Status of Glacial Lake Columbia During the Last Floods from Glacial Lake Missoula BRIAN E ATWATER U.S

QUATERNARY RESEARCH 27, 182-201 (1987)

Status of Glacial Lake Columbia during the Last Floods from Glacial Lake Missoula BRIAN E ATWATER U.S. Geological Survey at Department of Geological Sciences, University of Washington. AJ-20. Seattle, Washington 98195 Received March 1, 1986 The last floods from glacial Lake Missoula, Montana, probably ran into glacial Lake Columbia, in northeastern Washington. In or near Lake Columbia’s Sanpoil arm, Lake Missoula floods dating from late in the Fraser glaciation produced normally graded silt beds that become thinner upsection and which alternate with intervals of progressively fewer varves. The highest three interflood intervals each contain only one or two varves, and about 200-400 successive varves conformably overlie the highest flood bed. This sequence suggests that jokulhlaup frequency progressively in- creased until Lake Missoula ended, and that Lake Columbia outlasted Lake Missoula. The upper Grand Coulee, Lake Columbia’s late Fraser-age outlet, contains a section of 13 graded beds, most of them sandy and separated by varves, that may correlate with the highest Missoula-flood beds of the Sanpoil River valley. The upper Grand Coulee also contains probable correlatives of many of the approximately 200-400 succeeding varves. as do nearby parts of the Columbia River valley. This collective evidence casts doubt on a prevailing hypothesis according to which one or more late Fraser-age floods from Lake Missoula descended the Columbia River valley with little or no interference from Lake Columbia’s Okanogan-lobe dam. TJ 1987 University of Washington.

INTRODUCTION Fraser age suggest progressively smaller jokulhlaups into Lake Columbia or coex- This report concerns the relative timing isting waters, and a succeeding interval of of floods and ice-lobe movement at the late Fraser-age varves suggests existence south-central edge of the Cordilleran ice of Lake Columbia during many decades sheet during a late part of the Fraser (late after the last of these floods. This kind of Wisconsin) glaciation. Citing flood-formed sequence forms the upper part of a 115-m features in the Columbia River valley section of alternating flood beds and varves downriver from Foster Creek (Figs. 1 and in ,the Sanpoil River valley, as summarized 2A), some geologists have argued that at below from Atwater (1986). Similar stratig- least one late Fraser-age flood from glacial raphy crops out piecemeal in the upper Lake Missoula, Montana, descended the Grand Coulee and the nearby Columbia Columbia River valley in north-central River valley, in exposures described herein Washington after the Okanogan lobe had for the first time. These observations lead become incapable of blocking great down- to the report’s chief hypothesis-that the Columbia floods (Bretz et al., 1956, pp. last flood from Lake Missoula occurred 993-994; Bretz, 1969, p. 525; Waitt, 1977; many decades before the demise of Lake Waitt and Thorson, 1983, p. 64). This pre- Columbia’s Okanogan-lobe dam-which in vailing view is not necessarily compatible turn raises doubt about a Lake Missoula with evidence upriver from Foster Creek, source for certain down-Columbia floods of in areas where the lake formed by Oka- late Fraser age. nogan-lobe blockage of the Columbia River valley (glacial Lake Columbia) stood in the PREMISES low-altitude path of Lake Missoula floods (1) Fraser-age Lake Missoula originated (Fig. 2). There, varve-bounded beds of late after 19,000 yr B.P. This timing is consis-

182 0033-5894187 $3.00 Copyright 0 1987 by the University of Washington. All rights of reproduction in any form reserved. LAKE MISSOULA FLOODS 183

120” /16~ 114” I I 1 \ I I ’ Path of 3 ‘< c . jbkulhloups BRITISH C OLUMBIA of Clogue (197 m;;,,------.-. _..-.

-q d'illene Lohr

WASHINGTON < ? \\

FIG. I. Index map. Glacial paleogeography from Waitt and Thorson (1983). Lake Columbia at -500-m level. Lake Missoula at maximum level. Stratigraphic sections: BC, Burlingame Canyon (Waitt. 1980); LC, Latah Creek (Waitt, 1984); MC. Manila Creek (Atwater, 1986): NC. Ninemile Creek (Chambers, 1971): NF, Ninemile Flat: NPC, Nez Perce Creek (Table 1). tent with the available Canadian chro- the lake’s Fraser-age bottom sediment, and nology of the east-central part of the Cor- (b) the apparently complementary Mis- dilleran ice sheet (Clague et al., 1980), with soula-flood rhythmites of southern Wash- ash-layer and radiocarbon dating of Mis- ington (Waitt, 1980, 1985). Self-dumping of soula-flood rhythmites in southern Wash- Fraser-age Lake Missoula further explains ington (Waitt, 1985, p. 1284), and with the stratigraphy of varve-bounded floodlaid counts and radiocarbon dating of varves beds in Washington and Idaho (Waitt, 1984, among Missoula-flood rhythmites in the 1985; Atwater, 1984, 1986), and is consis- Sanpoil River valley (Atwater, 1986, pp. tent with mathematical models of the lake’s 29, 32). Morainal correlations of Richmond ice dam (Clarke et al., 1984, p. 296; Waitt, (in press) indicate the possibility that Lake 1985, pp. 1280- 1283). Missoula also existed during a Rocky (3) Fraser-age jloods from Lake Mis- Mountain alpine-glacial maximum about soula ranged greatly in volume. Custom- 22,000 yr B.P. The pertinent correlations arily unqualified use of epithet (cata- being speculative at present, Lake Mis- strophic, cataclysmic, colossal) conceals soula floods of Fraser age are herein re- great variation in the likely Fraser-age garded as entirely younger than 19,000 magnitude of Lake Missoula floods. The yr B.P. interflood period, as inferred from varve (2) Fraser-age Lake Missoula released a counts in the Sanpoil River valley, ranged flood each time it rose high against its ice- from about 1 to 50 yr during Fraser time sheet dam (the Purcell Trench lobe, Fig. (Atwater, 1986; summarized below). If 2). Such self-dumping explains (a) the cy- Lake Missoula’s minimum level and clic tilling and emptying of Lake Missoula average inflow rate were fairly constant, that Chambers (1971, 1984) inferred from then the volume of Fraser-age floods could 184 BRIAN ATWATER

have ranged 50-fold. The range could have beds of sand and silt during some Lake been greater if the least frequent floods Missoula floods of Fraser age. drained the lake to lower levels than did floods of much greater frequency. Such SANPOIL RIVER VALLEY partial drainage during frequent floods is Fraser-age Lake Columbia usually consistent with the stratigraphic distribu- drowned the lower Sanpoil River valley to tion of desiccated, frost-cracked varves in altitude -500 m (Figs. 1 and 2). The lake the Lake Missoula section of Chambers was not ordinarily self-dumping because (1984, pp. 191, 198). Thus, below I advo- subaerial outlets -especially divides near cate the seeming paradox that the Grand 465 m in the upper Grand Coulee-pre- Coulee, greatest of the scabland channels vented it from rising high against the Oka- (Bretz, 1932), locally accumulated mere nogan-lobe dam. The normal level of the

