier Peak ash. magnitude-even if Lake Missoula were only Grand Coulee, the greatest and functionally partially drained. Such may, perhaps, have the latest of all flood channels on the pla- been the origin of many violent floods that teau, therefore ceased to operate before the are supposed to have swept over the scab- Glacier Peak ash was distributed, 12,000 lands." However, he could not accept as years B.P. A later flood (Bretz et al. 1956, This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  LAKE MISSOULA FLOODS AND CHANNELED SCABLAND 521 but not noted by Richmond) found an The Telford-Crab Creek scabland tract open Columbia. Thus the Okanogan lobe's in the north-central part of the plateau has marked morainic record was made before yielded another controlling date. A bog in a 12,000 years B.P. Richmond's latest correla- rock basin near Creston (Hansen 1947) con- tion chart shows his last flood as early tains volcanic ash layers of both Glacier Pinedale, perhaps 20,000 B.P. Peak and Mazama ages. This evidence GRAND COULEE DAM WATERVILLE PLATEAU oWilson Creek se CO* ^^ Qui'ncy QUINCY BASIN Moses Lake Othello *Connell RINGOLD PLAIN FIG. 7.-Structural features involved in the history of Grand Coulee floods. Data from U.S. Bureau of Reclamation. This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms 522 J HARLEN BRETZ agrees with that from Grand Coulee; all tion, it found no preglacial valley head, but the plateau floods must have occurred before crossed 20 miles or so of the northern divide 12,000 years B.P. of the plateau before encountering such a The largest scabland tract, Cheney- valley, and it trenched that divide nearly Palouse, also has a contribution to this 1,000 feet deep. effort to date the Missoula floods. In the The Columbia Plateau of eastern Wash- wild, canyoned scabland where the main ington had earlier suffered local warpings part of the Cheney-Palouse floods broke and foldings that produced originally un- across the Palouse-Snake divide (fig. 7),drained basins and narrow anticlinal uplifts (Fryxell and Daugherty (1962 and 1968 (fig. 8). In the Grand Coulee system, one press announcements) have investigated a basin (Quincy) covers more than twenty rock shelter archaeological site (Marines townships and now is deeply aggraded with Ranch). The oldest burials were beneath basaltic sand and gravel. One great mono- shell midden with a C14 age of 12,000 B.P. cline (the Coulee monocline) lifts its high ±+ 100 years, and other burials were under western side about 1,000 feet. The highest of Mazama ash (6,500 years B.P.). Thus no three marked anticlines (Saddle Mountain) Cheney-Palouse discharge has occurred rises 2,000 feet above the bottom of its since about 12,000 years B.P. closely parallel synclinal valley (Lower Crab Creek), which served as one of Grand Cou- GRAND COULEE SYSTEM lee's distributary routes. The Grand Coulee system provides the Grand Coulee is a tandem canyon that greatest display of flood-made features, heads in a notch in the south wall of the although its areal share of the plateau preglacial Columbia Valley with a floor 650 scabland is small. The great coulee is the feet above that valley bottom. Its northern deepest, longest, and steepest in gradient. 25 miles, about 900 feet in depth and Its system possesses the largest number of ranging from 1 to 5 miles wide, cross the abandoned cataracts, the widest one, a high side of the Coulee monocline. The record of the highest, the greatest cascade, glacial stream, on encountering the down- and the largest number of distributary warp slope of the monocline, descended in a canyons. It is unique in that, at its initia- cascade that very shortly became a cataract FIG. 8.-Park Lake quadrangle topographic map, U.S. Geological Survey. The 10-foot contour intervals show that the summit of the Coulee monocline is actually a minor anticline standing about 200 feet higher than semiparallel U.S. Highway 2 less than 2 miles distant to the northwest and nearly 1,600 feet above Blue and Park lakes. The brink of the great cliff ranges between 800 and 1,100 feet above the lakes in the coulee bottom. The five named draws which head on the southeastern flank of this anticlinal divide show very little rejuvenation since the great wall was made. Knickpoints in them are very obvious. The great oversteepened wall paralleling the strike of the monocline ends north of Park Lake by turning a right angle to follow down the dip of the structure and there leads into the lower cliffs of the Dry Falls recessional gorge. North of this, the monoclinal slope into the Hartline basin is only a smooth structural slope with the preflood draws un- interrupted by knickpoints down almost to the Dry Falls platform. East of the coulee lakes and of all tilted basalt flows, this map shows, north of Dry Coulee, a terribly scarified scabland anastomosis of canyoned coulees 100-400 feet deep with four subfluvial cataract alcoves and recessional gorges 100-150 feet deep. This tract was flood river bottom throughout, eroded before the lake-occupied