Missoula Flood Dynamics and Magnitudes Inferred from Sedimentology of Slack-Water Deposits on the Columbia Plateau, Washington

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Missoula flood dynamics and magnitudes inferred from sedimentology of slack-water deposits on the Columbia Plateau, Washington

GARY A. SMITH Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131

ABSTRACT flood-water tracts. These deposits, which exceed 30-m thickness in many places, consist of repetitive graded beds (rhythmites), Sedimentological study of late Wisconsin, Missoula-flood slack- ranging in thickness from 0.1 m to 1.0 m. Continuity of these water sediments deposited along the Columbia and Tucannon Rivers in terraces with flood-constructed eddy bars at the mouths of the southern Washington reveals important aspects of flood dynamics. Most valleys and the prominence of upstream-directed ripple cross- floodfades were deposited by energetic flood surges (velocities > 6 m/sec) laminations clearly relate the fine-grained "slack-water" sedi- entering protected areas along the flood tract, or flowing up and then ments to the scabland floods. directly out of tributary valleys. True still-water fades are less volumi- Bretz (1969) recognized the importance of understanding the nous and restricted to elevations below 230 m. High flood stages attended origin of the slack-water deposits for resolving flood dynamics and the initial arrival of the flood wave and were not associated with sub- chronology. Significant advances in the understanding of the stratig- sequent hydraulic ponding upslope from channel constrictions. Among raphy of these deposits have taken place (for example, Waitt, 1980), 186 flood beds studied in 12 sections, 57% have bioturbated tops, and and mechanisms of slack-water deposition have been proposed (Ba- about half of these bioturbated beds are separated from overlying flood ker, 1973; Waitt, 1980, 1985a). The physical sedimentological study beds by nonflood sediments. A single graded flood bed was deposited at reported on here was undertaken to resolve the origin of the slack- most sites during most floods. Sequences in which 2-9 graded beds were water deposits, the number of rhythmites deposited in each flood, and deposited during a single flood are restricted to low elevations. These the relationship of rhythmite-depositing floods to the dramatic ero- sequences imply complex, multi-peaked hydrographs in which the first sional and depositional features of the scabland. flood surge was generally the largest, and subsequent surges were at- Controversy over interpretation of the slack-water beds was her- tenuated by water already present in slack-water areas. Slack-water- alded by Bretz (1969, p. 533): "There are altogether too many in one sediment stratigraphy suggests a wide range of flood discharges and section, and they are too thin to assign each [bed] to a separate flood volumes. Of >40 documented late Wisconsin floodstha t inundated the influx. Thicknesses of the total deposit seem too great, however, to Pasco Basin, only about 20 crossed the Palouse-Snake divide. Floods assign to one flood." Although evidence for more than one cataclys- younger than the set-S tephras from Mount St. Helens were generally mic flood had been recorded (Bretz and others, 1956; Bretz, 1969), smaller than earlier floods of late Wisconsin age, although most still most subsequent workers adopted the early view of Bretz (1929, p. crossed the Palouse-Snake divide. These late floods primarily traversed 529) that apparently conformable sequences of as many as 40 to 62 the Cheney-Palouse scabland because stratigraphy of slack-water sed- graded beds could possibly record a complex, pulsating hydrograph iment along the Columbia River implies that the largest flood volumes for a single great flood (Baker, 1973; Patton and others, 1979; Carson did not enter the Pasco Basin by way of the Columbia River. and others, 1978; Bjornstad, 1980). The observation that tephra layers within the sequence of graded beds accumulated with eolian sedi- INTRODUCTION ments (Bunker, 1980; Waitt, 1980) led to recognition of breaks within the depositional history. Waitt (1980) also interpreted brief ("de- Most geologists know about the legacy of the great outburst cades") subaerial hiatuses at other stratigraphic levels within the floods from glacial Lake Missoula in western Montana, the resulting slack-water sequence and, because of gross similarity of all beds, spectacular landforms of Washington's Channeled Scabland, and the argued that each bed was the record of a single flood. tireless efforts of J Harlan Bretz to convince skeptics of the cataclys- This stimulating but controversial hypothesis, requiring tens of mic scale required for the floods (see Baker, 1978, and Baker and floods from a self-dumping glacier-dammed lake (Waitt, 1980,1985a; Bunker, 1985, for a review). Bretz (1925,1928,1929,1930,1959,1969; Baker and Bunker, 1985), was the topic of subsequent spirited debate Bretz and others, 1956) focused his efforts on descriptions of large (for example, Baker and Bunker, 1985; Waitt, 1985a). Evidence fa- erosional channels; huge, now-dry cataracts; and gigantic, megarip- voring Waitt's (1980) hypothesis was subsequently found in the de- pled gravel bars that dominate the landscape of the Channeled Scab- posits of glacial lakes north of the Channeled Scabland (Fig. 1), where land, within a 30,000 km2 area of loess-mantled basalt in eastern as many as 89 flood beds of interpreted Missoula-flood origin are Washington. Baker's (1973) thorough study of the scabland channels interbedded with varved lacustrine silt and clay (Rigby, 1982; Atwater, and the coarse gravel deposits associated with them led to elegant 1984, 1986; Waitt, 1984, 1985a). Bed-for-bed correlations between paleohydraulic interpretations of the cataclysmic floods. these glacial-lake sections and the slack-water sediments of southern Until the 1980s, little attention was given to sand and silt Washington remained contested, however, and the alternative hy- forming conspicuous terraces in valleys adjacent to the main pothesis that multiple slack-water rhythmites could be generated in the

Geological Society of America Bulletin, v. 105, p. 77-100, 17 figs., 2 tables, January 1993.

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Figure 1. Map of the Pacific Northwest showing maximum late Wisconsin extent of the Cordilleran ice sheet, major glacier-dammed lakes (shown at maximum stage), the Channeled Scabland, and area inundated by Missoula floods.

Channeled Scabland by each flood was rejuvenated (Baker and Bun- responsible for hydraulic ponding of flood waters to depths possibly ker, 1985; Kiver and Stradling, 1985). exceeding 200 m in the Pasco Basin (Fig. 2A; Baker, 1973;Craig, 1987; The problem has been further complicated by the stratigraphic O'Connor and Baker, 1992). The Tucannon River joins the Snake River evidence of several flood episodes, each consisting of one or more 125 km upstream of the Snake-Columbia confluence in the Pasco outburst floods over periods of several thousand years at each glacial Basin. Because slack-water sediment in these valleys was presumed maximum (McDonald and Busacca, 1988; Moody, 1987; Bjornstad to be related to backflooding by hydraulic ponding in the Pasco Basin, and others, 1991). McDonald and Busacca (1988) recognized at least Bretz (1969) referred to them as "backflood deposits," a term also six such episodes, increasing the uncertainty in correlating erosional adopted by Waitt (1980). The term is not generally applicable to flood- and depositional products of individual floods or even flood episodes. deposited, rhythmically bedded sand and silt, however, because these The late Wisconsin flood episode occurred between about 15 ka and deposits also occur at seldom-noted sites along the major flood routes, 12 ka, and its depositional products are associated with as many as including the Columbia River. The term "slack water" is used here, three volcanic ashes correlated with set-S tephra erupted from Mount although it is also not perfectly suited for describing these sediments, St. Helens at about 13 ka (Mullineaux and others, 1978). The next because several existing definitions include the provision that slack- oldest episode occurred before —36 ka, and its deposits overlie set-C water sediments are deposited from suspension (Baker and others, tephras from Mount St. Helens (McDonald and Busacca, 1988; 1983; Kochel and Baker, 1988; Baker, 1989). The dominance of cross- Moody, 1987). Busacca and others (1989) infer that this older Wis- stratification within the deposits negates suspension deposition alone. consin phase produced the greatest cataclysmic flows. A general upward progression from massive or plane-bedded coarse sand, to ripple cross-laminated fine sand, and finally to massive SLACK-WATER SEDIMENTS: GENERAL FEATURES AND or laminated, very fine sands and silt, has led to comparison of the PREVIOUS INTERPRETATIONS slack-water deposits to the Bouma sequence of sedimentary structures in turbidites (Baker, 1973; Bjornstad, 1980; Bunker, 1980; Waitt, Missoula-flood slack-water sediments are best known from ex- 1980). The principal distinction from a typical turbidite is the occur- posures in the Walla Walla Valley (Bjornstad, 1980; Waitt, 1980), rence of poly directional, usually bidirectional, cross-laminations in lower Yakima Valley and Badger Coulee (Bunker, 1980; Waitt, 1980), the rippled interval of the slack-water deposits that, in backflooded and the Tucannon Valley (Baker, 1973; Patton and others, 1979; Fig. valleys, generally record upvalley flow followed by downvalley flow. 2A). The Walla Walla and Yakima Rivers join the Columbia River Baker (1973) envisioned tributary valleys to the main flood channels immediately upstream of Wallula Gap (Fig. 2A), a water gap that was as "stilling basins" that were repeatedly disturbed by water-surface

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Figure 2. (A) Generalized map of south-central Washington showing features related to the Missoula floods, localities mentioned in the text, and sites of measured sections along the Columbia River. Arrows show general flood paths. (B) Map of the lower Tucannon Valley illustrating measured section sites and flood-gravel bars near the confluence of the Snake, Palouse, and Tucannon Rivers. Arrows indicate general paths of flood waters spilling over the former Palouse-Snake divide from the north.

waves that propagated upvalley from the adjacent flood channel. ogy because turbidity currents are gravity driven and cannot, in These transient surges brought main-channel sediments into the the strict sense, have initial motion in upslope directions. They valley in the form of turbidity currents that first moved upvalley instead envisioned each rhythmite to be the product of one of and then were reflected downvalley. Successive surges during the many flood currents that surged upslope during each flood, and same flood generated multiple turbidites with bidirectional flow which may have had water-surface slopes resembling tidcil indicators. Baker and Bunker (1985) abandoned the turbidite anal- bores.