Mortmum oru rheltsred- by IO00 2 t +OJanogan loba from lots Fraser-age floods down thp = Columbia River volley c 5 ,’ 600- ;= z z 2 - c 2 600 -

Early Fraser-age F landslide dom sronc Coule 400 Uistoulo flood fsotures KC-

r. r MR-

Yorticat augg9ration X 52110 0 50 100 200km cl-u J

FIG. 2. Profiles along drainage between Lake Missoula and Wallula Gap during the Fraser glaciation (Fig. I). Configuration of Purcell Trench lobe from Waitt (1985, p. 1282). Upper limit of Missoula- flood features from Baker (1973, pp. 15-17, Pl. 1) except near Sanpoil River valley, where from Atwater (1986, p. 4). Modern river profiles from District Engineers (1934, PI. 2). Maximum depth of Coeur d’Alene Lake from Ned Homer, Idaho Dept. Fish and Game (oral communication, 1985). (A) Profile via site of Okanogan-lobe dam in Columbia River valley. Dams near Moses Coulee from Waitt (1982). Profile of Great Terrace shows range in altitude of kettled surfaces, some of them alluvial fans graded to main part of terrace. (B) Profile via the Grand Coulee and lower Crab Creek. Boulder diameter in Quincy Basin from Baker (1973, Pl. 1). LAKE MISSOULA FLOODS 185

NE -

Ytghest Lake - Missoulo Quincy Basin n

1000 Largest flood -moved boulder (intermediate diameter,m) I;0 km NC I+

400 Post-glacial dam ,

200

A Divide brtrwn Columbia River volley .$mpare with Columbia Rive!, aoutharn Cbonntl,d Seablond lAtrot,r, profile shorn in Figure 2A wPC,I Stratigraphic retion -- Acronym keyed 0 to flgura I or 4A FIG. 2.-Continued. lake rose to -715 m while the maximally by Chambers (1971, 1984, p. 191) in sets of advanced Okanogan lobe blocked the Lake Missoula varves near Ninemile upper Grand Coulee. Certain anomalously Creek, Montana. Higher in the Manila thin Missoula-flood beds mentioned below Creek section the number of interflood suggest that this high level was maintained varves declines further to just one or two. for only about two centuries in Fraser time About 200-400 additional varves overlie (Atwater, 1986, pp. 34-36). the highest flood bed without evident A 115-m glaciolacustrine section near hiatus. These varves, and perhaps the Manila Creek, a Sanpoil River tributary, highest several flood beds as well, postdate contains a bed for each of 89 Lake Mis- separation of Manila Creek valley from soula floods of Fraser age (Fig. 3). These Lake Columbia by a Sanpoil River sandur beds alternate with intervals of varves. A whose progradation probably explains the varved interval near the middle of the sec- abrupt thickening of varves and flood beds tion has yielded detrital wood with a 14C near altitude 490 m in the section (Atwater, age of 14,490 f 290 yr B.P. (USGS-1860). 1986, pp. 14, 23). Among the lower 45 flood beds, the The thickness and maximum grain size of number of interflood varves decreases up- most flood beds in the Manila Creek sec- ward from a maximum of 50-55 to about tion tend to vary directly with the number 15-25. This trend parallels that observed of interflood varves (Fig. 3). Partial excep- -.- 0 10 20 30 40 50 50 70 0 5 10 0 0.5 1.0 1.5 2.0 313 , , , , , , 1 i15. I-200-400 VARVE COUNTS INTERPRETATION z 503.5 - successive iXPLiNAT;GN ’ ’ ’ - rj E 03,5-e--Purcell Trench lobe retreat* from = varves 0 Varved rhythmlte of Lake Missoula I Clark Fork valley - at Ninemile Creek. Montana (demise of Lake

Sanpoil River outwash plain progrades near and eventually dams the mouth of Manila unrecovered core. or structural - thlnnlng

Purcell Trench lobe 4 probably reedvances. 460 - = 4

=--,

-- J -, Y 470 - Purcell Trench lobe 465 - 465 - possibly readvances, - temporarily 47o-l 460 - -- 2.6 m thick -

430 - 425 - 3.2 m,?k Purcell Trench lobe 420 - maximally advanced 410 - I- 2.9 m- thick

