AN ALTERNATIVE THEORY OF THE MISSOULA FLOODS

BY: RANDAL CARLSON [LINK]    

The Randall Carlson hypothesis is that the amount of energy required to melt the ice sheet and create the amount of liquid water described by researchers is 180,000 megatons. This is a staggering amount of energy. It is equivalent to 18 times the total nuclear arsenal of the USA and Russia combined. No terrestrial source of energy exists to discharge this amount of energy on an instantaneous basis. Therefore it had to be an extraterrestrial source such as asteroid or meteor perhaps a swarm asteroids that struck the ice sheet and caused it to melt all at once.

Below is his description of the Missoula floods that closely matches the event as described in the bibliography above where ten papers are listed that agree on certain commonalities of the event. In particular the melt and floods did not occur all at once but over a period of thousands of years with somewhere between 40 to 70 melt-and-flood events separated by 50 years or more. Therefore an extraterrestrial energy source that can deliver the power of ten times the total nuclear arsenal of the USA and Russia in an instant is neither necessary nor plausible particularly so given the complete absence of evidence for such an event. The daily energy need of a 6,000-year ice melt event that needs a total of 180,000 megatons of energy is less than 7,000 joules, an amount that can be easily provided by solar irradiance reaching the surface of a 10-million-square-km ice sheet. 

{Note: The energy balance is as follows: 180,000 megatons of TNT is 7.54E20 Joules and over a 6,000-year period it works out to 1.26E17 Joules/year or 3.98E9 watts. At 1000 watts per square meter of sunshine energy we need about 6 million square meters but we have 10 million square km at least on that ice sheet}.

Randall Carlson’s description of the Missoula floods is presented below. 