deep gash along the monocline had been gouged out and Dry Falls initiated. High Hill anticline, in the southern part of the map, stood a few hundred feet above the highest flood. A subfluvial cataract alcove and its recessional gorge at the truncated western end of High Hill are separated from the main coulee by a remarkable bladed ridge. Where narrowest (extreme southwest corner of the map) it rises more than 100 feet above the bottom of the reces- sional gorge on the east and more than 450 feet above Lake Lenore in the main coulee on the west. This bladelike ridge is the attenuated toe of all that survives of the monoclinal slope before it was gutted by Grand Coulee floods. The recessional gorge with its cataract east of the Blade is the record of a preflood streamway between the Coulee monocline and the High Hill anticline. Lake Lenore basin occupies most of the width of the steep toe of the moncoline as its hogback islands testify. This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms JOURNAL OF GEOLOGY, VOLUME 77 BRETZ, FIGURE 8 This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms JOURNAL OF GEOLOGY, VOLUME 77 BRETZ, FIGURE 9 This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  LAKE MISSOULA FLOODS AND CHANNELED SCABLAND 523 which receded back into horizontal basalt it encountered (Quincy basin). One of these flows of the high side of the monocline. This abandoned cataracts, 165 feet high, is today recession eventually lengthened the canyon utilized by the main canal that leads from all the way across to the Columbia Valley's the equalizing reservoir on the Upper Coulee southern wall and thus destroyed itself. floor to the same depression (Quincy) An imposing mid-coulee butte, Steamboat where the Columbia Basin Irrigation Proj- Rock (Bretz 1932, figs. 20, 21), about a ect's largest present development exists. square mile in summit area and as high as Thus the Lower Coulee is bypassed by the bounding Coulee cliffs, for a time was a man's puny river from Grand Coulee Dam "Goat Island" in the receding cataract. to the fertile but arid region of the Quincy Further evidence for this interpretation basin. The need for the bypassing is ob- is the existence under the present detrital vious, for the Lower Coulee is a closed de- floor, at least as far north as Steamboat pression dammed by its own huge bar de- Rock, of a debris-filled trench in the basalt posits in the basin. more than 200 feet deep, a greatly elongated For about 15 miles, the Lower Coulee's plunge pool of the vanished great cataract. course is superposed directly on the Coulee Just downstream from the site of the monocline with consequent erosional fea- original cascade, the coulee floor is of un- tures unknown elsewhere in the scabland trenched basalt with a typical scabland complex. The shattered rock in the steep river-bottom topography (Bretz, 1932 fig. limb (450-60°) of the monocline yielded so 6). This is the Upper Coulee. readily to the flood from the Upper Coulee For a few miles beyond, the glacial river that a small preglacial stream valley closely spread widely in a broad synclinal valley parallel but two miles or so farther east (Hartline basin), burying most of it in was left with only relatively minor altera- basaltic waste. But a lower upwarp, the tion. The flood channel moved over into High Hill-Trail Lake anticline at the south- the shattered belt, leaving a string of deep ern margin of the basin, was shortly en- lakes on subsidence. countered and the widened flood plunged The western wall of this monoclinal across that relief feature in several cataractscourse rises, 1,000 feet above the lakes, and on its way to the lowest structural feature its crest line is gabled in appearance from FIG. 9.-Coulee City quadrangle topographic map, U.S. Geological Survey; contour interval, 10 feet. All but about a square mile of this quadrangle has been swept by Missoula floodwater. What is probably the world's largest abandoned cataract lies in its northern half, and two large, compound subfluvial cataracts with recessional alcoves lie near midlength. Structural control is evident in (1) a small portion of the Coulee monocline's flank in the northwest (largely above flood levels), (2) a part of the gravel-filled Hartline basin along the eastern side of the quadrangle, and (3) the Trail Lake and a part of the High Hill anticlines (with Dry, Hudson, and Trail Lake coulees) which trend northeast from the southwest corner of the quadrangle. The nearly 500 feet of relief in the canyoned tracts south of the great cataract is all the product of flood erosion before that cataract (Dry Falls-Castle Lake Falls) was made. The Dry Falls cliff is about 400 feet high, and the complex of recessional gorges dominated by Castle Lake Falls totals about 300 feet in height above Deep Lake. Before the Lower Coulee's great gash was made in the flank of the Coulee monocline (Park Lake quadrangle) and the great cataract was eroded, earlier floods had traversed essentially the whole of the quadrangle and had given origin to