-L-,1 0 10 20 30 40 50 60 70 0 5 10 0 0.5 1.0 1.5 2.0 2.5 RANGE OF MEAN THICKNESS THICKNESS OF SUCCEEDING NUMBER OF VARVES PER VARVEO INTERVAL OF VARVES. IN CENTIMETERS. FLOOD BED, IN METERS WITHIN VARVED INTERVAL FIG. 3. Vertical trends in ranges of varve counts. in ranges of mean varve thickness. and in flood-bed thickness and grain size near Manila Creek (from Atwater, 1986, p. 22). Column at right shows events inferred from these data. Ordinate divided evenly into 89 increments of largely unequal stratigraphic thickness each corresponding to a known or inferred flood bed and to the known or inferred interval of varves beneath that bed. Varve-count ranges of Lake Missoula rhythmites (from Ninemile Creek section (Figs. 1 and 2) as logged by Chambers. 1971, App. III: 1984, p. 191) are fitted visually to the Manila Creek counts with presumption that nondeposition and erosion prevent the Lake Missoula data from recording all of interflood time (see Chambers, 1971, pp. 30, 59). R, reversal of overall decline in varve counts in upper part of Manila Creek section. LAKE MISSOULA FLOODS 187 tions to this rule are the aforementioned main thick (fed by Sanpoil River outwash) flood beds near altitude 490 m. These fine- both among and above the highest flood grained beds, separated by 15-30 varves, beds, and therefore show no sign that the are at least as thick as many of the coarser- Sanpoil River regarded to a base level grained flood beds separated by 40-55 below 500 m either during or shortly after varves. The other main exceptions are two the last Lake Missoula floods recorded in sets of anomalously thin flood beds low in the Manila Creek section. the section (Fig. 3). I suspect that the Correlatives of the highest flood beds floods responsible for these thin beds were and intervening varves near Manila Creek attenuated by the maximum extension of are probably lacking in the Ninemile Creek the Okanogan lobe (which enlarged am- section and in most sections of flood rhyth- bient Lake Columbia) and Columbia River mites in southern Washington. The Nine- lobe (which could have deflected floods in mile Creek section, at altitude 940 m, the Spokane River valley; Fig. 2); further, I stands 200 m above the highest thalweg on believe that the lack of such anomalous the floor of Rathdrum valley (Fig. 2). Lake beds in the lowermost part of the section Missoula had a capacity of -260 km3 in precludes substantial Fraser-age lowering that 200-m vertical interval (Singer and of divides in the upper Grand Coulee (At- Craig, 1984)-room for roughly five times water, 1986, p. 37). the annual inflow indicated by the product The scarcity of gravel in flood beds of of maximum volume of Lake Missoula the Manila Creek section (Fig. 3) is consis- (-2500 km3; Singer and Craig, 1984) and tent with their deposition in a deadend arm minimum Missoula-flood frequency (V-50 of a large lake. Given the consequent op- yr; Fig. 3). A Lake Missoula that emptied portunities for damping of Missoula-flood every year or two was thus unlikely to have effects, I infer that certain sandy flood beds surmounted the Ninemile Creek section. represent the largest of the Fraser-age As for flood-rhythmite sections in southern floods from Lake Missoula, hence also the Washington, these commonly stand many maximum thickening (and advance) of the tens of meters above local base level, as Purcell Trench lobe (Fig. 3; Atwater, 1986, shown in Figure 2 for the Burlingame p. 34). Lacustrine damping of flood effects Canyon section of Waitt (1980). The last should have diminished as Lake Columbia Lake Missoula floods probably were too shoaled. Flood beds in the Manila Creek small to overtop such sections-as sur- section nonetheless become generally mised by Waitt (1980, p. 675) and con- thinner and finer upsection-evidence of firmed by him (Waitt, 1985, p. 1284) and by an overall decrease of Missoula-flood mag- Baker and Bunker (1985, p. 31) from altitu- nitude in late Fraser time. dinal variation in the number of floodlaid The Manila Creek section thus corrobo- rhythmites overlying ash layers of Mount rates Waitt’s (1980, 1985) hypothesis that St. Helens set S. Lake Missoula floods of late Fraser time But correlatives of the upper 25 m of the became generally smaller (upsection thin- Manila Creek section could be present in ning and fining of flood beds) and more fre- areas at or below local late-Fraser-age base quent (upsection decrease in number of in- level along the profile of the glacially terflood varves) in response to overall late dammed and diverted Columbia River (Fig. Fraser thinning of the Purcell Trench lobe. 2). I believe that they are-that probable The approximately 200-400 varves above correlatives of the highest flood beds near the highest flood bed further imply that Manila Creek are exposed near Steamboat these trends culminated in the demise of Rock in the upper Grand Coulee, and that glacial Lake Missoula. But Lake Columbia varves younger than the last flood from apparently lived on; scores of varves re- Lake Missoula are widely exposed in the 188 BRIAN ATWATER upper coulee and in nearby segments of the section is a few meters below the silty ter- Columbia River valley. race, nearby patches of which have an average altitude of about 493 m. The lowermost part of the section rests unconform- UPPER GRAND COULEE ably on basalt-rich cobble gravel (Fig. 5B) A lake or sluggish river continuous with and on a dark gray diamict that is probably glacial Lake Columbia occupied the upper lodgment till. The uppermost 0.5 m of sec- Grand Coulee during late Fraser time tion shown in Figures 5A and 5B is mostly (Bretz, 1932, pp. 76, 78; Flint, 1935, pp. structureless sand and silt, probably collu- 188-189). Water that was standing or vial or eolian. But tens of successive slowly moving is indicated by laminated silt varves crop out on the Banks Lake bench that mantles most of the coulee floor; con- 50-100 m south of the area shown in tinuity of this water with Lake Columbia is Figure 5B. These varves dip south and lap indicated by a patchy, silty terrace that onto a south-dipping unconformity that stands well above the divides on the coulee truncates at least the lowest 4.5 m of the floor (altitude 465 m; Figs. 2B and 4A) as it section. descends the coulee’s upper 40 km from al- The Steamboat Rock section is domi- titude 500 m to altitude 480 m. Some of this nated by 13 graded beds. Overall these height and slope probably reflects isostatic beds become much thinner upward; the rebound (Flint, 1935, p. 189). If equal to the lowest complete bed is 3.5 m thick, the vertical separation between the normal highest 0.1 m. Grain size also decreases 479-m level of Banks Lake (man-made res- upsection: whereas the 3.5-m bed consists ervoir dammed just above Dry Falls) and chiefly of medium sand, the 0. l-m bed con- the highest varved silt (deposited in late tains no clast larger than coarse silt. Most Fraser-age lake or river whose spillway beds are reversely graded in their lowest was Dry Falls), the rebound amounts to 8 lo-30 cm and normally graded in their m at the Steamboat Rock section and 5 m highest IO-50 cm. Lamination in the sand, farther south at the Paynes Gulch section defined by median-grain-size differences (Fig. 4). Such rebound could have been estimated to be as much as 1 phi, is plane- caused not only by the wasting of the parallel in the center of most beds, rippled Okanogan lobe but also by the draining of above, and locally rippled below. The up- Lake Columbia and consequent incision of permost part of most beds is a normally its fill. graded layer of yellowish brown (1OYR 5/5) Bretz (1932, p. 78; 1969, p. 525) reported silt and silty clay whose contact with un- a lack of flood-formed features either derlying sand is abrupt (Fig. 5C). within or above the laminated late Fraser- The lowest 10 graded beds, and perhaps age silt of the upper Grand Coulee. Expo- also beds 11 and 12, are separated from one sures beside Banks Lake suggest an another by a deformed and eroded interval amendment: though the youngest of this of olive (5Y 5/4) silt-and-clay varves. The silt indeed postdates the last flood through base of each varved interval is conform- the coulee, older late Fraser-age silt of gla- able, the top distinctly disconformable. The ciolacustrine or glaciofluvial origin is inter- interval between graded beds 6 and 7 con- bedded with the deposits of 13 floods that tains the largest number of countable could have come from Lake Missoula. varves, 3 or 4; the thickness of each of these varves is about 2 cm. Though I ne- Steamboat Rock Section glected to examine carefully the varves This section, 0.8 km south of Steamboat above the south-dipping unconformity, I Rock, rises 10 m from a bench eroded by did note that they resemble the varves in Banks Lake (Figs. 4 and 5). The top of the the lower half of the Paynes Gulch section LAKE MISSOULA FLOODS 189