1. One of the great unresolved scientific mysteries of our time concerns an extensive body of evidence for extraordinary catastrophic flooding events in the very recent geological history of North America. From the Pacific Coast of Washington State, across the mountains and prairies to the Atlantic Coast of New England, from the region of the Great Lakes to the mouth of the Mississippi, from the arid deserts of the Southwest to the lush forests of the Southern Appalachians, the geo-morphological tracks of tremendous floods of truly prodigious scale are etched indelibly into the landscape.
2. Based upon irrefutable field evidence, these colossal floods utterly dwarf anything experienced by modern man within historical times, and yet, by geological standards they occurred exceptionally close to our own time, at the close of the most recent ice age, some 11 to 14 thousand years ago. Familiarity with the currently reigning dogmas regarding the cause of these great ice age floods would leave the casual observer with the impression that the explanation for this diluvial phenomenon has been more or less determined to the satisfaction of a majority of Earth scientists and the work remaining is only in sorting out a few particulars such as the exact number and timing of the floods.
3. However, it is our contention that the model of causation, which is accepted at present by the overwhelming majority of geologists who have investigated the phenomenon, has inherent difficulties. We argue that researchers have not yet grasped an accurate explanation and that the currently accepted hypotheses are beset with unexamined assumptions, inconsistencies and contradictory evidence.
4. The most impressive evidence for ancient mega-floods is found in the Pacific Northwest, primarily in Washington State, Idaho and western Montana. Here the flood features are attributed to a series of events referred to as The Missoula Floods, and these are blamed upon the repeated failure of a large ice dam that held back an enormous proglacial lake named Lake Missoula, allowing the lake to drain suddenly.
5. The lake is supposed to have occupied the mountain valleys of Western Montana, and to have been held in by a large valley glacier in the region of Lake Pend O’rielle in northern Idaho and finally to have drained to the west across southeastern Washington. The floodwater is then assumed to have entered the great valley of the Columbia River from whence it was conveyed to the Pacific Ocean. In the process of Lake Missoula’s repeated draining a massive complex of erosional and depositional features were created that have almost no parallel on Earth.
6. While they may have been the most spectacular, the Missoula Floods were not the only giant flood events to have occurred in North America as the great Ice Age drew to a close. The effects of mega scale flood flows have been extensively documented in the eastern foothills of the Rocky Mountains in both Canada and the U.S.; across the prairie states; in the vicinity of the Great Lakes; in Pennsylvania and western New York and in New England. All of the Canadian provinces preserve large-scale evidence of gigantic water flows. All regions within or proximal to the area of the last great glaciation show the effects of intense, mega-scale floods.
7. Complicating the problem is the fact that areas far removed from the immediate proximity of the glaciers have not been spared the ravages of gigantic floods. The arid American southwest preserves extensive evidence of vast flooding on a scale unprecedented in modern times. The Mojave Desert of Southern California is replete with evidence of mighty flood currents drowning entire landscapes. Likewise the Sonoran Desert in Arizona and New Mexico preserves evidence of mighty flood currents. One also finds in the southeastern United States, massive erosional and depositional features in the Appalachians that allow of no other explanation than that of colossal floods. Another great flood is attributed to the catastrophic draining of Lake Bonneville, which, during the latter part of the ice age occupied large intermontane basins in Utah. The Great Salt Lake is but a diminutive remnant of this giant lake.
8. The passage of catastrophic floods has left their mark in Pennsylvania and Western New York. The scientific documentation of these great floods reaches back into the nineteenth century, with repeated discoveries of various effects that could not be explained by invoking modern fluvial processes operating at a familiar scale, nor could they be explained by invoking glacial phenomenon. It appears that much of this continent wide flooding occurred during, or at the close of, the most recent ice age. The exact timing of the various events remains to be established. Much of the evidence points to episodic events stretching back tens of thousands of years.
9. However, it also appears that much of this continent wide mega flooding happened concurrently at the end of the last great ice age. Evidence for megascale flooding at the end of the most recent ice age, is not limited to North America, but has been documented from all over the world. This evidence supports the conclusion that large scale super-flooding events were globally ubiquitous throughout the ice age, but occurred with exceptional power and size at or near its conclusion.
10. Among the places around the planet from which proof is emerging of floods of extraordinary size – Siberia especially, in the Altai Mountains region near the Siberia/Mongolia border, hosts evidence for massive floods equivalent in scale and power to the largest western USA floods. Across northern Europe mega-flood evidence is found in abundance. South America, too, shows extensive evidence for massive catastrophic flooding in the recent geological past, as does Australia, New Zealand, the Middle East and Northern Africa.
11. However, for the time being, our focus will be on the great floods of North America. Eventually, however, it will be our goal to document and correlate this imposing mass of evidence for global catastrophe with a view to understanding its origin and causes. Then, we will be in a better position to address the question of social and cultural consequences. Emerging evidence of earlier mega flood events, apparently associated with global climate changes and transition phases from glacial to interglacial ages implies a non random distribution in time, perhaps periodic or cyclical. The geographic distribution of mega-scale flood events also appears to be non-random, certain areas being affected with greater intensity than others.
12. As stated, the Missoula Floods and Siberian floods were, as far as can be determined from field evidence at present, the greatest known freshwater floods in the history of the Earth. Other areas experienced floods of profound magnitude, but, not apparently on the scale of these two events, although the possibility of future discoveries should not be ruled out. The study of megafloods from tsunamis is a related but distinct area of palaeoflood hydrology, which in any comprehensive purview of catastrophism must be addressed. However, for now we shall limit our discussion to floods involving fresh water, meaning events related to glacial melting or rainfall.
