Status of Glacial Lake Columbia During the Last Floods from Glacial Lake Missoula BRIAN E ATWATER U.S
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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 upsec- tion 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 glacia- tion (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).