: old;T;’ 0 1 J ‘500 600 700 ALTITUDE (.I 1 C

Verlital ‘G exoggtrati0n Y 100 J B

of it* flaw - Rim of the Grand Cwltt and Steamboat Rock - Crow - coul~~ profile -0 Eartarn lamit of Okonogan Iabe during tht Frarw PG. Strotlgraphic swtion glacIalinn-- Dashed where location opprorimote, dotted rhara location Inferrod A Divide nur 465m --- Historic rentsrlina of Columbia River - End moraine of tote Fr~wr age FIG. 4. Paleogeographic setting of stratigraphic sections in the upper Grand Coulee and adjacent Columbia River valley. Stipple denotes -500-m glacial Lake Columbia and its outflow in the upper coulee. B, Barry; EC, Elmer City; GCD, Grand Coulee Dam (Flint and Irwin, 1939); KC, Kaiser Canyon; MR, Monaghan Rapids; PC, Paynes Gulch; SR, Steamboat Rock. (A) Plan view. Glacial geology chiefly from Hanson (1970, Fig. 19-2). (B) Cross-coulee profiles. Level of Lake Columbia equated with present altitude of highest silty terrace near profile, without adjustment for possible isostatic rebound. (C) Cross-sectional areas along the cross-coulee profiles. 190 BRIAN ATWATER

IN 4GRADED0 BEDS T Biolybaied Load structures 01 burrows

/ Rip-up closts I of varves

? -:*-3-4 “owes preserved ” locally

LT Uppermost port : of bed 4 I largely contorted -1.

FIG. 5. Steamboat Rock section; SW/4 NW/4 sect. 11, T.27N. R.?9E, Steamboat Rock SW 7.5-min quadrangle. (A) Generalized stratigraphy, and vertical trends in estimated grain size within flood beds. Contacts shown by solid lines in and left of column; wavy lines left of column denote disconformity. In flood beds (numbered), sand is stippled, silt and clay dashed. Varved intervals (V), unpatterned in column, are mostly too thin to sketch, but disturbed varves are locally prominent within flood beds as flame structures (beds 3 and 4) and rip-up clasts (bed 8). Grain-size trends dashed where commonly

(described below), and that they number at (3) Deposition of most of section by 13 least 30. successively smaller floods. At least the The Steamboat Rock section suggests a first 10 of these found the coulee floor al- fivefold sequence of glacial-age events. ready submerged, and this submergence (1) Deposition of the diamict by glacier was probably by waters continuous with ice. Probably this occurred during the Lake Columbia. The interflood period was Okanogan lobe’s last occupation of the at least 3 or 4 yr between the 6th and 7th Grand Coulee, before which time the upper flood. Probably most floods waxed more coulee “had been eroded to present slowly (gradual upward coarsening) than depths” (Bretz, 1932, p. 35). they waned (abrupt upward change from (2) Deposition of the basaltic cobble sand to silt), therein resembling the modern gravel by subglacial stream or Luke Mis- jokulhlaups described by Thorarinsson soula flood. If deposited by flood, the (1939, p. 227), Mathews (1973, p. 102), and gravel may belong to a bar that is pendant Bjornsson (1974, pp. 5, 7) (see also Waitt, to Steamboat Rock. 1984, p. 51, 1985, p. 1285). Many of these LAKE MISSOULA FLOODS 191

FIG. 5-Continwd interrupted by differences between adjacent laminae. (B) Outcrop viewed from south-southeast: Steamboat Rock in background. Ledges are silt and clay. chiefly the uppermost parts of flood beds. Prominent flood beds numbered. Gravel in foreground exhumed by waves of Banks Lake (normal level 479 ml. (Cl Abrupt contact of silt lz) over fine to medium sand 1s) in two flood beds. Loose dry sand (Is) conceals lowermost parts of flood beds. Lamination: r. ripple drift: p. plane parallel. Shovel 55 cm long. 192 BRIAN ATWATER floods deposited medium sand as flat beds standing or slowly moving water, in which of the upper flow regime-a combination formed the tens of successive varves above of grain size and bedform that suggests a the south-dipping unconformity. The inun- current velocity, averaged through the dation could have been continuous with water column, of at least 1 m/set (Rubin that recorded by the interflood varves, or it and McCulloch, 1980, p. 218). If this ve- could postdate a period when most of the locity applies to the entire cross-sectional coulee floor was subaerially exposed. The area below the terrace altitude of 493 m hypothesis of continued inundation re- (Fig. 40, peak flood discharge exceeded quires only that the lack of varves atop 130,000 m3/sec. Although speculative, this beds 10, 12, and 13 be due to nondeposition limiting estimate is probably too low be- on a flood-built shoal, and that the local cause the Steamboat Rock section is lo- erosion low in the section be due to one or cated both in the lee of Steamboat Rock more of the last 10 floods recorded in the (Fig. 4A) and on the point-bar side of the section. channel cross section (Fig. 4B); also flood- The Steamboat Rock section thus con- water may have risen many meters above tains evidence that 13 successively smaller the 493-m terrace. Yet 130,000 m3/sec is jokulhlaups swept through the Grand four times the 24-hr average for maximum Coulee in late Fraser time, and that this Columbia River discharge during the flood series is largely or wholly equivalent to the of 1894, the greatest Columbia River flood increasingly frequent Lake Missoula floods in the historic record (District Engineers, that I infer from the highest flood beds of 1934, p. 1738; discharge determined at The the Manila Creek section (Fig. 3). The first Dallas, downstream of Wallula Gap). More- 10 of the 13 floods probably occurred while over, 130,000 m3/sec is enough to have the upper coulee held much standing or transmitted 1 yr’s Lake Missoula inflow (at slowly moving water that was continuous -50 km3/yr) in 4 days, and it equals the with Lake Columbia. I cannot tell whether peak discharge calculated for a 36-km3 jo- such ambient water was present during the kulhlaup by means of the Clague and succeeding 3 floods. But the entire series of Mathews (1973) equation as modified by 13 floods had surely ended decades before Beget (1986): this water finally disappeared-as shown further in the Paynes Gulch section. Q = 0.0067v0.69, where Q is maximum instantaneous dis- Paynes Gulch Section charge (in m3/sec) and V is volume drained A boulderly gravel in the form of a point (in m3). (This modified equation describes a bar extends 5 km along the east side of the regression line (r2 = 0.86) through data for upper Grand Coulee’s floor approximately 21 modern jokulhlaups with peak discharge midway between the Columbia River valley in the range 150-50,000 m3/sec and total and Dry Falls (Figs. 4A and 4B). This volume in the range 0.003-7.0 km3.) Thus gravel, overlapped to the west by well-lam- many of the sandy beds in the Steamboat inated silt, probably is the silt-draped Rock section connote jokulhlaups of mag- boulder bed of Bretz (1932, p. 78). Low- nitude appropriate to those Lake Missoula ering of Banks Lake exposes exhumed tops outbursts recorded high in the Manila of the boulders beside wave-freshened ver- Creek section. tical outcrops of the silt. Some vertical ex- (4) Local erosion of at least the loM)er posures nearly intersect the silty terrace, half of the section. This erosion may have here at altitude 486 m. The Paynes Gulch taken place during any of the last 10 floods. section (Fig. 6) is representative of such or it may totally postdate them. exposures, the stratigraphy of which is sim- (5) Continued or renewed inundation of ilar along at least 1 km of the southern part at least the lower half of the section bJ of the bar. LAKE MISSOULA FLOODS 193