13. The Missoula floods were the most powerful of the great North American floods. The vast scale, the complexity and the sheer magnitude of the forces involved bestow upon these mighty events a preeminent ranking in any accounting of Earth’s great catastrophes. Even a preliminary acquaintance with the awe-inspiring after effects of this extraordinary deluge can provoke a deep sense of wonder and astonishment. Through a more prolonged acquaintance with this landscape and the story that it tells, comes a humbling realization of the almost inconceivable power of the natural forces involved.
14. No flood events even remotely close in scale are documented from anywhere within historical times. They were one of the most significant geological occurrences in the history of the earth. Their magnitude and the release of energies involved rank them with the greatest forces of nature of which we are aware. What renders these diluvial events of exceptional importance and interest is that they occurred only yesterday in the span of geological time, and, most significantly, well within the time of Man.
15. Let us place the great floods in context. The final phase of the last ice age, the Late Wisconsin, as it is called in reference to North America’s version of the Great Ice Age, came to a conclusion only some 12,000 to 14,000 years ago. While the effects of the ice age were global, the Late Wisconsin itself was the last episode of major ice expansion in North America at the close of the larger cycle of glacial climate called simply the Wisconsin. The final phase known as the Late Wisconsin appears to have lasted from approximately 25 or 26 thousand years before present to around 10 to 12 thousand years before present, depending upon how one defines the precise point of termination.
16. The entire Wisconsin Ice Age lasted for around 100,000 years. While the timing and extent of glacial recessions and expansions throughout the Wisconsin Ice Age is still being worked out, it is clear that the fluctuations of climate and glacial mass during this time were considerably greater than that experience within historical times. Three ice ages in North America that were earlier than the Wisconsin have been documented by geologists and named after the states in which their glacial effects are best preserved. From oldest to youngest they were the Nebraskan, the Kansan and the Illinoian. Each of these glacial ages was separated from the next by distinct interglacial periods. The warm interval preceding the Wisconsin Ice Age and following the Illinoian is called the Sangamonian (Eemian).
17. The European counterpart of the Wisconsin Ice Age is called the Würm, which has been extensively documented in the Alps. The signature of the Wisconsin Ice Age was, obviously, the presence of huge volumes of glacial ice where no such ice now exists. In North America this was most of Canada and a substantial amount of the northern United States. The northern boundary of the great North American ice sheet reached to the Arctic Ocean. From there south to the area now occupied by the Great Lakes the entire region was entirely buried under glacial ice. At the southern glacial margin the ice reached almost to the Ohio River in the eastern half of the U.S. New York lay under a half mile to a mile of ice. Most of the states of Wisconsin and Minnesota were buried as well as the Dakotas.
18. The ice reached out of Canada across what is now the border, from Montana to the Pacific Ocean, with several major incursions further south in Idaho along the Rocky Mountains and in Washington State. Great glaciers also occupied many areas of the Cascades and the Sierra Nevada mountains. In all, some 6 million square miles was buried beneath a mantle of ice, about the same size as that now occupying the South Polar Region on Antarctica. Reference to this map will help to give you the big picture of the Late Wisconsin Ice Age.
19. At the peak of the Late Wisconsin, around 18,000 to 15,000 years before present, the great ice mass reached from the Atlantic to the Pacific. However, there were actually two separate ice sheets that began separately some 5 to 7 thousand years earlier and eventually grew until they coalesced near the final stage of the Late Wisconsin. The easternmost and the larger of the two was named the Laurentide Ice sheet after a region in Quebec where it appears the ice first began accumulating. This ice sheet appears to have formed from the convergence of two centers of nucleation and outflow, one center to the east of present day Hudson Bay and one to the west.
20. A separate ice sheet formed over the Canadian Rockies and has been designated the Cordilleran Ice Sheet by glaciologists after the collective term for the great mountain chain that forms both the Rocky Mountains and the Andes. As the Late Wisconsin reached its maximum it appears that these three ice sheets coalesced in an essentially single mass. One controversial question relates to the timing and extent of an ice free corridor between the Laurentide and Cordilleran Ice sheets, either prior to their convergence, or after, during the retreat phase.
21. A supposition would be that humans could have utilized such an ice free corridor to migrate to the lower United States from Alaska, after crossing the Bering Land Bridge, which, of course, was exposed during the lowered sea levels of the Ice Age. As described in more detail elsewhere, through most of the late Nineteenth century and the first half of the Twentieth, it was believed that the most recent ice age was essentially an unbroken episode of global cooling and ice growth which for the most part continued uninterrupted for some 150 thousand years, or longer. It was also believed that the transitions into and out of an ice age were protracted episodes lasting tens of thousands of years.
22. However, during the second half of the Twentieth Century, with improved dating, and with more precise and detailed stratigraphy available, it became apparent that the climate changes associated with the onset and termination of ice ages occurred much more rapidly than believed by earlier workers. As the Twentieth Century drew to a close, high-resolution records bore witness to climate changes that occurred with astonishing speed and severity. The most recent episode of widespread catastrophic flooding occurred at the termination the Late Wisconsin. Some of these floods were associated directly with melting of the glacial ice. Others are only indirectly linked to glacial melting.
23. The most powerful of the terminal ice age floods was the complex of events known as the Missoula Floods, a much more complex series of floods rather than a single large scale event. The effects of the Missoula Floods can be found imprinted upon the landscape of the Pacific Northwest from western Montana to the Pacific Ocean, and, in addition to Montana include the states of Idaho, Washington and Oregon. Our intention will be to convey an understanding of these awesome floods and to raise some questions concerning important issues that have not yet been addressed under the current state of research.
24. The other catastrophic floods which occurred during this period of transition out of the ice age, roughly from 13,000 to 11,000 years ago, will be examined in an effort to understand the phenomenon accompanying the end of the Great Ice Age, and which, hopefully, will shed light on the most important question, which remains “What factor, or combination of factors, brought about the abrupt and extreme climate changes which terminated the ice age, and provoked catastrophic melting of the ice complex?”