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The case for the hypothetical multi-peaked hydrographs for OBJECTIVES AND STUDY SITES the Pasco Basin and the backflooded valleys has been argued by Baker and Nummedal (1978), Carson and others (1978), and Pat- I studied the sedimentology of Missoula-flood slack-water de- ton and others (1979). Envisioned causes of discrete flood surges posits to learn (1) the nature of flows and sediment-transport processes include (1) flood-discharge variations at the ice dam, or related to responsible for rhythmite deposition; (2) the relationship of rhythmite the emptying of the irregular dendritic drainage system of deep genesis to hydraulic ponding along the flood tract; (3) the nature of valleys (separated by constrictions) that was flooded to form gla- sedimentological features that might record intraflood surging phe- cial Lake Missoula; (2) variable arrival times in the Pasco Basin nomena; and (4) evidence for, or against, hiatuses between flood beds. of separate masses of flood water traveling different routes Two geomorphically distinct suites of study sites were selected to through the Channeled Scabland; and (3) water-level fluctuations achieve these objectives. The precise locations of measured sections that might occur as a result of cataract recession in scabland are provided in Appendix I. channels, spilling of flood waters across drainage divides, or even The first group of study sites is located along the Columbia River temporary channel blockage by transported icebergs or landslides in southern Washington (Figs. 2A and 3). These four sites display caused by undermining of scabland-channel walls. As noted by Missoula-flood rhythmites that formed along the major flood tract, Baker and Bunker (1985, p. 19), however, the multiple-bed-per- rather than in backflooded tributary valleys. An explanation for rhyth- flood hypothesis lacks confirming demonstration of the number of mite genesis must include an understanding of deposition at both types flood beds that could be generated by a single outburst flood. of localities. Waitt (1980, 1985) maintained that each rhythmite recorded the Eight stratigraphic sections located along the Tucannon River upvalley surge of a rapidly deepening flood over dry land in dead-end (Fig. 2B and 4) constitute the second suite of sites. The general features tributary valleys, where suspended load was deposited as the hydrau- of the Tucannon slack-water deposits were described by Baker (1973) lically ponded waters deepened, crested, and then drained. This in- and Patton and others (1979) and also partly illustrated by Waitt terpretation was consistent with Waitt's (1980) contention that only (1985b) but have not been targeted for rigorous, detailed study. Two one rhythmite was deposited by a flood. Waitt (1985b, p. 357) con- aspects of the Tucannon Valley make it geomorphically different from curred with the hypothetical arguments for intraflood surging, but felt better-known sites of slack-water sedimentation in the Walla Walla that this phenomenon should be reflected as minor deviations in grad- (Bjornstad, 1980) and Yakima-Badger Coulee (Bunker, 1980, 1982; ing within rhythmites rather than producing many similar-appearing Waitt, 1980) valleys. First, although general upvalley decreases in beds. The persuasive logic supporting this interpretation was best grain size and bed thickness have been described for all three of these stated by Waitt (1985a, p. 1279): "The supposed cause of such surges valley systems, there has not been a detailed consideration of lateral (abrupt local changes in flood-surface level), however, could not have facies variations that are important to understanding the processes dominated current velocity and sedimentation at the base of deeply responsible for rhythmite genesis. These facies changes occur over ponded water. An inrushing flood of hundreds of cubic kilometers on shorter distances in the Tucannon Valley because the gradient of the the one hand and transient intraflood surging on the other are wholly inundated portion of this valley is nearly three times steeper than the different processes; transportation competence at the base of the two Walla Walla Valley, and more than four times steeper than the Yakima flows must differ by orders of magnitude. The proposition of many River-Badger Coulee valley. Second, inundation of the Tucannon graded beds per Missoula flood is unfounded." "How could a Valley did not occur as a result of hydraulic ponding behind Wallula prodigious incoming flood sweeping mainly over dry land on the Gap. The mouth of the Tucannon River faces the Paiouse-Snake one hand, and a later 'surge' causing a modest increase to already River divide crossing (Figs. 2B and 5) where flood waters left the deeply ponded water on the other hand, produce bottom currents preflood Palouse River valley, which extended westward through so similar as to deposit almost identical beds?" (Waitt, 1985b, Washtucna Coulee and spilled southward along a 16-km-wide front p. 357). over the drainage divide into the Snake River canyon (Bretz, 1925, Although recognizing the need for an upward revision in the 1929, 1969; Busacca and others, 1989). Flow diverged eastward and number of floods from glacial Lake Missoula, especially in light of westward in the Snake canyon and also surged violently up the op- Atwater's (1984,1986) description of flood beds separated by glacial posite canyon wall and over the 100-m-high ridge of basalt east of the Lake Columbia varves, Waitt's critics remained unwilling to accept mouth of the Tucannon River (Figs. 2B and 5). Study in the Tucannon that each slack-water rhythmite in the backflooded valleys of southern Valley offers the potential of evaluating how many floods crossed the Washington recorded only one flood. "An important defect in the Palouse-Snake divide. Waitt (1980) hypothesis is that, while it infers that subaerial exposure In addition to detailed investigations along the Tucannon and succeeded each bed's deposition, evidence of subaerial exposure does Columbia Rivers, other well-known sites of flood rhythmites were not exist atop of all rhythmites. . . . However, he ignored a . . . pos- examined in reconnaissance. These include (Fig. 2A) sections in Bad- sible explanation for the data: multiple floods did indeed occur, but one ger Coulee, described by Bunker (1980,1982); near Mabton and Zillah or more of them deposited multiple rhythmites" (Baker and Bunker, in the Yakima Valley, described by Waitt (1980); and at several sites 1985, p. 24). Waitt (1985a) countered that the sparseness of evidence in the Walla Walla Valley, but most notably at Burlingame Canyon, for the inferred hiatuses between flood beds was a result of (1) the described by Waitt (1980, 1985a), Bjornstad (1980), and Waitt and aridity of the region, which limited pedogenic modification during Atwater (1989). interflood intervals of decades inferred from northern Washington varve data (Atwater, 1984,1986,1987; Waitt, 1985a); (2) the low slopes AGE OF FLOOD DEPOSITS at most rhythmite exposures, which lessened the likelihood of interflood development of erosional unconformities; and (3) the monoto- With the exception of the Columbia Hills section, where no nous light-brown color of rhythmite tops that hindered recognition of diagnostic evidence of deposit age was found, all study sites clearly root casts, insect burrows, and other signs of bioturbation. represent deposition during the late Wisconsin flood episode. The

80 Geological Society of America Bulletin, January 1993

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- PALEOCURRENT DIRECTION

NONFLOOD DEPOSITS

Figure 3. Graphic sections described along the Columbia River. Location of sections shown in Figure 2A. Elevations above sea level and above the modern river bed (in parentheses) are shown at the top of each section. Facies codes are explained in the text. Extent of bioturbation (% Biot.) is estimated by comparison to diagrams in Droser and Bottjer (1986). Grain size is indicated by 8 ° CSyff mcvc " sand a horizontal scale shown at the bottom of each section.

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Figure 4. Graphic sections described along the Tucannon River. Location of sections shown in Figure 2B. Elevations above sea level and above the modern river bed (in parentheses) are shown at the top of each section. Fades codes are explained in the text. Extent of bioturbation (% Biot.) is estimated by comparison to diagrams in Droser and Bottjer (1986). Grain size is indicated by a horizontal scale shown at the bottom of each section.

g ° csvffmc - sand

82 Geological Society of America Bulletin, January 1993

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TUCANNON 6 older set-C ash than for clasts overlying the latest Wisconsin deposits. A thin tephra not noted by Moody (1987) or Kiver and others (1991) is present within the Tucannon rhythmites at sections 2 and 4 (Fig. 4) and was previously exposed at now-graded roadcuts near section 7 (E. V. McDonald, 1990, personal commun.). Geochemical and min- eralogical analyses of samples from the Tucannon Valley demonstrate that this ash is set-S tephra (Table 1), requiring deposition of these rhythmites during the late Wisconsin flood episode.

DEPOSITIONAL FACIES

Sediments in the measured sections are classified into seven facies, four representing flood processes and three recording nonflood alluvial, eolian, and hillslope processes. Characteristics of these facies are described below.

Flood Facies FA

In sections that I studied, the coarsest sediments are poorly sorted, coarse gravel (clasts as much as 1.2 m across) with a sandy matrix (Figs. 4 and 6A). Beds are 0.4-1.0 m thick, massive, commonly reversely graded at the base, and merge upward, and laterally upvalley, into flood-facies FB over distances of a few meters. The massive, poorly sorted character of these beds suggests very rapid, or even en masse, deposition from highly concentrated sediment flows, perhaps as thick traction carpets at the base of the flood waters.

Note variable thickness and grain-size scales

DOWNVALLEY FLOW DIRECTION

- PALEOCURRENT DIRECTION

'«Sii' NONFLOOD DEPOSITS

Figure 4. (Continued).

Figure 5. Oblique aerial view to the northwest showing the area of distinctive 13 ka Mount St. Helens ashes are present in the Wanapum confluence of the Tucannon River (lower left), Palouse River (PR), and Dam and Hanford sections and have been verified geochemically and Snake River (right center to upper left). Flood waters crossed the former mineralogically at the Wanapum Dam locality by Moody (1987). The Palouse-Snake divide from the upper right and formed large gravel bars Wallula section overlies the set-S ashes, on the basis of approximate (gb) in the Snake River Canyon, now inundated to a depth of 25 m under correlation to a terrace 1 km to the east-southeast. Busacca and others Lake Herbert G. West. Erosional features and distribution of flood (1989) and Kiver and others (1991) attribute the Tucannon rhythmites gravel indicate that flood waters also crossed the basalt ridge in the to the >36 ka flood episode because Moody (1987) contends that foreground, traversed by the highway, between the Snake and Tucan- weathering-rind thickness on basalt clasts within the B-horizon of soils non Rivers. Locations of Tucannon sections 1 and 2 are indicated; section overlying the rhythmites are more similar to those found below the 3 is just out of view below the bottom of the photo.

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Figure 6. Illustrations of facies characteristics summarized in Table 2. (A) Massive flood facies FA (Tucannon section 3) in two reverse- to normal-graded beds, resting on flood facies FD. Note rip-up clasts of light-colored, cohesive FD sediment. Photo shows 3.5 m of section. (B) Plane- and cross-bedded, coarse, pebbly sand layers of flood facies FB, intercalated with light-colored fine sand and silt of flood facies FD. Note upward coarsening in bed that trowel (40 cm high) rests against and the interstratification of facies FB and FD along foresets to left of trowel, indicating episodic migration of the bedform separated by periods of quieter water. Lower part of flood sequence I in the Hanford section. (C) Flood facies FC (flood bed 5 at Tucannon section 7) resting on slope wash (NB) and grading upward through a massive bioturbated interval into loess (NA). Note the normal grading and bidirectional ripple cross-laminations (flow directions indicated by arrows); downvalley direction is to the right. (D) Flood facies FD at Tucannon section 3 consisting of an upward-thinning and -fining sequence of graded fine-very fine sands separated by thinly laminated, light-colored silts. Camera case, resting on contact with underlying FB, is 24 cm high. Note ripples in base of lowest bed, numerous flame structures, and upward increase in bioturbation to produce a massive silt marking the top of a flood sequence. (E) Poorly sorted, silty, pebbly sand with local diffuse laminations (for example, below coin) in nonflood facies NB at Tucannon section 7. (F) Massive, lenticular gravel and cross-bedded and scour-fill-bedded coarse sand typical of nonflood facies NC, intercalated with thin Missoula-flood beds (arrows) at the Columbia Hills section (beds 2-6 are labeled; see Fig. 3). Hat in lower left of photo for scale.

•

Flood Facies FB Flood Facies FC

Flood facies FB consists of coarse or medium sand to fine Flood facies FC composes most or all of classically described gravel, commonly with dispersed cobbles as large as 20 cm in Missoula-flood slack-water rhythmites. Beds are normal graded, or diameter. Normal grading is common within beds as thick as 1 m, reverse to normal graded and range from medium tp silty very-fine and reverse grading is present in some cases (Fig. 6B). The beds sand. Dominant sedimentary structures are horizontal laminations are horizontally bedded, or low- to high-angle planar-tabular and climbing-ripple cross-lamination (types A and B of Ashley and cross-bedded. Upward transitions from horizontal to low-angle, others, 1982; Fig. 6C). Paleocurrent directions are almost everywhere to high-angle cross-bedded subunits are typical. Cross-bedded bidirectional (upvalley followed by downvalley) in the Tucannon sec- subunits have erosive bases, attributed to reverse eddies caused tions (Fig. 6C) and, in the Columbia River sections, vary from uni- by flow separation over the dune crest, and thickness of suc- directional (usually downvalley) to recording multiple flow reversals. cessive cross-bed sets generally decreases upward. Flow direc- In the latter case, nearly symmetrical ripples related to interference tions for cross-bedded intervals may be unidirectional or poly- patterns are common (see also Fig. 10 below). General flow charac- directional. This facies is formed by decelerating, turbulent teristics are probably similar for FB and FC, although the finer grain flows. size of FC requires lower current velocities.