ALTITUDE SCHEMATIC VARVE (meters) LlTHOLOGr THICKNESS 485- -v- - - -- . -----_ ------NUYBER OF VARVES 484- .---_ -111 h5

Fines upward -210* ,~me4a~ Icm

27-28

Boulder ?,

FIG. 6. Paynes Gulch section; SW/4 NE/4 sect. 7. T.26N. R.29E. Steamboat Rock SW 7.5-min quadrangle. (A) Outcrop viewed from west. Boulders exhumed by 479-m Banks Lake. (B) Estimated grain size of prominent nonvarved strata (patterned). vertical trend in thickness of varves (unpatterned), and paleocurrent directions inferred from cross lamination. Silt dashed, very fine sand stippled: both chiefly structureless. Range in varve thickness shown with wavy line where varves commonly pinch and swell because of ripples (Fig. 7B).

The Paynes Gulch section contains about sedimentary structures other than slight 180 silty rhythmites, most or all of them normal grading in the lowest of these beds. varves, and 5-10 beds of silt or very fine The highest 0.5 m is bioturbated and un- sand without any sort of rhythmic&y (Fig. stratified. No deposits or landforms sug- 6B). The varves predominate at altitudes gesting flood are superposed atop the sec- 479-483 m, the silt and sand beds at alti- tion. tudes 483-485 m. The varves thin upward The Paynes Gulch sequence suggests a from an average of 8 cm in the lowest 1.5 m twofold sequence of glacial-age events. (Fig. 7) to 1-2 cm at altitudes 482-483 m (1) Formation of the bouldery bar. (Fig. S), then thicken slightly to about 3 cm Though many of the boulders may have near altitude 484 m (Fig. 6B). Most thick been delivered by the Okanogan lobe, the varves contain basal very fine sand with point-bar form indicates that the deposit ripple lamination that climbs southward, was at least shaped by flood, as previously downcoulee (Fig. 7A). Some of these lo- inferred by Bretz (1932) and by Hanson et cally pinch and swell from sand-starved, al. (1979). Probably this shaping occurred silt- and clay-draped ripples whose forms during the scores of Lake Missoula floods climb southward through successive varves that postdate the Okanogan-lobe maximum (Fig. 7B). Sand interbedded with thin and are recorded in the Manila Creek sec- varves in the middle of the section also tion at altitudes 433-504 m (Fig. 3). contains evidence of south-directed cur- (2) Inundation of the west flank of the rents (Fig. 8). By contrast, most silt and bar for abordt 180 yr. During some of this sand beds high in the section lack primary time and especially in the first few decades, BRIAN ATWATER

FIG. 7. Thick varves in lower part of Paynes Gulch section. (A) Tabular varves, altitude 479.5 m. Upward sequence in each of the four complete varves: thin basal layers of very fine sand and silt with gently south-climbing pseudo-lamination; thick, mostly upward-fining layer of yellowish brown (IOYR 514) silt; and thin cap of olive gray (5Y 5/3) clay (c). (B) Wavy varves through which climb draped. sand-starved ripples; altitude 480.0-480.6 m. Sand (light) accumulated chiefly in the lee of the pre- vious year’s clay-draped ripple crest. LAKE MISSOULA FLOODS 195

FIG. 8. Thin varves and a cross-laminated sand layer (3). altitude 482.3 m in Paynes Gulch section. downcoulee currents were seasonally Steamboat Rock section. This correlation strong enough to form ripples of very fine is likely because the varves are the sand but seasonally weak enough to permit youngest well-laminated deposits at each the draping of these ripples with clay. Hy- section and thereby appear to be part of the pothetically, such currents could have been silt that so widely mantles the floor of the generated by inflow from coulee-rim upper coulee. Moreover, the correlation is drainage basins, especially if those basins consistent with the lack, in the Paynes contained relicts of the Okanogan lobe. But Gulch section, of exposed correlatives of the coulee-rim drainage basins north of the the 13 flood beds of the Steamboat Rock Paynes Gulch section are so small (total section. This lack befits a threefold differ- area -300 km*) that the more likely cause ence in cross-sectional area (Figs. 4B and of the downcoulee currents was outflow 4C) and the absence of an upstream ob- from Lake Columbia. Such outflow must stacle comparable to Steamboat Rock. A have been small or lacking at least season- flood that laid sand and silt in the lee of ally during the last several decades that are Steamboat Rock may have chiefly scoured represented by the upper 2 m of the sec- the site of the Paynes Gulch section, and tion, so that most of the silt and sand there what deposit it left at that site had little could accumulate without lamination. chance of surviving any but the last few of Much of that part of the section may have the floods represented in the Steamboat been formed by eolian deposition in slug- Rock section. gish or standing water, with Lake Columbia Varves of the Paynes Gulch section prob- draining chiefly through the remains of its ably correlate also with tall sections of Okanogan-lobe dam in the Columbia River varves in the Columbia River valley. These valley. sections provide further evidence that Varves of the Paynes Gulch section prob- Lake Missoula jokulhlaups ceased, at least ably correlate with the tens of successive temporarily, before the demise of Lake Co- varves above the unconformity in the lumbia. 196 BRIAN ATWATER

COLUMBIA RIVER VALLEY sandy graded beds at Kaiser Canyon contain basal downvalley-dipping foresets that Periodic floods from glacial Lake Mis- indicate an easterly source for the flood- soula into glacial Lake Columbia are abun- water; but foresets in the intervening dantly recorded in the Columbia River varves dip exclusively upvalley, away from valley within the Fraser-age limit of both a westerly (Okanogan lobe) source and to- the Okanogan and Columbia River lobes. ward Lake Columbia’s subaerial outlets. Within the limit of the Okanogan lobe, At Grand Coulee Dam and Kaiser Canyon sandy graded beds alternate rhythmically the alternating flood beds and varved in- with sets of varves at Grand Coulee Dam tervals underlie lodgment till; at Elmer City (Flint and Irwin, 1939, p. 666) and farther they postdate it (Table 1; Fig. 9). Additional downvalley at Elmer City and Kaiser sections of alternating flood beds and Canyon (Table 1; Figs. 2A, 4A, and 9). The varved intervals are present farther up-