CHANELED SCABLAND

MISSOULA FLOODS BIBLIOGRAPHY 

1. Bretz, J. Harlen. “The Lake Missoula floods and the channeled scabland.” The Journal of Geology 77.5 (1969): 505-543.  This paper reviews the outstanding evidence for (1) repeated catastrophic outbursts of Montana’s glacially dammed Lake Missoula, (2) consequent overwhelming in many places of the preglacial divide along the northern margin of the Columbia Plateau in Washington, (3) remaking of the plateau’s preglacial drainage pattern into an anastomosing complex of floodwater channels (Channeled Scabland) locally eroded hundreds of feet into underlying basalt, (4) convergence of these flood-born rivers into the Columbia Valley at least as far as Portland, Oregon, and (5) deposition of a huge delta at Portland. Evidence that the major scabland rivers and the flooded Columbia were hundreds of feet deep exists in (1) gravel and boulder bars more than 100 feet high in mid-channels, (2) subfluvial cataract cliffs, alcoves, and plunge pools hundreds of feet in vertical dimension, (3) back-flooded silts high on slopes of preglacial valleys tributary to the scabland complex, and (4) the delta at Portland. Climatic oscillations of the Cordilleran ice sheet produced a succession of Lake Missoulas. Following studies by the writer, later investigators have correlated the Montana glacial record with recurrent scabland floods by soil profiles and a glacial and loessial stratigraphy, and have approximately dated some events by volcanic ash layers, peat deposits, and an archaeological site. Several unsolved problems are outlined in this paper.
2. Baker, Victor R., and Daniel J. Milton. “Erosion by catastrophic floods on Mars and Earth.” Icarus 23.1 (1974): 27-41.  The large Martian channels, especially Kasei, Ares, Tiu, Simud, and Mangala Valles, show morphologic features strikingly similar to those of the Channeled Scabland of eastern Washington, produced by the catastrophic breakout floods of Pleistocene Lake Missoula. Features in the overall pattern include the great size, regional anastomosis, and low sinuosity of the channels. Erosional features are streamlined hills, longitudinal grooves, inner channel cataracts, scour upstream of flow obstacles, and perhaps marginal cataracts and butte and basin topography. Depositional features are bar complexes in expanding reaches and perhaps pendant bars and alcove bars. Scabland erosion takes place in exceedingly deep, swift floodwater acting on closely jointed bedrock as a hydrodynamic consequence of secondary flow phenomena, including various forms of macroturbulent votices and flow separations. If the analogy to the Channeled Scabland is correct, floods involving water discharges of millions of cubic meters per second and peak flow velocities of tens of meters per second, but perhaps lasting no more than a few days, have occurred on Mars.
3. Atwater, Brian F. “Periodic floods from glacial Lake Missoula into the Sanpoil arm of glacial Lake Columbia, northeastern Washington.” Geology 12.8 (1984): 464-467. At least 15 floods ascended the Sanpoil arm of glacial Lake Columbia during a single glaciation. Varves between 14 of the flood beds indicate one back-flooding every 35 to 55 yr. This regularity suggests that the floods came from an ice-dammed lake that was self-dumping. Probably the self-dumping lake was glacial Lake Missoula, Montana, because the floods accord with inferred emptyings of that lake in frequency and number, apparently entered Lake Columbia from the east, and produced beds resembling backflood deposits of Lake Missoula floods in southern Washington.
4. Clarke, G. K. C., W. H. Mathews, and Robert T. Pack. “Outburst floods from glacial Lake Missoula.” Quaternary Research 22.3 (1984): 289-299. The Pleistocene outburst floods from glacial Lake Missoula, known as the “Spokane Floods”, released as much as 2184 km3 of water and produced the greatest known floods of the geologic past. A computer simulation model for these floods that is based on physical equations governing the enlargement by water flow of the tunnel penetrating the ice dam is described. The predicted maximum flood discharge lies in the range 2.74 × 106−13.7 × 106 m3 sec−1, lending independent glaciological support to paleohydrologic estimates of maximum discharge.