TABLE I. MICROPROBE GLASS AND MINERAL ANALYSES OF ASHES IN TUCANNON VALLEY AND COMPARISON TO TEPHRA STANDARDS

Glass analyses Na20 Fe203 K20 MgO CaO A12OJ Ti02 Si02 GS90T2*1 (17 analyses) 4.12 ± 0.16 1.36 ± 0.09 2.30 ± 0.09 0.29 ± 0.02 1.47 ± 0.05 13.37 ± 0.10 0.17 ±0.03 76.84 ± 0.23 Correlation coefficients Mt. St. Helens set S: ash So 0.833 0.990 0.982 0.746 0.983 0.981 0.934 0.994 avg. 0.930 ash Sg 0.935 0.8% 0.935 0.925 0.934 0.970 1.000 0.989 avg. 0.948 Mt. St. Helens set C: ash Cw 0.900 0.843 0.985 0.999 0.875 0.947 0.565 0.997 avg. 0.889 ash Cy 0.948 0.774 0.944 0.914 0.947 0.959 0.646 0.998 avg. 0.891 GS90T3 (16 analyses) 4.17 ± 0.16 1.42 ± 0.06 2.31 ± 0.08 0.29 ± 0.01 1.46 ± 0.05 13.42 ± 0.07 0.17 ± 0.03 76.68 ±0.18 Correlation coefficients Mt. St. Helens set S: ash So 0.823 0.951 0.987 0.739 0.995 0.985 0.942 0.992 avg. 0.927 ash Sg 0.947 0.933 0.930 0.934 0.923 0.974 0.991 0.991 avg. 0.953 Mt. St. Helens set C: ash Cw 0.888 0.809 0.980 0.991 0.865 0.951 0.561 0.999 avg. 0.880 ash Cy 0.936 0.743 0.949 0.905 0.936 0.963 0.641 1.000 avg. 0.884

Mineral analyses MnO Na20 FeO K20 Si02 CaO A1203 Ti02 MgO Cumminetonite GS90T2* (n = 4) 0.35 ± 0.10 20.28 ± 0.86 0.02 ± 0.01 53.62 ± 0.62 1.22 ± 0.08 1.82 ± 0.29 0.20 ± 0.04 18.69 ± 0.64 0.68 ± 0.05 Mt. St. Helens set S: Ash So (n = 10) 0.30 ± 0.07 21.30 ± 0.52 0.00 ± 0.01 53.91 ± 0.68 1.18 ± 0.28 1.81 ± 0.37 0.19 ± 0.06 18.50 ± 0.57 0.63 ± 0.03 ash Sg (n = 8) 0.34 ±0.11 21.25 ± 1.17 0.01 ± 0.01 53.46 ± 0.66 1.38 ± 0.35 2.10 ± 0.57 0.26 ± 0.10 18.23 ± 0.63 0.63 ± 0.06 Mt. St. Helens set C: ash Cw (n = 22) 0.35 ± 0.10 18.88 ± 0.49 0.01 ± 0.03 53.51 ± 0.59 1.71 ± 0.41 2.86 ± 0.56 0.23 ± 0.07 19.16 ± 0.36 0.69 ± 0.05 ash Cy (n = 12) 0.43 ± 0.14 18.58 ± 0.47 0.03 ± 0.04 53.14 ± 0.78 1.99 ± 0.49 3.24 ± 0.75 0.24 ± 0.07 18.87 ± 0.50 0.69 ± 0.05 Hypersthene GS90T2* (n = 10) 0.01 ± 0.02 22.09 ± 0.42 0.01 ± 0.01 52.71 ± 0.44 0.53 ± 0.05 1.17 ± 0.26 0.12 ± 0.04 22.09 ± 0.44 0.86 ± 0.05 Mt. St. Helens set S: ash So (n = 14) 0.02 ± 0.02 22.06 ± 0.73 0.01 ± 0.01 53.21 ± 0.55 0.57 ±0.11 1.21 ± 0.22 0.12 ± 0.03 22.36 ± 0.27 0.84 ± 0.06 ash Sg (n = 4) 0.02 ± 0.03 21.93 ± 0.14 0.01 ± 0.01 53.24 ± 0.63 0.76 ± 0.23 0.97 ± 0.42 0.14 ± 0.10 22.66 ± 0.31 0.82 ± 0.06 Mt. St. Helens set C: ash Cw does not contain hypersthene ash Cy does not contain hypersthene

Note: electron microprobe analyses by Washington State University, Geoanalytical Laboratory, Scott Cornelius, analyst. *Ash sampled in Tucannon section 2. +Ash sampled in Tucannon section 4.

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Flood Fades FD be distinguished from other fine-grained sediment. Grain size is variable, ranging from fine (or, rarely, up to medium) sand to silt. The Hood fades FD is also present as part of many classically de- facies is characterized by multiple (as many as 20), sharp-based, 1- to scribed rhythmites in the Walla Walla and Yakima Valleys, and its 5-cm-thick, graded strata that typically become finer and thinner up- characteristics, at one extreme, are gradational to those of FC. This ward through total thicknesses of 5 to 50 cm (Fig. 6D). Structural facies, however, represents dilferent hydraulic conditions and should variations include (1) laminated silt; (2) internally massive, graded

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Figure 7. Illustrations of features indicating hiatuses between, or depositional continuity across, flood-bed contacts. (A) Bioturbated facies FD(?) separating facies-FB intervals and marking break in deposition between flood sequences II and III at Tucannon section 1. Note rodent burrows (rb) in coarse sand to right of trowel (40 cm long) and in the overlying, thoroughly bioturbated sandy silt. Cross-bedding indicates upvalley flow to the right. (B) Mudcracked bedding surface, indicative of a hiatus, developed on 4 cm of facies FD overlying rippled FC at top of flood bed 11, Wanapum Dam section. Pencil is 14 cm long. (C) Continuity in deposition indicated by gradual transition, at level of pencil (14 cm long), from upward-fining and -thinning facies FD into facies FC capped by FD. Note starved ripples to left of pencil; downstream is to the right. Bed 9 and upper part of bed 8 at Wallula section. (D) Outcrop in the Pasco Basin showing facies FD marking a flood-bed top between IB units. Rodent burrows (rb) and small, finger-like insect burrows (ib) transect contacts, indicating that bioturbation occurred after deposition of both beds in this flood sequence. (E) Facies FD, capping flood bed within a flood sequence in the Hanford section; multidirectional ripple cross-laminations (small arrows indicate transport directions) are separated by more-resistant silt drapes. Note lack of bioturbation and conformity (open arrows) of coarse sand in FB of overlying flood bed with ripplecrest s and troughs in the FD interval. (F) Unbioturbated FC and FD interval at top of floodbe d 2 in Tucannon section 1 between FB intervals of flood beds within the same flood sequence. Compare with similar bioturbated facies marking a flood-sequence top in A.

•

horizontal strata typically associated with flame structures (Fig. 6D); extensive loess of the Palouse Formation, and I concur with Bunker (3) drape-laminated beds (terminology of Ashley and others, 1982) and Waitt that these deposits are of eolian origin. All volcanic ash overlying ripples of FC; and (4) alternating ripple cross-laminated and layers observed in this study are within this facies. Extensively bio- massive or drape-laminated intervals wherein cross-laminations com- turbated FC and FD may closely resemble NA, although local pres- prise as much as 80% of the facies (thus, transitional to FC; Fig. 7E), ervation of laminations characteristic of FC and FD and generally or as little as only a few percent represented as starved-ripple lami- darker hues for the loess are helpful discriminants. nations (Fig. 7C). Flow directions indicated by cross-laminations and flames are unidirectional within a single graded stratum and typically indicate downvalley flows in the Tucannon sections, although more- Nonflood Facies NB variable flow directions are recorded in the Columbia River sections (Fig. 7E). Drape laminations and the more-even, continuous thin- Massive or discontinuously laminated, lenticular beds of poorly laminated silt are indicative of deposition from suspension within sorted silty sand, with scattered basaltic and exotic pebbles as large ponded waters. The repetitive nature of the thin graded strata (Fig. 6D) as 10 cm across, are interlayered between some flood-deposited rhyth- and indications of prevalent downvalley transport, suggest that these mites in the Tucannon Valley (Figs. 4 and 6E). Downslope-oriented beds were deposited by low-volume density currents caused by influx flame structures are present in some circumstances where these de- of water and sediment from preflood drainages into a hydraulically posits overlie flood facies FC. These sediments are indistinguishable ponded lake. Flood facies FD, alone, provides unambiguous evidence from mixed eolian and colluvial mantles that are present on the mod- for hydraulically ponded, relatively still waters. ern canyon-wall hillslopes above the rhythmite terraces. I interpret this facies to represent slope wash. Exotic pebbles are derived from Flood-Fades Associations flood-related ice-rafted debris (Bretz, 1929) that is present on hillslopes in the Tucannon drainage to elevations at least 60 m higher than the An individual flood bed, or rhythmite, consists of (1) a single highest known rhythmites. continuously graded unit, generally ranging from a medium sand to gravel base upward to a fine sand or silt top (Fig. 6C) or (2) a distinct couplet (rhythmite in the strict sense) composed of a basal coarse- Nonflood Facies NC grained bed and an upper, conspicuously finer-grained layer (Fig. 6B). A variety of stacking patterns of the four flood facies have been Least abundant of the recognized nonflood deposits are those observed to compose single flood beds. Abundances from the mea- attributed to nonflood alluviation. The moderately to poorly sured sections are noted in Table 2. Flood facies FC alone, or with sorted sand and gravel exhibit one of two characteristic sets of facies FD, represents the majority of flood beds. Flood beds are sedimentary structures (Fig. 6F). Lenticular, scour-and-fill bed- designated by Arabic numbers on Figures 3 and 4. ded strata typical of shallow, flashy, braided channels are more common than sequences dominated by planar-tabular or trough Nonflood Facies NA cross-bedding. Cross-bedding, where present, indicates deposition from flows moving downvalley, or downslope from adjacent Massive beds of bioturbated fine sand and silt with dispersed valley margins. Poorer sorting, rarity of horizontal bedding or medium and coarse sand grains (Fig. 6C) have been noted between planar-tabular cross-bedding, dominance of scour-and-fill struc- some flood beds by Bunker (1980,1982) and Waitt (1980). These units tures, and lack of upslope transport features distinguish nonflood are texturally and structurally indistinguishable from the regionally facies NC from flood facies FB.