TABLE 1. SIX GLACIOLACUSTRINE SECTIONS WITHIN THE ICE-SHEET LIMIT IN THE COLUMBIA RIVER VALLEY, NORTHEASTERN WASHINGTON (FIGS. I, 2, AND 4A) Name Location” Evidence for stratigraphic position (Fig. 9)b

Sectiorts MYthin the limit of the Okunogun lobe Kaiser Canyon 30N/30E-22 and -23 In SE/4 NW/4 sect. 23, till overlies varves that, (KC) (Alameda Flat) in sets of at least 40, alternate with flood beds (sequence first described by Flint and Irwin (1939, p. 676) and Jones et al. (1961, p. 28)) Monaghan Rapids 30Ni30E-26; NW14 Diamicts, largely tabular and bounded by well- (MN SW/4 (Alameda laminated silt and sand, are confined to lower FlaUc part of section; section butts against and partly overlaps a haystack-bearing till regarded as younger by Flint and Irwin (1939, p. 676) Barry (B) 29N/30E-I; SW14 See footnote b (Nespelem) Elmer City (EC) 29N/3 1 E-20; SW/4 Undeformed section set 300 m below and 1.5 SW/4 (Grand Coulee km within glacial margin; contains at )tast 6 Dam) sandy flood beds separated by sets of about 25 varves

Sections within the limit of the Columbia River lobe Ninemile Flat (NF) 29N/35E-9; NE/4 SW14 Thirteen sandy flood beds, 5 of them usually (Wilmont Creek) drowned by reservoir, alternate with sets of 40-55 varves (V. L. Hansen, written communication, 1983); beds at altitude 391-398 m resemble the upper set of anomalously thin flood beds in the Manila Creek section (F-10 through F-13, Fig. 3); overlain by landslide deposits but not necessarily by the gravelly sand reported by Jones et al. (1961, p. 20) Nez Perce Creek 3 1N/36E-34; SE/4 SE/4 Top of section forms probable lake-bottom (NW (Hunters) terrace (1Township/Range section; part of section (USGS IS-min quadrangle). b Chief evidence for correlation among the Monaghan Rapids, Barry, and Nez Perce Creek Sections, and for their correlation with the Paynes Gulch and uppermost Manila Creek sections: lack of evidence of glacial over- ride; apparent lack of Missoula-flood beds; pronounced upsection thinning of varves. c Illustrated by Flint and Irwin (1939, PI. 6, Fig. I). who mistakenly assigned the subject to sect. 23. LAKE MISSOULA FLOODS 197

l-COLUMBIA RIVER VALLEY+-GRAND COULEE+~SANPOItt,COL R VALLEY-(

I* OKANOGAN LOBE-----$Vt?Y FCOL. R LOBE-l MC 500 : LOW-LEVEL LAKE COLUMBIA

Burled thalwag I” bedrock at Grand Coulee Dam I L /: NO HORIZONTAL SCALE

EXPLANATION DEPOSITS CONTACT ~ Canformable - D~sconformoble upward thinning of YO~VBS TIME- STRATIGRAPHIC CORRELATION Sandy flood beds and intervening -- Last in main 01 Sole series Of lllllll varved intervals ]dkulhioup’i from take Misrou~o Diomict oceuring during the Fraser glociotlo” xxx* Probably lodgement till ~ox\mum extenslan of Qkanogon ears Probably subaqueous flaw till Lobe .. . . Gravel, cobbles, 01 boulders -- Probably floadlold at IC and PG, not show” above disconfarmltler of KC,MR,E, and EC FIG. 9. Correlation of stratigraphic sections in and near the Columbia River valley of northeastern Washington. Arrows at top indicate downvalley and upglacier directions. Figures I and 2 show location of sections; Table 1 gives acronyms for all sections except Manila Creek (MC: Fig. 3), Paynes Gulch (PC; Figs. 6-8). and Steamboat Rock (SR: Fig. 5). valley in areas vacated by the Columbia to about 10 cm (Barry, Monaghan Rapids) River lobe, most notably beside Ninemile or 2 cm (Nez Perce Creek) at the top. All Flat (Table 1; Figs. 1, 2, and 9). three sections contain basal cobble gravel: Also present in valley reaches vacated the Monaghan Rapids section also contains by the Okanogan and Columbia River basal diamict probably deposited by sub- lobes, however, are tall varved sections of aqueous debris flow (Table 1). Terrace- late Fraser age that contain little or no sign forming cobble gravel unconformably of Missoula flood. Two such sections con- overlies the Barry and Monaghan Rapids taining about 200 varves are located near sections without upward coarsening or Barry and Monaghan Rapids, between thickening of the subjacent varves. None of Elmer City and Kaiser Canyon; a third, the three sections contains punctuating with about 500 varves, is near the mouth of strata that resemble the Missoula-flood Nez Perce Creek, 25 km upvalley from beds at localities mentioned above. De- Ninemile Flat (Table 1; Figs. 1,2A, 4A, and posits of relatively small Missoula floods 9). The varves in all these sections thin up- could be mistaken for some of the sections’ ward from I to 5 m near the base (Fig. 10) thickest varves, the clayey parts of which 198 BRIAN ATWATER