Dry, Hudson, and Trail Lake erosional gashes. The highest surviv- ing basalt of the scabland stands just west of the Hudson Coulee cataract alcoves and suffered but little. Its altitude corresponds closely with the easternmost scabland on the southern part of the Hartline basin fill and thus indicates a relief of nearly 500 feet before the great cataract was born. The Bacon syncline once carried a prescabland gravel fill comparable to that in the Hartline basin. Stream-lined, attenuated remnants lie southeast and east of Hudson Coulee. As far as shown on the map, the only human uses of this desolate area south of Coulee City are transverses (highways, dirt roads, jeep trails, and abandoned railroad), gravel pits, springs, the main irrigation canal, and perhaps jackrabbit hunting. Recorded only by name are the tragic attempts by two brothers to farm a portion of Hudson Coulee and the location of a particular (Deadman) spring. The total vertical range of flood-eroded scabland on this quadrangle is at least 650 feet. This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms 524 J HARLEN BRETZ truncation of the now-hanging little ravines miles distant. But the volume was so great that originally descended the monoclinal that a portion rose to overflow southward slope (Bretz 1959, fig. 6). But the out- past the eastern nose of Saddle Mountain standing erosional form of the Lower Coulee anticline by way of Othello Channels is at the head and in the broad uncanyoned (Bretz et al. 1956, fig. 12), a small edition area (part of the Hartline basin) which of Drumheller. In both the Othello and separates the two canyoned portions of Drumheller cascades, a residual hill of Grand Coulee. It is Dry Falls-Castle Lake, sedimentary rock survived the tremendous an abandoned compound cataract with erosion the basalt received. Othello has a maximum height of 400 feet and a width plucked rock basin 135 feet deep (Eagle or of 31 miles (fig. 9). It had taken origin and Scootenay Lake) on the summit of the pre- been maintained only because it was ini- glacial divide. tiated a little north of the coulee's entrance Drumheller did not carry all the flood into the shattered belt. Plunge pool lakes overlow from Quincy basin. On the west side in alcoves at the base of the cliff mark of this basin is a broad, low, north-south several more concentrated strands of the ridge (Evergreen-Babcock) separating it floodwater (Bretz 1932, fig. 50). from the closely adjacent Columbia Valley. The distributary system of cataracts In three places, saddles in the ridge took and canyons which took much of the spill some of the floodwater, and three cataracts across Hartline basin's southern rim and (Bretz 1959, figs. 13,14,15,16,17) were born out of the east side of the Lower Coulee all as their take fell over the Columbia Valley returned to the northeastern corner of cliffs. Both Frenchman Springs cataract Quincy basin a few miles east of the major and Potholes cataract were 400 feet high coulee's mouth. This large basin also re- above the bottom of their plunge pools. ceived several distributary rivers from the Both were double falls with a narrow upper stretches of the Cheney-Palouse tract dividing septum. The total width of Pot- and from all of the Telford-Crab Creek holes cataract was 1 miles. Crater cataract tract (see fig. 1). Seven of these were con- was 200 feet high. All three had the same verged into one (Upper Crab Creek) before upper limit and must have operated simul- reaching Quincy basin. taneously. The debouchure of the main Grand Cou- A startling fact that spurred critics of the lee discharge out of its rock-walled course early "Spokane Flood" to propose even into Quincy basin involved a great slacken- more startling alternatives (Bretz 1928c) ing of flood current and the consequent was that the upper limits reached initially deposition of a known maximum of 186 feet by discharge over these three cataracts of gravel and sand over the twenty town- were identical with Drumheller's upper ships involved. The topography of the de- limit, 1,300 feet A.T., although 30 miles posit is that of great, broad bars with distant. It was the relative altitudes of bars separating channels (Bretz et al. 1956, fig. and channels in Quincy basin and of the 4) which lead southward toward the cataract upper limits which led to the con- Drumheller cascade (Bretz et al. 1956, pls. cept of a succession of floods down Grand 7, 10, fig. 1). Drumheller is 9 miles wide and Coulee. Another startling fact is that at has a maximum depth of 300 feet in basalt. some time during Lake Missoula's repeated The gradient of the water escaping from outbursts, flooding back from Columbia the basin at Drumheller was 15 feet per Valley into Quincy basin occurred through mile. That water entered the synclinal the saddles used by the westward cataract valley (Lower Crab Creek) between the discharge to the Columbia. At such times Frenchman Hills and Saddle Mountain there could have been no Okanogan dam anticlines, and much of it there turned west and no scabland rivers, not