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TABLE 2. VERTICAL FACIES SEQUENCES IN FLOOD BEDS moval of stratigraphic evidence for the postulated hiatus (for example, Waitt, 1985a, p. 1285). Amalgamation is most notable near the mouths Facies Columbia Tucannon Total River River of backflooded tributary valleys, where scour during flooding clearly sections sections occurred. In the exposures studied, however, erosion of flood-bed tops or intraflood sediment was observed to occur only very locally. FA-FB-FD 0 1 1 FA-FD 0 4 4 Careful examination of the flood-bed contacts along the entire face of FB 2 4 6 FB-FC 1 8 9 the exposure, several tens of meters to more than 100 m long, often FB-FC-FD 0 J 5 revealed evidence of hiatuses adjacent to local sites of amalgamation FB-FD 7 6 13 FC 16 45 61 (Fig. 8B), but in all cases where more than one bed is interpreted to FC-FD 31 39 70 FD-FC-FD 1 0 1 occur in a flood sequence, a continuous rhythmite top of facies FC or FD 1 6 7 FD was found intact and with preserved depositional structures at the contact (for example, Fig. 7F). Erosion by floods also cannot account for continuity of deposition between flood beds (for example, Fig. 7C) EVIDENCE FOR OR AGAINST HIATUSES BETWEEN or conformity of flood-bed contacts (for example, Fig. 7E). In the latter SUCCESSIVE FLOOD BEDS case, it is inconceivable that flood erosion of nonflood facies or an entire horizon of bioturbated flood facies would perfectly exhume the Flood beds are grouped into flood sequences, which are defined ripple forms in unlithified, poorly consolidated sediment. Equally un- as one or more flood beds that are bounded by nonflood sediments, likely is the occurrence of exclusively flood facies in sections 3 m or horizons of bioturbation, or desiccation structures, but that lack such more thick (for example, Fig. 3, Hanford section), consisting of sev- features between beds, and are thus inferred to record deposition eral flood beds of subequal thickness that lack any evidence of bio- during a single flood. Flood sequences are denoted by roman numerals turbation, including rodent burrows that are, in other cases, traced in Figures 3 and 4. In flood sequences containing more than one bed, downward for 1 to 2 m below the paleo-surface. the beds are subequal in thickness and similar in grain size and are not There has been considerable debate over the significance of soft- adequately described as minor deviations in grading within single beds sediment-deformation features at rhythmite contacts as evidence (compare with Waitt, 1985b, p. 357). against hiatuses in deposition (Bjornstad, 1980, p. 66; Baker and Bun- The definition of flood sequences is critical to the one-bed-per- ker, 1985, p. 19; Waitt, 1980, p. 662; Waitt, 1985a, p. 1285). I concur flood versus many-beds-per-flood debate. The recognition of flood- with Waitt (1985a) that flame and load structures merely require a wet sequence boundaries is most straightforward where nonflood sedi- substrate and do not indicate deposition of multiple beds during single ments intervene between flood facies (Figs. 3, 4, 6C, and 6F) and floods. In the studied sections, flame structures also occur at the base where desiccation structures occur (Fig. 7B). Through careful outcrop of beds comprising slope-wash facies NB, and at the base of flood beds preparation with mason's trowel and brushes, I found it much easier resting on eolian facies NA. Of 35 cases of flame and load structures to detect evidence of bioturbation than was the case stated by Waitt at flood-bed contacts, however, 31 occur within flood sequences, (1985, p. 1280). Discrete insect and rodent burrows are very common suggesting that these deformational features, though not uniquely and generally increase in abundance upward through a flood sequence associated with intraflood deposition, are more commonly related to until, in many cases, depositional sedimentary structures are entirely those conditions. Intense deformation related to load-induced dewa- obliterated (Figs. 6C, 6D, and 7A). Abrupt upward transitions from tering also occurs within flood sequences (Fig. 8A). In the Tucannon bioturbated to nonbioturbated, or extensively bioturbated to only Valley, a roughly cylindrical collapse pipe, of uncertain origin, dis- slightly bioturbated, sediment define buried landscape surfaces and, rupts most, though not all, of the thin graded beds in the facies-FD top hence, flood-sequence boundaries. The degree of bioturbation was of a flood bed, indicative of deformation during a flood (Fig. 8B). Also estimated on the basis of degree of preservation of depositional struc- notable in this circumstance are broken and rotated blocks of fine- tures, analogous to the method of Droser and Bottjer (1986), and is grained sediment, indicating sufficient cohesiveness that rip-up clasts represented in Figures 3 and 4. Careful scrutiny of burrow networks could be incorporated within units of the same flood (compare with is required, because single rodent burrows commonly extend down- Waitt, 1985a, p. 1274). Injection dikes (Fig. 8B) and wholesale dis- ward through more than one flood sequence. In some instances, bio- ruption of bedding at many sites in the Tucannon Valley (Figs. 14 and turbation is restricted to insect burrows that transect flood-bed con- 15 below), always at intersequence flood-bed contacts, suggest that tacts (Fig. 7D), and burrowing must postdate deposition of both beds this form of deformation was preferred by firmer substrates. and does not serve as evidence for a hiatus between deposition of The most-conspicuous deformational features in the slack-water successive rhythmites. deposits are clastic dikes, as much as 1 m wide, most of which were Evidence against hiatuses between flood beds, and thus in sup- infilled from above during multiple periods of fissure opening. The port of multiple beds per flood, included lack of nonflood facies, dikes, which are polygonal in plan, have been described in some detail desiccation features, or distinct horizons of bioturbation between by Lupher (1944), Baker (1973), Black (1979), Carson and others flood beds. Two other types of bed transitions are also significant to (1979), and Bjornstad (1980) and are of unclear origin, although Baker, this interpretation. In rare cases, transitions from facies FC to FD and Black, and Bjornstad favored initial formation as a loading response back to FC occur without distinctive breaks, suggesting continuity in to voluminous Missoula-flood waters. Because the clastic dikes ex- sedimentation (Fig. 7C). The second distinctive bed transition is basal tend uninterrupted through the deposits of many late Wisconsin floods facies FB or FC sediment resting conformably upon ripple cross- at any one exposure, it seems required that the clastic dikes be un- laminations of facies FC or FD (Fig. 7E). related to the floods. It could be argued that absence of nonflood sediments or bio- Flood sequences in the studied sections contain one to as many turbated horizons marking paleoground surfaces results from amal- as nine flood beds, although 75% of the floods are recorded by only gamation of beds deposited during different floods and erosive re- one bed (Fig. 9). Sections at Mabton, Zillah, Badger Coulee, and

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Figure 8. Soft-sediment deformation features. (A) Intense deformation at contact between flood beds 8 and 9 at Wallula section. Deposition of coarse FC basal part of bed 9 (a) caused expulsion of water from lower part of reverse-graded bed 8 (b) and disruption of fine-grained FD sediment at top of bed 8 (c). FD interval at top of bed 9 (d) was deposited after the lower sediments were deformed. Sediments below the dashed line belong to an older flood sequence (note bioturbation; e) and were not sufficiently saturated to be disturbed. (B) Alternating FB/FD beds, representing parts of flood sequences II, III, and V at Tucannon section 3, overlain by facies-FA/FB bed; bioturbated intervals marking flood-sequence tops are marked by b's. Collapse feature on left (marked by arrows) exhibits upward decrease in deformation of host sediment, indicating formation during deposition of uppermost FD interval (also illustrated in Fig. 6D). Also note downward-tapering injection dike, right of center.

Burlingame Canyon, which have been variously cited as exhibiting ciently powerful to contribute to scabland erosion at this site. The evidence for one or many beds per flood (Bjornstad, 1980; Bunker, absence of a depositional record of pre-set-S floods, conversely, sug- 1980, 1982; Waitt, 1980), were examined during the course of this gests that earlier floods in the latest Wisconsin flood episode did scour study, with emphasis on close scrutiny of flood-bed contacts on care- this area. Rhythmites consist mostly of flood facies FC and FD. fully cleaned exposures. All flood beds at Mabton and Zillah and 25 The presence of facies FD indicates relatively still waters ponded easily accessible beds at Burlingame Canyon meet criteria for repre- behind Sentinel Gap (Fig. 2A). Influxes of current-driven sediment senting only one bed per flood, in concurrence with Waitt's (1980) interrupted slack-water sedimentation to produce the multiple beds assessment. The same conclusion was reached in Badger Coulee. within flood sequences III, V, and VIII (Fig. 3). Flood beds 14 and 17 Bunker (1982) argued that massive-appearing rhythmite tops in Bad- are notable for their dominance of facies FD and the occurrence of this ger Coulee could be traced laterally to local areas of preserved plane laminations and cross-laminations resulting from flood deposition. 100! This relationship can be seen at all slack-water localities that I have visited, but rather than being indicative of continuous deposition (Bun- 90 co ker, 1982), these relationships argue for hiatuses in deposition during m o 80 which bioturbation disrupted nearly all of the depositional structures, z LU COLUMBIA RIVER SECTIONS accounting for the more generally massive rhythmite tops. The con- D 70- O ditions leading to development of multiple beds per flood as opposed LU to the more common one bed per flood are suggested by lateral vari- en 60 I Q O ations in flood-sequence stratigraphy, which are discussed in the next o 50 two sections. 40 o TUCANNON RIVER SECTIONS COLUMBIA RIVER VALLEY cr 30 LU CQ S 20 Wanapum Dam Section D 10 The Wanapum Dam section (Fig. 3) is within flood-produced scabland adjacent to the Columbia River, 10 km upstream from Sen- 123456789 tinel Gap. The base of the section rests on basalt and consists of loess FLOOD BEDS PER FLOOD SEQUENCE enclosing two bioturbated layers of the set-S ash. Nine flood sequences are recorded by 17 flood beds. The preservation of a multiple- Figure 9. Bar graph of number of flood beds per flood sequence in flood record requires that none of the post-set-S floods were suffi- studied sections. 75% of the flood sequences contain only one flood bed.

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Figure 10. Examples of complex ripple cross-lamination reversal patterns in facies FC in Columbia River sections. Arrows indicate transport directions and s's mark nearly symmetrical ripples produced by multidirectional current interference. (A) Bed 11 at the Wanapum Dam section (height 25 cm; downvalley is to right). (B) Bed 6 at the Columbia Hills section (height 40 cm; downvalley is to left); note upward grain-size increase.

facies at the base of the recorded flood sequence (Fig. 3). These progressively finer grained sediment, with fine sand and silt dominat- relationships, combined with apparent nonerosive bases to these beds, ing along the eastern margin of the basin (Bjornstad and others, 1991). suggest no surge of flood water and sediment past this point during The Hanford section is near the center of the basin in a depositional those two floods, but rather, relatively passive inundation by rising, tract dominated by coarse sands. The illustrated section (Fig. 3) hydraulically ponded waters. records 11 flood sequences represented by 22 flood beds. The base of Most ripple cross-laminations record flow in only the down- flood sediments is not exposed. stream direction, consistent with deposition in a relatively low-energy, Another section was described in an excavation 100 m away, and protected site in the scabland peripheral to the main flood flow. A the gross aspects of the two sections and their correlation are shown notable exception is flood bed 11, which not only records multiple in Figure 11. Flood sequences can be correlated between the two current reversals, but also exhibits basal cross-laminations, indicating sections. Most correlative sequences contain the same number of beds initial upstream transport (Fig. 10A). This bed is not at the base of the in both sections. The most notable exception is sequence II, which flood sequence, however, and the ripple pattern, suggesting interfer- shows conspicuous stratigraphic variations along the excavation ence between upstream- and downstream-directed flows, may record walls. The reverse-graded base of bed 14 in the first section may (1) eddy effects or (2) the arrival of flood water from the Crab Creek correlate to a discrete, thin, fine-grained bed in the second section. scabland-channel system, 5 km to the south (Fig. 2A), causing inter- Beds in other sequences correlate one for one between the two sec- ruption of the primarily suspension deposition in ponded water re- tions, although thickness differences are apparent. Correlation of flood corded by facies FD in the upper part of flood bed 10. Correlative beds and sequences supports the criteria applied for defining them; if rhythmites exposed on the opposite side of the river, 5.5 km to the local flood-bed amalgamation were responsible for removing evidence northwest, contain more-common upstream-directed ripple cross- for hiatuses in deposition, then it is highly unlikely that this evidence laminations, but always overlying downstream-directed ripples within is only preserved between the same rhythmite pairs in different sec- the same flood bed, or low in flood beds within, not at the base of, flood tions. Remarkable in both sections is the lack of any bioturbation sequences. features in the lower 3 m and the common occurrence of facies-FB coarse sand resting conformably on very-fine sand ripples of under- Hanford Section lying facies FD within beds of the same flood sequence (Fig. 7E). Dark basaltic sands and fine gravel of facies FB predominate in The illustrated Hanford section (Fig. 3) is one of two that were the Hanford section (Figs. 3 and 6B). Many beds are notably reverse studied in adjacent excavations at U.S. Ecology Incorporated's low- or reverse to normal graded. All cross-bedding in facies FB indicates level radioactive-waste-disposal facility within the U.S. Department flow to the east-southeast and southeast, approximately parallel to of Energy's Hanford Site. Flood-deposit stratigraphy and lateral re- expected main-channel flow. lationships are complex and not clearly resolved in the Pasco Basin, Each flood bed in the lower part of the section is capped by facies largely because of lack of dissection and the transient nature of ex- FD, consisting of climbing-ripple laminae separated by drape laminae cavations at the Hanford Site. Hood-gravel bars cover most of the (Fig. 7E). Two to four current reversals are typically recorded in each northern Pasco Basin and occur within a few kilometers of the modern FD layer, and flow directions are markedly poly modal. The abrupt Columbia River channel farther south. Gravel gives way eastward to transition in grain size and structure from facies FB to FD suggests

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ponding of water adjacent to the most prominent constriction in the Missoula flood path. Gradual FC to FD transitions at some flood-bed contacts (for example, Fig. 7C) may record the more effective attenuation of far-traveled flood pulses within the Pasco Basin at this locality than at the Hanford section. Ripple cross-laminations record dominantly downvalley flow, although multiple flow reversals occur within several beds, and upvalley flow is indicated in the upper parts of some flood sequences. Because no flood waters entered the Co- lumbia River downstream from this point, current reversals must be the result of eddies or reflection of flood waves from the steep hillslopes flanking Wallula Gap.