areas may have also produced the Nez Perce Creek section, for the varves of this section likewise extend to altitude 490 m. Additionally or alternatively, the Nez Perce Creek section could have accumulated be- hind a dam of Missoula-flood gravel located at the mouth of the Spokane River. At least two varved sections in the Co- lumbia River valley thus reveal the same history that most simply explains the FIG. 10. Laminated silt and very fine sand in part of varves in the uppermost part of the Manila an extremely thick varve. lower part of Monaghan Creek section and in the Paynes Gulch sec- Rapids section. Long-wavelength, steeply climbing tion. For many consecutive decades in late ripples propogate through the prominent rhythmites. These rhythmites may represent diurnal fluctuations Fraser time, the Okanogan lobe impounded in meltwater discharge: if so, they are loosely analo- a -500-m Lake Columbia that received no gous to the modern glaciomarine rhythmites described floods from Lake Missoula. by Cowan and Powell (1986). SOURCES OF LATE FRASER FLOODS are commonly injected into one or more The Columbia River valley between succeeding varves (Jones et al., 1961, p. Foster Creek and Crab Creek (Figs. 1 and 41; V. L. Hansen, oral communication, 2A) contains the published evidence of 1983). But there is little chance for such great floods that might be more recent than confusion higher in the sections, where the demise of Lake Columbia. Waters only upward thinning modifies the rhyth- (1933, p. 817) cited the kettles on the Great micity of the varves. Terrace as evidence of a berg-bearing flood The varves of the Barry and Monaghan at least 30 m deep with respect to the ter- Rapids sections, and perhaps many in the race surface. He attributed that flood to Nez Perce Creek section as well, formed breakup of the Okanogan-lobe dam. Al- either within or beside Lake Columbia. The though Flint (1935, p. 191) argued that Barry and Monaghan Rapids sections be- Waters’s kettles could have been produced long to the “upper lacustrine unit” that without flood, bedforms and boulders Jones et al. (1961, p. 28, PI. 5) map as high found on the terrace reaffirm Waters’s hy- as altitude 490 m. This is about 100 m pothesis (reports cited by Waitt and above the late Fraser-age lake or lakes Thorson, 1983, p. 64). Assuming prior dis- (Lake Brewster; Fig. 1) into which formed section of the Great Terrace, Waitt and the Great Terrace (Fig. 2A; Waters, 1933, Thorson (1983, p. 64) inferred one or more p. 815; Waitt and Thorson, 1983, pp, down-Columbia floods as deep as 215 m 62-64) and to which seems graded the and called them catastrophic outbursts gravel atop the Barry and Monaghan from Lake Missoula. Additional flood- Rapids sections (Flint, 1935, pp. 187-188). formed features overlooking the Great Ter- The varves in these sections thus cannot race that were formerly ascribed to a still represent upvalley transgression by Lake deeper flood of late Fraser age (Waitt, Brewster; rather, they suggest downvalley 1977) are now believed to predate the spread of -500-m Lake Columbia-into a Fraser maximum of the Okanogan lobe be- canyon dissected during a temporary de- cause they cannot be traced upvalley from struction of the Okanogan-lobe dam (Jones the lobe’s southern limit (Waitt, 1982, p. et al., 1961, p. 28), or into giant kettles that 20). Far downvalley from that limit, a boul- opened after mere contraction of that dam. dery bar about 30 m high extends most of Spread of Lake Columbia into deglaciated the way across the valley of lower Crab LAKE MISSOULA FLOODS 199

Creek. Because much Lake Missoula retreat of the Okanogan lobe from the Co- floodwater followed the course of lower lumbia River valley. Perhaps the floods Crab Creek while the Columbia River was from Lake Missoula did not end with the diverted by the Okanogan lobe (Fig. 2B), last in the series of jokulhlaups represented Bretz et al. (1956, pp. 993-994) inferred in the Manila Creek section (Fig. 3). that the flood responsible for this bar (Bev- Rather, perhaps the lake was formed anew erly bar) descended the Foster Creek-Crab by readvance of the Purcell Trench lobe Creek reach of the Columbia River valley. (Lake Missoula’s ice dam; Figs. I and 2) Initially Bretz proposed correlation with following a period of retreat that is repre- Waters’s (1933) torrent from Lake Co- sented in the vicinity of Lake Columbia by lumbia (Bretz et al., 19.56, p. 994); later he the scores of successive varves described implicated Lake Missoula and assigned the above. Precedent for readvance of the Pur- Beverly-bar flood to an interval during cell Trench lobe is suggested by certain which the Okanogan lobe temporarily va- varve-count trends in the Manila Creek cated the Columbia River valley (Bretz, section (labeled R in Fig. 3); these trends 1969, p. 525). reveal one or two temporary reversals in The evidence presented herein-from the late Fraser-age decline in interflood pe- the Sanpoil River valley, the upper Grand riod and consequently, by the reasoning of Coulee, and the Columbia River valley Thorarinsson (1939, p. 223), imply one or above Foster Creek-disallows a Lake two interruptions in the late Fraser-age Missoula source for these late Fraser-age thinning of Lake Missoula’s ice dam. floods except under certain conditions. None of these conditions is required if (1) Tandem jiikulhlaups from Lakes Mis- the Okanogan lobe stably dammed Lake soula and Columbia. As the Okanogan lobe Columbia through all the last floods from thinned in the Columbia River valley, Lake Lake Missoula but retreated from the Co- Columbia may have become susceptible to lumbia River valley before later non-Mis- self-dumping. Perhaps jokulhlaups from soula floods, of which Clague’s (1975, p. Lake Columbia were triggered and fed by 235) late Fraser-age jokulhlaups from the some of the floods represented high in the Columbia River drainage basin in south- Manila Creek section and in the Steamboat eastern British Columbia are likely ex- Rock section. Such drainage of Lake Co- amples. This scenario best explains the lumbia could account for the lack of varves upper part of the Manila Creek section and atop flood beds high in the Steamboat Rock is compatible with the other sections de- section. scribed herein. It is also compatible with (2) Temporary retreat of the Okanogan the flood-formed features on the Great Ter- lobe from the Columbia River valley. This race provided that these were formed by could be the withdrawal proposed by Jones non-Missoula flood during or after demise et al. (1961, p. 28) and Bretz (1969, p. 525). of Lake Columbia, in the spirit of Waters’s It could account for the lack of varves atop (1933, p. 817) proposal; likewise, as stated beds 10, 12, and 13 of the Steamboat Rock by Bretz et al. (1956, p. 994). such a non- section (Fig. 5A); also it could account for Missoula flood may have produced Beverly the grave1 in the lowermost parts of three bar. Thus the last outburst from glacial of the sections in the Columbia River valley Lake Missoula may have run into a glacial (Fig. 9). Yet as noted above, the Manila Lake Columbia whose Okanogan-lobe dam Creek section indicates that this kind of re- passed little or none of the floodwater. treat, if it occurred at all, must have been brief. ACKNOWLEDGMENTS (3) Readvance of the Purcell Trench Susan H. Rose assisted with field work in the Grand lobe into the Clark Fork valley after final Coulee and nearby parts of the Columbia River valley. 200 BRIAN ATWATER