even in Grand along that valley to reach the Columbia 20 Coulee. This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms  LAKE MISSOULA FLOODS AND CHANNELED SCABLAND 525 At Beverly, where Lower Crab Creek's tributary channels (Koontz Channels) all glacial river from Drumheller Channels hang at varying altitudes above the present joined the Columbia River valley, a well- Columbia. They entered the broad lowland marked bar almost 100 feet high and 1 of the Columbia River in the Pasco basin, miles long has been built across the mouth which must have been semiponded at of the synclinal valley by a Columbia flood. different times up to the varying levels of This bar is double, its structure well shown these hanging channels. in cuts along Chicago, Milwaukee, and Pasco basin contained the major part Saint Paul Railroad (Bretz et al. 1956, of the flood backed up by the hydraulic figs. 1, 11, pi. 12). A flooded Columbia damming at Wallula Gap 30-35 miles rolled this bouldery gravel 125 feet above south of the Koontz Channels. This also river level of today and back into the trib- was the ponding which backed up Yakima utary glacial river's mouth after Grand Valley and which left the great flock of ice- Coulee had completely ceased to function. bergs stranded on the eastern lower slopes The last Missoula flood thus was later than of Rattlesnake Hills. The Koontz Channels the last Okanogan lobe's invasion of the continued to be deepened while the Pasco plateau. basin water level was being lowered as much Pardee (1918) has described an extensive as 250 feet. Their relations to a range of 250 deposit of white silt, the Nespelem forma- feet in the level of the Pasco pool are evi- tion, in the Columbia Valley upstream from dence of perhaps three different Missoula the head of Grand Coulee and on much of floods, during each of which Grand Coulee the floor of the Upper Coulee itself, but functioned. unknown farther down either drainage course. Only quiet water could have pro- EVERGREEN-BABCOCK RIDGE vided for the deposition of this silt, and The west side of this broad ridge has a only an empty coulee from that time on can pronounced structural bench which slopes explain its survival on the floor. The last southward 30 feet per mile from Crater to Missoula flood was over by this time, but Potholes and has a maximum width of 1 an Okanogan lobe dam must have held up mile (Bretz et al. 1956, pi. 11, figs. 1 and 2, this latest (and local) ponding in the Colum- pl. 13, fig. 2). For 4 miles north of The bia Valley. Potholes the bench surface is scabland and If these conclusions are correct, the flood the 100-foot cliff above it is clean-cut. But that built Beverly bar (clearly later than north of the 1,300-foot contour crossing, the last of Grand Coulee's discharges and the bench is much dissected and has no clearly a record of an open Columbia scabland or cliffs. The rise here to the Valley around the northwestern part of the summit of Babcock is over a completely plateau) preceded the last advance of the soil-covered surface much less steep than Okanogan lobe. the cliff, although held up by the same ba- The damming for the Nespelem silt salt flow. episode may well have been only a partial Floodwater in the Columbia Valley has reclaiming of that lobe's maximum stand clearly swept over that part of the bench and certainly was not the one which pushed below 1,300 feet A.T. But both cliff and over into Grand Coulee's head portion. Yet scabland record two different times of such its failure as a dam may have released the flooding. North of about 1,250 feet A.T. on very last flood down Columbia Valley, and the bench, the cliff is talus-covered almost not a maximum flood at that. to the summit and has a mantle of soil with Most of Othello's discharge went south- sage and grass cover. The terrace surface west across a plain determined by the flat- below this subdued cliff, although definitely lying Ringold superbasalt sedimentary for- scabland, has few bare rock outcrops and, mation in the Pasco basin. The several dis- like the cliff, is mantled by coarse, weath- This content downloaded from 146.187.217.145 on Mon, 17 Feb 2025 04:35:24 UTC All use subject to https://about.jstor.org/terms 526 J HARLEN BRETZ ered debris and soil. Farther south and across the ravaged divide that swept its below that altitude, the cliff is vertical debris across and 500 vertical feet up on the bare rock with an actively growing talus far (south) side of the Snake Canyon, pick- and the structural bench carries strongly ing up well-worn cobbles from the Snake expressed scabland. Clearly, here is scab- riverbed to be mingled with the rubbly, land of two different ages, a record of two coarse gravel yielded in the divide summit's floods down the Columbia, widely separated erosion. in time, and the earlier one the greater. Fryxell reports that the enormous gravel deposits made here in Snake Canyon con- FLOOD CROSSINGS OF THE PALOUSE stituted a dam for some time afterward, SNAKE DIVIDE upstream from which the terrace record of Palou