Figure 11. Correla- Columbia Hills Section tion of two closely spaced (100 m apart) sections in The Columbia Hills section is located along an ephemeral first- the Pasco Basin. Section on order tributary to the Columbia River at an elevation 140 m above the right corresponds to the modern river. Six flood sequences are present, each consisting of a Hanford section in Figure single bed, four of which are reverse graded (Figs. 3 and 10B). The 3. Flood-bed (arabic) and rippled flood beds (facies FC) consist of micaceous arkosic sand sep- flood-sequence (roman) arated by nonflood basaltic alluvium (facies NC; Fig. 6F). Ripple numbers refer to those in cross-laminations indicate flow reversals in several beds (Fig. 10B), the Hanford section. Solid with initial flow always upslope relative to the present ephemeral lines show correlations of channel, but oriented approximately downstream, relative to the Co- flood sequences; thin lumbia River. Facies FD is not present, suggesting that protracted dashed lines show correla- ponding did not occur at this site. tions of flood beds. Thick dashed lines show correla- Discussion tions of three Mount St. Helens set-S ash layers be- Slack-water rhythmites deposited along the Columbia River are tween the two sections. sedimentologically similar to those previously described from backflooded tributary valleys, in contrast to statements to the contrary by Waitt (1980, p. 660). Deposition occurred beneath polydirectional currents, but with initial transport in the downvalley direction, an important distinction from backflooded-valley sequences. Current reversals must, at least in part, result from eddies or reflection of flood currents, rather than wholesale change in flow direction. Hydraulic ponding for adequate durations to generate flood facies FD occurred only at the three lowest-elevation sections, with respect to the modern channel elevation; strong evidence for protracted still- water conditions is lacking at the Columbia Hills section. This latter locality records only one bed per flood, whereas multiple beds occur in many or all flood sequences at the three lower sites. Although varying numbers of flood sequences were measured at each site, there is a suggestion that the number of beds deposited per flood is also sudden changes in hydraulic conditions. I interpret this transition to related to elevation. In order by increasing elevation above the river, represent the passage of a violent flood surge, followed by an interval 75% of the Wallula section flood sequences contain multiple beds, as of hydraulic ponding when agitated surface waves and bottom cur- do 50% of those at Wanapum Dam, 27% at the Hanford section, and rents were reflected in a complex fashion from the basin margin and none at the Columbia Hills. This suggests that surges subsequent to intrabasinal highs. The return to facies-FB deposition, typically with the initial inrush of flood water were not as voluminous or powerful, little or no disruption of ripples formed during relatively low-energy and pulsating-flood records were restricted to low elevations. deposition (for example, Fig. 7E), represents further incursions of The thinner and less-extensive post-set-S rhythmites in the Walla flood water and sediment into the basin. Most of these later flood Walla and Yakima Valley have been taken as indicating that these late pulses were sufficiently voluminous to have suffered little attenuation floods were of smaller discharge and volume than previous events in by whatever volume of ponded water existed in the basin, so that they the late Wisconsin flood episode (Waitt, 1980; Baker and Bunker, are as thick as those recording earlier pulses (Figs. 3 and 11). 1985; O'Connor and Baker, 1992). This interpretation is supported by the preservation of slack-water sediments within the scabland at Wallula Section Wanapum Dam and at a low elevation above the Columbia River at Wallula (also noted by Bjornstad, 1980, and O'Connor and Baker, The Wallula section illustrates parts of four post-set-S ash flood 1992). It is also noteworthy that the well-known gravel-megaripple sequences at a low-elevation position 3.7 km upstream from Wallula locality at Crescent Bar (Fig. 2A) consists of 100 m of coarse, foreset- Gap (Fig. 2A), and the abundance of facies FD records the expected bedded gravel, locally overlain by several meters of finer-grained

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TUCANNON RIVER VALLEY SECTIONS

Section 1

The farthest downvalley section along the Tucannon River is located near the eddy bar at the river mouth (Figs. 2B and 5). It contains the incompletely exposed record of six flood sequences represented by at least 13 flood beds (Fig. 4). The overall facies character is similar to the Hanford section in the Pasco Basin. Distinct bioturbated horizons define flood-sequence contacts (Fig. 7A), whereas flood-bed boundaries within sequences are nearly planar and lack evidence of biogenic disturbance (Fig. 7F).

Section 2

Section 2 is located in the deposits of a valley-wall alluvial fan (Figs. 2B and 5) and contains a high proportion of nonflood facies. At least seven, and probably eight, flood sequences occur, with the second sequence containing multiple rhythmites and evidence of standing water. This section illustrates the complexity of interfingering of flood Figure 12. Typical character of exposures at Tucannon section 3 and nonflood facies at extreme valley margins, which has not been showing erosional surfaces between flood sequences, local nonflood al- illustrated in previous studies, and is also stratigraphically important luvium (NC), and intercalation of coarse- and fine-grained beds in 1.5- because it includes one thin layer of Mount St. Helens S ash near its m-thick flood sequence V. base (Fig. 4; Table 1).

Section 3

Section 3 was described from roadcuts near Starbuck that have been illustrated previously (Baker, 1973; Patton and others, 1979; rhythmites (Waitt, 1980; Moody, 1987). The set-S ashes occur low in Waitt, 1985b). Flood-sequence bases are markedly erosive (Fig. 12) this rhythmite sequence. At the Hanford section, the post-set-S flood and overlain by flood facies FA and FB. At least 35 flood beds, sequences are thinner, finer grained, generally contain only one flood comprising 9 flood sequences, appear in this outcrop. Lateral varia- bed, and contain little or no facies FD. These collective observations tions are extreme, as a consequence of erosion between, or during, suggest that the last flood to activate or scour the collossal bars at floods, and no single vertical section can accurately represent the flood Crescent Bar, or to cause erosion in the scabland above 200-m ele- stratigraphy. The section illustrated in Figure 4 shows a composite of vation at Wanapum Dam, or to generate complex hydrographs or deep four sections measured over a distance of400 m. Figure 13 illustrates flooding in the Pasco Basin, occurred prior to the set-S eruptive some of the lateral facies complexity at this site. period. Section 3 has very coarse facies-FA beds, which are not recog- On the basis of the Mount St. Helens set-S ashes, I loosely nized in other exposures (Fig. 4). Abundance of facies FA is probably correlate the upper seven flood sequences at the Hanford section with related to the location of this section directly southeast of the spillover the upper eight at the Wanapum Dam section. The slight difference in point for flood waters that crossed the basalt ridge between the Snake number of flood sequences between these sites may result from (1) River and the lower Tucannon Valley (Figs. 2B and 5). Flood gravel erosion of the upper sequence at the Hanford section or (2) lack of transported by these flows drapes the 90-m-high canyon wall northeast significant deposition by the last flood at this elevation in the Pasco of section 3, suggesting that large volumes of gravel were likely carried Basin. On the basis of exclusively facies FD in the uppermost flood as thick traction carpets driven by the extreme shear stresses of flood sequence at Wanapum Dam, I interpret this last flood to have been a flows descending the steep canyon wall. This gravel was rapidly de- relatively low-discharge event. Despite being located farther down- posited by the flows as they reached the valley floor. The presence of stream and at a higher elevation than the Wanapum Dam section, more than one FA bed within a flood sequence (for example, sequence post-set-S flood beds at the Hanford section are considerably thicker II in Fig. 4) indicates, therefore, that (1) more than one flood surge and coarser grained. This observation is incompatible with a simple crossed this septum during single floods or (2) gravel deposited on the model of downstream and upslope attenuation of flood energy along canyon wall by the initial flood surge repeatedly slumped downward the Columbia River, but is consistent with the introduction of the most into the subsequent slack-water stilling basin, perhaps in response to voluminous and powerful flood waters along flood routes that joined undercutting by later upvalley flood surges. the Columbia River downstream of Wanapum Dam, but largely up- Correlation of multiple-bed flood sequences II, III, IV, and V stream from the site of the Hanford section (for example, Crab Creek, across the outcrop face (Fig. 13) reveals repetitive facies patterns that Othello Channels; Fig. 2A). Although it has been long recognized that are interpreted to represent repetition of similar events in successive scabland channels converge toward the Pasco Basin along several floods. Each flood sequence is represented by four "members." different routes, recent modeling of flows in the basin (Craig and "Member a" consists of a basal FA or FB bed, with a very irregular Hanson, 1985; Craig, 1987) has presented the simple case of all water thickness and erosional base (Figs. 12 and 13). Cross-strata record entering through Sentinel Gap, which is not substantiated by the both upvalley and downvalley flow, and, in the case of flood sequence stratigraphic data. III, sediment transported downvalley rests erosively upon sediment

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Figure 13. Lateral facies variations over a distance of 400 m in floodsequence s II-VI at Tu- cannon section 3. Repetitive vertical facies patterns delineated as a through d are explained in the text.

sand

transported upvalley (Fig. 13). "Member b" is composed of facies-FD cant thickness variations (sequence V), suggesting attenuation of flood strata that thicken and thin over depositional irregularities at the top energy in the ponded waters recorded by facies FD. "Member d" of "member a." Ripple cross-laminations and flame structures within contains upwardly fining and thinning strata of facies FD (Fig. 6D). the multiple thin, graded beds indicate primarily downvalley flow, Convolute bedding and load structures are prominent, and grain size although some upvalley currents are also recorded. "Member c" is a is generally finer and beds thinner than in facies FD within "member continuous interval of interbedded FC and thin FD layers recording b." Ripple laminations and asymmetric load structures are rare but interruption of relatively still-water conditions by three or four up- always indicate downvalley flow, except for rare occurrences of upvalley-directed flood pulses. The facies-FC beds in "member c" are valley-directed ripples in the first bed immediately overlying the up- typically much finer than the beds in "member a" and thin and fine permost facies-FC bed in "member c" (Fig. 6D). The nonflood allu- upvalley (flood sequences III and IV) or fine upvalley with insignifi- vium, above flood sequences III and V (Fig. 13), contains no

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Figure 14. Erosional and depositional features at Tucannon section 4. (A) Erosion surface (dashed line) separating coarse flood beds (above) from fine-grained flood beds, loess, and gravel of probable colluvial origin (below). Irregular nature of this surface is due to deformation of underlying fine sediments, pieces of which are incorporated into the overlying flood deposits. Note injection dikes (id) of coarse material into fine-grained sediments and a more typical clastic dike (cd), which is truncated by the erosion surface. Illustrated part of outcrop is about 5 m high. (B) Example of thin FD flood bed, outlined by dashed lines, encased in loess in lower unit (Fig. 4). Starved ripples indicate both downvalley (left) and upvalley (right) flow. Clastic dike interrupts bedding in center of view.