Vicki L. Hansen made available her unpublished work Clague, J. J., and Mathews, W. H. (1973). The magni- on the Ninemile Flat and Nez Perce Creek sections. tude of jbkulhlaups. Journal of Glaciology 13, James E. Evans supplied Figure 10, and Bernard 501-504. Hallet pointed out a helpful reference. For careful re- Clarke, G. K. C., Mathews, W. H., and Pack, R. T. views I thank Victor R. Baker, Eugene P. Kiver. (1984). Outburst floods from glacial Lake Missoula. Robert E. Powell. and Richard B. Waitt. Quaternary Research 22, 289-299. Cowan, E. A., and Powell, R. D. (1986). “Deposition REFERENCES of Cyclically Interlaminated Sand-and-Mud in an Ice Proximal Glacimarine Environment,” p. 74. Atwater, B. F. (1984). Periodic floods from glacial American Quaternary Association (AMQUA), 9th Lake Missoula into the Sanpoil arm of glacial Lake Biennial Conference, Program and Abstracts. Columbia, northeastern Washington. Geology 12, District Engineers (1934). “Columbia River and Minor 464-467. Tributaries.” U.S. House of Representatives, 73rd Atwater, B. F. (1986). “Pleistocene Glacial-Lake De- Congress, 1st Session, House Document 103, Vol. posits of the Sanpoil River Valley, Northeastern II. Washington.” U.S. Geological Survey Bulletin Flint, R. F. (1935). Glacial features of the southern 1661. Okanogan region. Geological Society of America Baker, V. R. (1973). “Paleohydrology and Sedimen- Bulletin 46, 169-194. tology of Lake Missoula Flooding in Eastern Wash- Flint, R. F., and Irwin, W. H. (1939). Glacial geology ington.” Geological Society of America Special of Grand Coulee Dam, Washington. Geological So- Paper 144. ciety of America Bulletin 50, 661-680. Baker, V. R., and Bunker, R. C. (1985). Cataclysmic Hanson, L. G. (1970). “The Origin and Development late Pleistocene flooding from glacial Lake Mis- of Moses Coulee and Other Scabland Features on soula: A review. Quaternary Science Reviews 4, the Waterville Plateau, Washington.” Unpublished 1-41. Ph.D. dissertation, University of Washington, Beget, J. E. (1986). Comment on “Outburst Floods Seattle. from Glacial Lake Missoula.” by G. K. C. Clarke, Hanson, L. G., Kiver. E. P., Rigby. J. G.. and Strad- W. H. Mathews, and R. T. Pack. Quaternary Re- ling, D. F. (1979). “Surficial Geologic Map of the search 25, 136-138. Ritzville Quad, Washington.” Washington Division Bjomsson, H. (1974). Explanation ofjokulhlaups from of Geology and Earth Resources Open-File Report Grimsviitn, Vatnajokull, Iceland. Jiikull24, l-26. 79-10, scale 1:250,000. Bretz, J. H. (1932). “The Grand Coulee.” American Jones, F. O., Embody, D. R., and Peterson, W. L. Geographical Society Special Publication 15. (1961). “Landslides along the Columbia River Bretz, J. H. (1969). The Lake Missoula floods and the Valley, Northeastern Washington.” U.S. Geological Channeled Scabland. Journal of Geology 77, Survey Professional Paper 367. 505-543. Mathews, W. H. (1973). Record of two jbkulhlaups. In Bretz, J. H., Smith, H. T. U., and Neff, G. E. (1956). “Symposium on the Hydrology of Glaciers.” Inter- Channeled Scabland of Washington-New data and national Association of Scientific Hydrology Publi- interpretations. Geological Society of America Bul- cation 95, pp. 99- 110. letin 67, 957- 1049. Richmond, G. M. (in press). Tentative correlation of Chambers, R. L. (1971). “Sedimentation in Glacial deposits of the Cordilleran ice sheet in the Northern Lake Missoula.” Unpublished M.S. thesis, Univer- Rocky Mountains. In “Quaternary Glaciations in sity of Montant, Missoula. the United States of America, Part I of Quaternary Chambers, R. L. (1984). Sedimentary evidence for Glaciations in the Northern Hemisphere.” Per- multiple glacial Lakes Missoula. Za “Northwestern gamon. London. Montana and Adjacent Canada, 1984 Field Confer- ence and Symposium,” pp. 189- 199. Montana Geo- Rubin. D. M., and McCulloch, D. S. (1980). Single and superimposed bedforms: A synthesis of San logical Society, Billings. Clague, J. J. (1975). Sedimentology and paleohy- Francisco Bay and flume observations. Sedimen- drology of late Wisconsinan outwash, Rocky Moun- tary Geology 26, 207-231. tain Trench, southeastern British Columbia. In Singer, M. P., and Craig, R. G. (1984). Volume-area- “Glaciofluvial and Glaciolacustrine Sedimentation” depth relations in the Lake Missoula basin. Geolog- (A. V. Jopling and B. C. McDonald, Eds.). pp. ical Society of America Abstracts u’ith Programs 223-237. Society of Economic Paleontologists and 16, 333. Mineralogists Special Publication 23. Thorarinsson, S. (1939). The ice-dammed lakes of Ice- Clague, J. J., Armstrong, J. E., and Mathews, W. H. land with particular reference to their values as in- (1980). Advance of the late Wisconsin Cordilleran dicators of glacier oscillations. Geografiska Annaler Ice Sheet in southern British Columbia since 22,000 21, 216-242. yr B.P. Quaternary Research 13, 322-326. Waitt. R. B., Jr. (1977). Missoula flood suns Oka- LAKE MISSOULA FLOODS 201

nogan lobe. Geological Society of America Ab- Waitt, R. B. (1985). Case for periodic, colossal jokulh- stracts with Programs 9, 770. laups from Pleistocene Lake Missoula. Geological Waitt, R. B., Jr. (1980). About forty last-glacial Lake Society of America Bulletin 96, 1271-1286. Missoula jokulhlaups through southern Washington. Waitt, R. B., Jr.. and Thorson, R. M. (1983). The Journal of Geology 88, 653-679. Cordilleran Ice Sheet in Washington, Idaho, and Waitt, R. B., Jr. (1982). Surlicial deposits and geo- Montana. In “Late Pleistocene Environments” morphology. In “Geologic Map of the Wenatchee (S. C. Porter. Ed.). In “Late Quatemary Environ- 1: 100,000 Quadrangle, Central Washington” (R. W. ments of the United States” (H. E. Wright, Jr., gen- Tabor et al.. Authors). U.S. Geological Survey Mis- eral Ed.), pp. 53-70. Univ. of Minnesota Press, cellaneous Investigations Map I-131 1, pp. 15-21. Minneapolis. Waitt, R. B., Jr. (1984). Periodic jokulhlaups from Waters, A. C. (1933). Terraces and coulees along the Pleistocene Lake Missoula-New evidence from Columbia River near Lake Chelan, Washington. varved sediment in northern Idaho and Washington. Geological Society of America Bulletin 44, Quaternary Research 22, 46-58. 783-820.