conclusive evidence for having been deposited following a hiatus after soft-sediment deformation is present along this surface (Fig. 14A), deposition of underlying "member d" sediments. Perhaps this allu- including downward injection structures and diapiric rise of underly- vium, deposited 15 m above the modern river channel, represents ing sediment, suggesting that the erosion surface was either formed or progradation of fluvial sediments into the draining, hydraulically pon- significantly modified by the flood that deposited the overlying sedi- ded lake. ment. It is, therefore, uncertain whether all of the flood sequences are The recognition of the repetitive flood-sequence stratigraphy at the product of late Wisconsin flooding, or if only the deposits above section 3 (Fig. 13) is important for two reasons. First, if it were argued the erosion surface, which enclose a set-S ash layer (Fig. 4; Table 1), that the ubiquitous erosional features at this site (for example, Fig. 12) are related to the last flood episode. The lack of soil features below the preclude unambiguous assignment of more than one flood bed to a erosion surface or soil clasts in the overlying flood bed favors assign- flood event, it becomes very difficult to explain the repetitive pattern ment of both flood-sequence groups to the same flood episode. Only of cumulative deposition of each four or five floods. Second, by in- flood sequences VII and VIII contain more than one flood bed. Dis- ferring that this repetitive pattern results from successive floods, it tinct bioturbated horizons and, near the top of the section, loess layers becomes necessary to postulate similar complex flood histories for separate all other flood-bed pairs. each flood. The essential elements of this scenario are (1) initial up- The five lowest flood sequences are very significant because they valley surge of flood water ("member a"), followed by (2) relatively consist only of facies-FD flood beds (Fig. 14B), implying that the site quiet slack-water ponding and deposition of fine-grained sediment by was inundated by ponded water but was not subjected to strong small downslope-directed density currents and minor upvalley-di- upvalley flood surges. Flood bed 14, above the erosion surface, is rected bottom currents (' 'member b''), interrupted by (3) three or four mostly facies FD and also suggests relatively passive flooding of the flood surges upvalley from the confluence with the Snake River valley. Ponding is recorded by facies FD in most of the other flood ("member c"), succeeded by (4) continued still-basin deposition, sequences, but this facies is notably absent above the set-S ash. mostly by downvalley transported density currents, beneath deepening hydraulically ponded waters ("member d"). The lack of an up- Section 5 ward-coarsening and -thickening sequence in the top of "member d" suggests that drainage of the lake was abrupt and caused incision of In this section, which rests on pre-late Wisconsin loess, most flood sediments in the center of the valley. This dissection may not flood beds are simple graded units separated by bioturbated horizons have occurred at a uniform rate, however, and may account for the (Figs. 4 and 15). These beds make up seven flood sequences that drape presence of alluvial facies NC above two flood sequences on the a loess knoll with only slight thickening of beds on the upvalley side. slack-water-deposit terrace. Contacts are planar, except at the base of flood bed 7, where there is 1.5 m of erosional relief on the upvalley side of the buried loess knoll Section 4 in association with extreme soft-sediment deformation like that seen along the erosional surface in section 4 (Fig. 15B). This section contains two groups of flood sequences separated by Only two flood sequences, V and VII, contain more than one bed. an erosional surface with as much as 5 m of relief. Considerable In each case, the lower of the two beds consists of just a few centi-

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Figure 15. Bedding characteristics of slack-water deposits at Tucannon section 5. (A) Flood beds (between dashed lines) with nonerosive contacts draping loess knoll, with minor thickening of some beds on the upvalley (right) side of the knoll. (B) View just to the right of A showing complexity of loaded erosion surface (dashed line) at base of flood bed 7. Note clastic dikes that cut across the erosion surface.

meters of medium sand at the base, overlain by facies FD. This ripples. These record six floods that transported sediment to this point, suggests the establishment of slack-water ponding before the arrival 30 km upstream and 180 m above the mouth of the Tucannon River. of a significant flood surge. A relatively thick facies-FD section in flood bed 1 contains many rippled intervals indicating downvalley trans- Discussion port, probably representing density currents derived from continued flow of the Tucannon River into the hydraulically ponded lake. The restricted occurrence of multiple-bed flood sequences, the number of floods recorded, the distribution of facies, and the disparate Section 6 elevations of different flood-produced features provide insights into the nature of flooding in the Tucannon Valley. Most flood sequences Eleven floods are recorded by each of eleven beds with obviously contain only one flood bed (Fig. 9), with evidence for deposition of bioturbated tops and/or intervening nonflood facies (Fig. 4). The base multiple beds in single floods restricted to the lower part of the valley of the late Wisconsin flood section is exposed and rests on older loess. (Fig. 4). Because pulsing floods are not recorded upvalley of section Slope-wash facies are notably more numerous between flood beds at 5, flood surges subsequent to the initial inrush of water were either of this section and at those farther upvalley, than is the case at down- smaller volume or were attenuated by flood water already present in valley sites; this probably results from closer proximity of slack-wa- the lower valley. ter-terrace exposures to the canyon walls in the narrower upper val- I conclude that at least 25 floods forced water to surge up the ley. All flood beds are composed entirely of facies FC, and four beds Tucannon Valley, on the basis of the presence of 23 flood sequences are reverse graded at the base. The lack of facies FD at this section, at section 4 and the record of 2 additional post-set-S ash floods at and at the farther upvalley sites, suggests the passage of upvalley flood section 2 (Fig. 4). Fewer than half of these floods deposited sediment surges followed by downvalley draining without the establishment of above 230 m, where nonflood sediments represent an increasing pro- a hydraulically ponded lake. portion of progressive upvalley sections. The base of flood-deposited sediment is concealed downvalley from section 4, where records of Section 7 only 6 to 9 floods are exposed. If concealed flood sequences in the lower valley are as thick as those exposed at sections 1 and 3, how- As at section 6, eleven facies-FC flood beds record eleven late ever, it seems unlikely that more than 25 floods produced significant Wisconsin floods at section 7 (Fig. 4). Except near the top of the deposits near the mouth of the Tucannon River. This number agrees section, each flood bed is not only bioturbated in its upper part, but reasonably with the 20 or 21 floods interpreted by Waitt (1983) to be also separated from the next higher bed by lenticular slope-wash recorded in the Snake River canyon at Clarkston, 130 km upstream deposits (Figs. 6C and 6E). It is likely that the eleven flood beds at from the mouth of the Tucannon. This number is less, however, than sections 6 and 7 are correlative. the >39 floods recorded at Burlingame Canyon in the Walla Walla valley (Bjornstad, 1980; Waitt, 1980), where evidence of hiatuses Section 8 exists atop each flood bed. The distribution of facies FD relative to other facies suggests This section was described near the farthest upvalley extent of varying flood dynamics. Facies FD, recording hydraulically ponded recognizable flood rhythmites in the Tucannon Valley. Within an water, does not occur above 235 m, and is rare above 220 m. The exposure composed mostly of slope-wash facies, there are six thin, presence of other flood facies to an elevation of 335 m, and ice-rafted distinctly finer grained, stratified beds that contain upvalley-directed erratics to 400 m (Bretz, 1929), indicates that flood surges extended

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hydraulically ponded waters, to about 370-385 m (Baker, 1973; Waitt, 1980; Craig, 1987; O'Connor and Baker, 1992). Heretofore not explained is the evidence for even higher flood stages, to 400 m, in the Snake-Tucannon system, despite the lack of constrictions downstream from the mouth of the Tucannon River that could produce hydraulic ponding at a level higher than that recorded in the Pasco Basin. Sedimentological evidence from the distribution of facies FD, furthermore, restricts sustained ponding to elevations below 230 m. For floods that crossed the Palouse-Snake divide at 415 m, sufficient head was available to drive transient flood surges to the 400-m elevation in the Tucannon Valley. The elevation of erratics is, therefore, of uncertain significance to determining the depth of hydraulically ponded water that remained in the valley after this initial surge, if floods responsible for depositing erratics also deposited the observed slack-water sediment. Buoyant icebergs probably rode high in the flood surges and, because of their keels, were easily stranded on the valley walls. Ice-rafted erratics may record the maximum water-surface elevation of flood surges, therefore, but not the level of hydraulically ponded water, an important distinction in efforts to reconstruct hydrographs. With the exception of flood facies FD, the slack-water rhythmites are the product of rapidly moving flood currents, not slowly rising hydraulically ponded lakes. The velocity of these currents can be estimated from the grain size and sedimentary structures in the deposits. The estimates are crude, however, because of uncertainty in sediment concentration, which affects grain-settling velocities and bottom shear stresses, and also uncertainty in the flow depths for deposition of individual beds, which negates effective modeling of bedforms as simply a function of velocity. Flood-surge velocity was Figure 16. Sequence of FD strata overlying, partly intercalated estimated by application of the suspension criterion in the manner of with, and overlain by cross-bedded gravel in the Shoulder Bar, 4.5 km Komar (1985), which evaluates the maximum current velocity per- west of the confluence of the Snake and Palouse Rivers. mitting grains of an observed diameter to fall to the bed and be tractively transported. In the example considered by Komar (1985X it was found that the most reasonable results were obtained from calculations high into the valley but drained relatively quickly, so that ponded using the coarsest l/10th percentile of the grain-size distribution. For water was restricted to lower elevations. Conversely, floods that de- application to the Tucannon rhythmites, two calculations were made posited the lower five sequences in section 4 failed to direct energetic for each stratigraphic section by using the mean, and one standard surges into the Tucannon Valley, although ponded water rose to deviation above the mean, of the coarsest grains in all beds in the passively backflood the valley to at least 210-m elevation. Flood se- section (Fig. 17). Section 3 was not considered because facies-FA beds quences V and VII at section 5 record floods that were characterized were emplaced as mass flows and are not subject to the suspension by an early phase of similar passive flooding, followed by energetic criterion. The calculations suggest flow velocities on the order of 3 flood surges. Three types of floods are indicated: (1) flood surges that m/sec to more than 6 m/sec near the mouth of the river, diminishing extend as much as 30 or more kilometers upstream and then drain out rapidly upvalley to velocities between 0.5-1.0 m/sec. Values below 0.5 of the valley or that are partly sustained by a hydraulically ponded lake m/sec are probably required for some flood beds in sections 6,7, and that extended no more than 10 km upstream, and in which subsequent 8, where type-B climbing ripples occur at the base (Ashley and others, attenuated flood surges deposited sediment only in the lowest part of 1982). Although these velocities are averaged over each section and the valley; (2) passive backflooding of hydraulically ponded water beds cannot be unambiguously correlated between sections, a crude without an initial energetic flood surge, or a surge restricted to the estimate of the time required for a flood surge to travel from the mouth vicinity of the river mouth; (3) initial passive backflooding, followed of the river to the upvalley limit of recognized flood deposits can be by energetic flood surges. Floods of the first type are also recorded obtained by integrating the velocity averages over this 30-km length along the Snake Canyon, where facies FD is locally preserved at the of the valley. The resulting calculations (Fig. 17) suggest that one-way toes of foresets in the great gravel bars (Fig. 16). transit times on the order of hours were required. In contrast, the simplest modeled hydrographs (Craig, 1987; O'Connor and Baker, The high elevations of ice-rafted erratics, relative to tractively 1992) for Wallula Gap require tens of hours to several days for the deposited and ponded-water facies, requires separate consideration. largest floods to crest. The above calculations, therefore, further em- These erratics, recognized throughout the scabland and backflooded phasize the importance of energetic flood surges, rather than hydraulic valleys, have been assumed to represent grounding sites of icebergs, ponding, to generate slack-water deposits. themselves part of the failed ice dam, which had floated on the surface of hydraulically ponded lakes. For reconstruction of flood stages and The velocities required for most floods that inundated the Tu- discharge calculations, the elevations to which these erratics occur has cannon Valley, the discharges required for those floods that crossed been taken to indicate the degree of inundation of the Pasco Basin by the 100-m-high basalt ridge that partly separates the Snake and Tu-

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ley than are found in valleys that drain directly into the Columbia River. Post-set-S ash floods were generally smaller than previous ones, as also noted for sections along the Columbia River. There are seven flood sequences above the ash at section 2, and five are present above the ash at section 4. The beds at section 4 are thinner and have finer grained bases than most of those found below the ash. Based on the distribution of facies FD, ponded water associated with five of these late floods rose to at least 200-m elevation (section 2) but never reached 230 m (section 4). Because facies FC indicates surging of flood waters more than 7 km up the Tucannon Valley, I believe that each of these seven floods crossed the Palouse-Snake divide.

CONCLUSIONS

Number of Beds Deposited in Each Flood KILOMETERS UPSTREAM FROM CONFLUENCE WITH THE SNAKE RIVER Deposition of only one bed during each flood was the norm, but not the rule (Fig. 9). Scrutiny of bed contacts indicates hiatuses in •• Velocity estimate based on the average mean deposition between most beds but also provides equally compelling grain size at the base of each flood bed in the evidence for deposition of multiple-bed flood sequences in many cases (for example, Fig. 7). Waitt's(1980,1984,1985a) arguments against the section. multiple-bed-per-flood hypothesis are not entirely substantiated by the I | Velocity estimate based on one stand, dev. detailed stratigraphic data collected in this study. Restriction of mul- above the average mean grain size at the base of tiple-bed flood sequences to low elevations in the Columbia and Tu- each flood bed in the section. cannon Rivers indicates, however, that successively lower surge volumes or attenuation of flow energy by standing water effectively Figure 17. Estimates of current velocity, using the suspension cri- restricted the presence of stratigraphic evidence for pulsating floods. terion in the manner of Komar (1985), for deposition of basal sediment Re-examination of sites that were studied by Waitt (1980) in the in floodbed s at Tucannon sections 1,2,4,5,6,7, and 8. Upvalley velocity Yakima and Walla Walla Valleys firmly supports his contention of variation is shown based on the mean (solid symbols) and one standard hiatuses following the deposition of each flood bed at those sites. deviation above the mean (open symbols) of the average grain size in each Thus, as argued by Waitt, at least 40 late Wisconsin floods inundated bed in each section. One-way transit times (t) from the river mouth to the Pasco Basin. 30 km upvalley are indicated for each set of velocity data, but should be treated as only crude estimates because the velocity for each section is Deposition of Slack-Water Sediment averaged for all exposed beds, and beds are not correlative between all sections. Two different processes contributed to the deposition of the rhythmites: dynamic flooding by energetic flood surges and passive flooding by slowly rising, hydraulically ponded water. The first pro- cess accounts for deposition of most rhythmite sediment, from mod- cannon Rivers, and the high level of erratics on the valley walls suggest erately to rapidly moving currents that extended to elevations equal that most Tucannon rhythmites were not deposited under waters to that of the initial flood-wave crest, which surged up valley walls rising behind Wallula Gap or surging 125 km up the Snake River to the along, and adjacent to, the main flood routes. Relatively fine-grained mouth of the Tucannon, but were generated by flood waters that sediment, which remained suspended in the main thread of flood flow, crossed the Palouse-Snake divide. Many floods that inundated the settled and was tractively deposited by decelerating currents moving Pasco Basin and surged up the Walla Walla and Yakima Valleys may up tributary valleys, or by eddies in more protected sites along the not have been of sufficient volume or discharge, or have been favor- main flood tract. Ponding upstream from valley constrictions led to ably routed, to cross the Palouse-Snake divide. Some water from these settling of silt from suspension and deposition of thin turbidites rep- floods would have spilled through the narrow, flood-carved Palouse resenting continued influx of sediment into the transient lakes that may River canyon, but most water would continue down Washtucna Cou- have persisted for several days (Craig, 1987; O'Connor and Baker, lee toward the Pasco Basin (Fig. 2). These floods would have little 1992). immediate effect in the Tucannon Valley because voluminous surges Facies recording dynamic flooding generally occur at higher el- would not be directed at the river mouth. Backflooding from the Pasco evations than those generated by passive flooding, indicating that Basin, perhaps enhanced by flow through the Palouse canyon, could, rhythmite deposition was more fundamentally linked to the passage of however, produce facies-FD flood beds like those seen in the lower flood surges that subsided after a matter of hours than to hydraulic unit at section 4. From this reasoning, about twenty of the tens of late ponding behind Wallula Gap. Successive cross-stratified cosets of Wisconsin floods released from glacial Lake Missoula (Waitt, 1985a) facies FB and FC recording flow reversals are not separated by sus- directed water over the Palouse-Snake divide, generating a smaller pension deposits as required for Waitt's (1980) hypothesis that the number of relatively coarse-grained rhythmites in the Tucannon Val- upvalley surge and subsequent draining of flood waters be separated

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by hours (Waitt, 1980) or days (Baker, 1973; Craig, 1987) of ponded Palouse-Snake divide; these may be the only floods that inundated the conditions. A few floods are recorded at some sites by only passive- Cheney-Palouse scabland tract during the last flood episode. More flood facies, indicating a rise of ponded waters to higher elevations detailed study of the slack-water sediments, especially those that are than were reached by relatively small initial flood waves. arguably correlative because of intimate association with the Mount Recognition of two different flood processes in the genesis of the St. Helens tephras, may provide further insights into the flood routing, rhythmites is a departure from previous models for rhythmite depo- which is critical for modeling the hydrograph for the Pasco Basin. sition. This model differs from that of Baker (1973), which attributes Current models have not considered flood routing in determining the all rhythmite deposition to the rise of ponded water, and that of Waitt possible discharge histories for a typical flood. (1980), which asserts that ponding contributed to the deposition of all Crude approximations of hydrograph shapes may also be inter- rhythmites. This model is similar to the tidal-bore analogy of Baker preted from the rhythmite stratigraphy. Multiple-bed flood sequences and Bunker (1985), although it is expanded here to include deposition require complex, multi-peaked hydrographs for some floods, whereas not only in backflooded valleys, but also along the main flood tract. the occurrence of single-bed flood sequences at low elevations sug- Another distinction is the recognition that high stages for most floods gests relatively simple hydrographs for others. Atwater (1986; 1992, were associated with the initial flood surge, rather than hydraulic written commun.) indicates that some sublacustrine flood beds within ponding. a few kilometers of the Columbia River in northern Washington contain internal grain-size variations that may reflect hydrograph fluctu- Implications for Hydrograph Reconstruction and ations or, alternatively, complex reflection of flood-generated tur- Paleohydraulic Models bidites within glacial Lake Columbia. The overall impression of The elevation to which water was ponded behind Wallula Gap is Atwater's stratigraphic sections farther away from the Columbia critical to discharge calculations and model hydrograph reconstruc- River, however, is that floods into Lake Columbia had a simple hy- tion. Previous studies have relied heavily on the elevation of berg- drograph shape. The more complex hydrographs implied by multiple- rafted erratics and high-level channels along divides to determine the bed flood sequences along the Columbia and Tucannon Rivers suggest volume of the hydraulically ponded lake and, thus, attribute the high- either that (1) hydrograph complexity was principally a function of the est water-surface elevations to the lake surface. The restricted ele- multiple paths that flood waters took after passing the vicinity of vation of facies FD and the relatively high current velocities required Spokane and glacial Lake Columbia, rather than being controlled by for deposition of the so-called slack-water sediments along both the jókulhlaup dynamics at the source, or (2) repeated flood surges were Columbia and Tucannon Rivers imply, however, that the highest more effectively attenuated by the deep waters of dead-end arms of water surfaces were associated with the initial flood surge and were Lake Columbia than they were by rising waters in the scabland chan- achieved within hours of the onset of flooding at a site and not es- nels and valleys of southern Washington. The repetitive nature of tablished by passive rise of hydraulically ponded water over a period intra-sequence bedding patterns at Tucannon section 3 (Fig. 13) in- of days. Alternatively, one might argue that the late Wisconsin floods, dicates that the conditions controlling hydrograph shape were almost whose deposits were the subject of this study, were smaller than those exactly reestablished during subsequent floods. Intuitively, it is easier of an earlier flood episode that produced the high-elevation erratic to envision successive floods following the same established courses strandlines and erosional features, but left no preserved ponded-water than to imagine nearly exact replication of patterns of subglacial drain- facies along the Columbia and Tucannon Valleys. There is no assur- age or complete ice-dam failure. ance, however, that the icebergs were stranded on the shoreline of a Although observed jókulhlaups wax more slowly than they wane transient lake, rather than along the path of a surging flood wave, as because of the dynamics of subglacial ice-tunnel enlargement and is argued above for the Tucannon Valley. Also, flood erosion on top closure (Thorarinsson, 1939; Sturm and others, 1987), the stratigraphy of Gable Mountain (330 m; see also Bretz, 1925), in the center of the of multiple-bed flood sequences generally suggests that the first flood Pasco Basin, indicates the minimum level to which energetic flood surge that reached a site had the greatest discharge, although the erosion occurred, regardless of when the responsible floods occurred. occurrence of many reverse-graded beds suggests that each hydro- These observations have important implications for the required dis- graph pulse may have been symmetric. One might alternatively in- charges of the late Wisconsin, and perhaps earlier, floods because the terpret upward thinning and fining of beds within a sequence to reflect available storage in the Pasco Basin for ponding to 230-m elevation is attenuation of large pulses late in the flood by rising water within the only about 16% of that available if ponded to 375 m (data from Craig, area of inundation, and a subsequent lack of correspondence between 1987). flood-bed thickness and grain size with flood-pulse discharge. This The late Wisconsin rhythmite stratigraphy offers insights on flood interpretation does not, however, explain why the highest upvalley routing. Post-set-S ash floods entered the Pasco Basin primarily by incursions of flood water appear, with few exceptions, to have oc- routes that joined the Columbia Valley downstream of Wanapum curred early in the flood. The exceptions (that is, section 5 in the Dam. The presence of eight post-set-S ash flood sequences at Tucannon Valley) are significant and indicate that, in some cases, deep Wanapum Dam and seven in the Tucannon Valley suggests that most backflooding from the Pasco Basin occurred before large floods dis- of the discharge of these late floods was directed through the Cheney- charged over the Palouse-Snake divide. Similar flood dynamics may Palouse scabland and that they were not restricted to the Columbia also account for thick, coarse beds at the top of some flood sequences River Valley as argued by Kiver and others (1991). This interpretation at the Hanford section (Fig. 11). fits with evidence that glacial Lake Columbia was impounded behind the Okanogan Lobe of the ice sheet until after the last of the Missoula Scale of Late Wisconsin Floods floods (Atwater, 1987), thus directing floods across the Channeled Scabland. Of the scores of late Wisconsin floods that are inferred to Busacca and others (1989) and O'Connor and Baker (1992) ar- have occurred, probably only 20 to 25 carried large discharges over the gued that the distribution of late Wisconsin slack-water rhythmites is

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inconsistent with erosional and depositional features of the scabland APPENDIX I: LOCATIONS OF MEASURED SECTIONS channels. With the recognition of many flood episodes (McDonald and Busacca, 1989), each characterized by many floods, it is difficult to Listed below are Universal Transverse Mercator grid coordinates for each measured section site and the U.S. Geological Survey 7^' quadrangle on which establish which flood-generated erosional and depositional features each section can be located. are correlative and can, therefore, be used to constrain flood dynamics Wanapum Dam—5196860 m N, 274910 m E, UTM zone 11; Vantage and paleohydraulic models. Busacca and others (1989) infer that the quadrangle Cheney-Palouse scabland tract and Palouse-Snake divide crossing Hanford—5156420 m N, 304960 m E, UTM zone 11; Gable Butte quadrangle were principally formed by the >36 ka flood episode and were routes Wallula—5102680 m N, 352670 m E, UTM zone 11; Wallula quadrangle for few relatively diminutive late Wisconsin floods. The stratigraphic Columbia Hills—5069200 m N, 690120 m E, UTM zone 10; Luna Gulch data reported here, however, indicate that at least 20 late Wisconsin quadrangle floods did follow this route and directed energetic flood surges up the Tucannon 1—5155500 m N, 409957 m E, UTM zone 11; Starbuck West Tucannon Valley. quadrangle Tucannon 2—5155130 m N, 411000 m E, UTM zone 11; Starbuck West The scales of the floods were quite variable, however, and it is quadrangle difficult to envision the characteristics of an ideal late Wisconsin, Tucannon 3—5153280 m N, 412560 m E, UTM zone 11; Starbuck West glacial Lake Missoula flood. If one infers that flood-bed thickness and quadrangle grain size are proportional to flood discharge, then clearly there is a Tucannon 4—5151420 m N, 414990 m E, UTM zone 11; Starbuck East quadrangle general trend toward smaller floods through time (Waitt, 1980; At- Tucannon 5—5150700 m N, 417790 m E, UTM zone 11; Starbuck East water, 1986, 1987; Figs. 3 and 4). Most notable are the youngest, quadrangle relatively fine-grained, post-set-S ash flood deposits that, at Wanapum Tucannon 6—5150620 m N, 419790 m E, UTM zone 11; Starbuck East Dam, Wallula, and Crescent Bar, are found in sites where previous quadrangle floods were either violently erosive or had formed giant gravel bars. Tucannon 7—5150950 m N, 425580 m E, UTM zone 11; Delaney quadrangle Furthermore, although the floods occurring after the Mount St. Helens Tucannon 8—5146650 m N, 430000 m E, UTM zone 11; Tucannon S eruptive period were relatively smaller, they still inundated large quadrangle areas, had maximum stages to at least 200-m elevation in the Pasco Basin and adjacent valleys, and at least seven of these floods were routed, in part, through the Cheney-Palouse scablands and across the Palouse-Snake divide. REFERENCES CITED Ashley. G. M., Southard, J. B., and Boothroyd, J. C., 1982, Deposition of climbing-ripple beds: A flume simulation: Sedimentology, v. 29, p. 67-79. Significance of Conclusions Atwater, B. F., 1984, Periodic floods from glacial Lake Missoula into the Sanpoil arm of glacial Lake Columbia, northeastern Washington: Geology, v. 12, p. 464-467. Atwater, B. F., 1986, Pleistocene glacial-lake deposits of the Sanpoil River valley, northeastern Wash- ington: U.S. Geological Survey Bulletin 1661, 39 p. ' 'There is much to be learned from the field evidence that can help Atwater, B. F., 1987, Status of glacial Lake Columbia during the last floods from glacial Lake Missoula: us to improve the [paleohydraulic] models that are available" (Craig, Quaternary Research, v. 27, p. 182-201. Baker, V. R., 1973, Paleohydrology of catastrophic Pleistocene flooding in eastern Washington: Geo- 1987, p. 307). Detailed stratigraphic and physical sedimentological logical Society of America Special Paper 144, 79 p. Baker, V. R., 1978, The Spokane Flood controversy and the Martian outflow channels: Science, v. 202, studies of Missoula-flood slack-water sediments lead to revisions in p. 1249-1256, understanding of the processes of rhythmite genesis that, in turn, Baker, V.R., 1989, Magnitude and frequency of palaeofloods,mBeven, K., and Carling, P., eds., Floods: Hydrological, sedimentological, and geomorphological implications: London, England, John resolve recent controversy over the number of beds deposited by a Wiley and Sons, p. 171-183. Baker, V. R., and Bunker, R. C., 1985, Cataclysmic late Pleistocene flooding from glacial Lake Missoula: flood and provide new interpretations of late Wisconsin flood routing, A review: Quaternary Science Reviews, v. 4, p. 1-41. flood magnitude, and hypothetical hydrograph shape. Despite the Baker, V. R., and Kochel, R. C., 1988, Flood sedimentation in bedrock fluvial systems, in Baker, V. R., Kochel, R.C., and Patton, P. C.,eds., Flood geomorphology: New York, John Wiley and Sons, remaining uncertainties in correlating various depositional and ero- p. 123-137. Baker, V. R., and Nummedal, D., eds., 1978, The channeled scabland: Planetary geology program: sional features at different sites with individual flood events, or even Washington, D.C., National Aeronautics and Space Administration, 274 p. flood episodes, the slack-water rhythmite stratigraphy and sedimen- Baker, V.R., Kochel, R. C., Patton, P. C., and Pickup, G., 1983, Palaeohydrologic analysis of Holocene flood slack-water sediments, in Collinson, J. D., and Lewin, J., eds., Modern and ancient fluvial tology offer more controls on dynamics of the Missoula floods than systems: International Association of Sedimentologists Special Publication 6, p. 229-239. Bjornstad, B. N., 1980, Sedimentology and depositional environment of the Touchet Beds, Walla Walla have been applied in existing paleohydraulic models. River basin, Washington: Richland, Washington, Rockwell Hanford Operations, RHO-BWI-SA- 44, 104 p. Bjornstad, B. N., Fecht, K. R.,andTallman, A. M., 1991, Quaternary stratigraphy of the Pasco Basin, south-centra] Washington, in Morrison, R. B., ed., The geology of North America, Volume K-2: Quaternary nonglacial geology: conterminous U.S.: Boulder, Colorado, Geological Society of ACKNOWLEDGMENTS America, p. 228-238. Black, R. F., 1979, Clastic dikes of the Pasco basin, southeastern Washington: Richland, Washington, Rockwell Hanford Operations, RHO-BWI-C-64, 65 p. This research project was supported by the Northwest College Bretz, J H., 1925, The Spokane flood beyond the Channeled Scablands: Journal of Geology, v. 33, p. 97-115. and University Association for Science (Washington State University) Bretz, J H., 1928, Bars of the Channeled Scabland: Geological Society of America Bulletin, v. 39, p. 643-702. under Grant DE-FG06-89ER-75522 with the U.S. Department of En- Bretz, J H., 1929, Valley deposits immediately east oftheChanneled Scabland ofWashington. II: Journal ergy. Additional support was provided by Westinghouse Hanford of Geology, v. 37, p. 505-541. Bretz, J H., 1930, Valley deposits immediately west of the Channeled Scabland: Journal of Geology, Company, Geosciences Group. I am especially indebted to Bruce v. 38, p. 385-422. Bretz, J H., 1959, Washington's Channeled Scabland: Washington Division of Mines and Geology Bjornstad, Karl Fecht, Eugene Kiver, Dale Stradling, Richard Waitt, Bulletin 45, 57 p. Jim O'Connor, and Victor Baker, who inspired my interest in the Bretz, J H., 1969, The Lake Missoula floods and the Channeled Scabland: Journal of Geology, v. 77, p. 505-543. problems of interpreting the Missoula flood slack-water sediments Bretz, J H., Smith, H. T. U„ and Neff, G. E„ 1956, Channeled Scabland of Washington: New data and interpretations: Geological Society of America Bulletin, v. 67, p. 957-1049. through many discussions and field excursions. David Romero as- Bunker, R. C., 1980, Catastrophic flooding in the Badger Coulee area, south-central Washington [M.A. sisted in the collection of field data. The manuscript was improved by thesis]: Austin, Texas, University of Texas. Bunker, R. C., 1982, Evidence of multiple late-Wisconsin floods from glacial Lake Missoula in Badger constructive reviews by Brian Atwater, Norm Smith, Richard Waitt, Coulee, Washington: Quaternary Research, v. 18, p. 17-31. Busacca, A. J., McDonald, E. V., and Baker, V. R., 1989, The record of pre-late Wisconsin floods and Vic Baker, and Eric McDonald. late Wisconsin flood features in the Cheney-Palouse Scabland, in Breckenridge, R. M., ed., Glacial

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Lake Missoula and the Channeled Scabland: International Geological Congress Field Trip Guide- O'Connor, J. E., and Baker, V. R., 1992, Magnitudes and implications of peak discharges from Glacial book T310, American Geophysical Union, p. 57-67. Lake Missoula: Geological Society of America BuUetin, v. 104, p. 267-279. Carson, R. J., McKhann, C. F., and Pizey, M. H., 1978, The Touchet Beds of the Walla Patton, P. C., Baker, V. R.,and Kochel, R.C., 1979, Slack-water deposits: A geomorphic technique for Walla Valley, in Baker, V. R., and Nummedal, D., eds., The Channeled Scabland: Wash- the interpretation of fluvial paleohydrology, in Rhodes, D. D., and Williams, G. P., eds., Adjust- ington, D.C., Planetary geology program, National Aeronautics and Space Administration, ments to the fluvial system: Dubuque, Iowa, Kendall/Hunt, p. 225-253. p. 173-177. Rigby, J. G., 1982, The sedimentology, mineralogy, and depositional environment of a sequence of Craig, R. G., 1987, Dynamics of a Missoula flood, in Mayer, L., and Nash, D., eds., Catastrophic Quaternary catastrophic flood-derived lacustrine turbidites near Spokane, Washington [M.S. flooding: Boston, Massachusetts, Allen and Unwin, p. 305-332. thesis]: Moscow, Idaho, University of Idaho. Craig, R. G., and Hanson, J. P., Erosion potential from Missoula floods in the Pasco basin, Washington: Sturm, M., Beget, J., and Benson, C., 1987, Observations of jokulhlaups from ice-dammed Strandline Richland, Washington, Pacific Northwest Laboratory, PNL-5684,183 p. Lake, Alaska: Implications for paleohydrology, in Mayer, L., and Nash, D., eds., Catastrophic Droser, M. L., and Bottjer, D. J., 1986, A semiquantitative field classification of ichnofabric: Journal of flooding: Boston, Massachusetts, Allen and Unwin, p. 79-94. Sedimentary Petrology, v. 56, p. 558-559. Thorarinsson, S., 1939, The ice-dammed lakes of Iceland with particular reference to their value as Kiver, E. P., and Stradling, D. F., 1985, Comments on "Periodic jokulhlaups from Pleistocene glacial indicators of glacier oscillations: Geografiska Annaler, v. 21, p. 216-242. Lake Missoula—New evidence from varved sediments in northern Idaho and Washington: Qua- Waitt, R. B., 1980, About forty last-glacial Lake Missoula jokulhlaups through southern Washington: ternary Research, v. 24, p. 354-356. Journal of Geology, v. 88, p. 65S-679. Kiver, E. P., Moody, U. L., Rigby, J. G., and Stradling, D. F., 1991, Late Quaternary stratigraphy of Waitt, R. B., 1983, Tens of successive, colossal Missoula floods at north and east margins of Channeled the Channeled Scabland and adjacent areas, in Morrison, R. B., ed., Hie geology of North Scabland: U.S. Geological Survey Open-File Report 83-671, 29 p. America, Volume K-2: Quaternary nonglacial geology; conterminous U.S.: Boulder, Colorado, Waitt, R. B., 1984, Periodic jokulhlaups from Pleistocene glacial Lake Missoula—New evidence from Geological Society of America, p. 238-245. varved sediment in northern Idaho and Washington: Quaternary Research, v. 22, p. 46-58. Kochel, R. C., and Baker, V. R., 1988, Paleoflood analysis using slack water deposits, in Baker, V. R., Waitt, R. B., 1985a, Case for periodic, colossal jokulhlaups from Pleistocene glacial Lake Missoula: Kochel, R. C., and Patton, P. C., eds., Rood geomorphology: New York, John Wiley and Sons, Geological Society of America BuUetin, v. 96, p. 1271-1286. p. 357-376. Waitt, R. B., 1985b, Reply to Comment on "Periodic jokulhlaups from Pleistocene glacial Lake Mis- Komar, P. D., 1985, The hydraulic interpretation of turbidites from their grain sizes and sedimentary soula—New evidence from varved sediment in northern Idaho and Washington": Quaternary structures: Sedimentology, v. 32, p. 395-407. Research, v. 24, p. 357-360. Lupher, R. L., 1944, Clastic dikes of the Columbia basin region, Washington and Idaho: Geological Waitt, R. B., and Atwater, B. F., 1989, Stratigraphic and geomorphic evidence for dozens of last-glacial Society of America Bulletin, v. 55, p. 1431-1462. floods, in Breckenridge, R. M., ed., Glacial Lake Missoula and the Channeled Scabland: American McDonald, E. V., and Busacca, A. J., 1988, Record of pre-late Wisconsin giant flood in the Channeled Geophysical Union, International Geological Congress field trip guidebook T310, p. 37-55. Scabland interpreted from loess deposits: Geology, v. 16, p. 728-731. Moody, U. L., 1987, Late Quaternary stratigraphy of the Channeled Scabland and adjacent areas [Ph.D. dissert.l: Moscow, Idaho, University of Idaho, 419 p. Mullineaux, D. R., Wilcox, R. E., Ebaugh, W. F., Fryxell, R., and Rubin, M., 1978, Age of the last major MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 6,1992 scabland flood of the Columbia Plateau in eastern Washington: Quaternary Research, v. 10, REVISED MANUSCRIPT RECEIVED JUNE 3, 1992 p. 171-180. MANUSCRIPT ACCEPTED JUNE 13,1992

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