5. Waitt Jr, Richard B. “Case for periodic, colossal jokulhlaups from Pleistocene glacial Lake Missoula.” Geological Society of America Bulletin 96.10 (1985): 1271-1286. Two classes of field evidence firmly establish that late Wisconsin glacial Lake Missoula drained periodically as scores of colossal jökulhlaups (glacier-outburst floods). (1) More than 40 successive, flood-laid, sand-to-silt graded rhythmites accumulated in back-flooded valleys in southern Washington. Hiatuses are indicated between flood-laid rhythmites by loess and volcanic ash beds. Disconformities and nonflood sediment between rhythmites are generally scant because precipitation was modest, slopes gentle, and time between floods short. (2) In several newly analyzed deposits of Pleistocene glacial lakes in northern Idaho and Washington, lake beds comprising 20 to 55 varves (average = 30–40) overlie each successive bed of Missoula-flood sediment. These and many other lines of evidence are hostile to the notion that any two successive major rhythmites were deposited by one flood; they dispel the notion that the prodigious floods numbered only a few. The only outlet of the 2,500-km³ glacial Lake Missoula was through its great ice dam, and so the dam became incipiently buoyant before the lake could rise enough to spill over or around it. Like Grímsvötn, Iceland, Lake Missoula remained sealed as long as any segment of the glacial dam remained grounded; when the lake rose to a critical level ∼600 m in depth, the glacier bed at the seal became buoyant, initiating underflow from the lake. Subglacial tunnels then grew exponentially, leading to catastrophic discharge. Calculations of the water budget for the lake basin (including input from the Cordilleran ice sheet) suggest that the lakes filled every three to seven decades. The hydrostatic prerequisites for a jökulhlaup were thus re-established scores of times during the 2,000- to 2,500-yr episode of last-glacial damming. J Harlen Bretz’s “Spokane flood” outraged geologists six decades ago, partly because it seemed to flaunt catastrophism. The concept that Lake Missoula discharged regularly as jökulhlaups now accords Bretz’s catastrophe with uniformitarian principles.
6. Baker, Victor R., and Russell C. Bunker. “Cataclysmic late Pleistocene flooding from glacial Lake Missoula: A review.” Quaternary Science Reviews 4.1 (1985): 1-41.Late Wisconsin floods from glacial Lake Missoula occurred between approximately 16 and 12 ka BP. Many floods occurred; some were demonstrably cataclysmic. Early studies of Missoula flooding centered on the anomalous physiography of the Channeled Scabland, which J. Harlen Bretz hypothesized in 1923 to have developed during a debacle that he named ‘The Spokane Flood’. Among the ironies in the controversy over this hypothesis was a mistaken view of uniformitarianism held by Bretz’s adversaries. After resolution of the scabland’s origin by cataclysmic outburst flooding from glacial Lake Missoula, research since 1960 emphasized details of flood magnitudes, frequency, routing and number. Studies of flood hydraulics and other physical parameters need to utilize modern computerized procedures for flow modeling, lake-burst simulation, and sediment-transport analysis. Preliminary simulation models indicate the probability of multiple Late Wisconsin jökulhlaups from Lake Missoula, although these models predict a wide range of flood magnitudes. Major advances have been made in the study of low-energy, rhythmically bedded sediments that accumulated in flood slack-water areas. The ‘forty floods’ hypothesis postulates that each rhythmite represents the deposition in such slack-water areas of separate, distinct cataclysmic floods that can be traced from Lake Missoula to the vicinity of Portland, Oregon. However, the hypothesis has numerous unsubstantiated implications concerning flood magnitudes, sources, routing and sedimentation dynamics. There were multiple great Late Wisconsin floods in the Columbia River system of the northwestern United States. Studies of high-energy, high altitude flood deposits are necessary to evaluate the magnitudes of these floods. Improved geochronologic studies throughout the immense region impacted by the flooding will be required to properly evaluate flood frequency. The cataclysmic flood concept championed by J. Harlen Bretz continues to stimulate exciting and controversial research.
7. Atwater, Brian F. “Status of glacial Lake Columbia during the last floods from glacial Lake Missoula.” Quaternary Research27.2 (1987): 182-201. The last floods from glacial Lake Missoula, Montana, probably ran into glacial Lake Columbia, in northeastern Washington. In or near Lake Columbia’s Sanpoil arm, Lake Missoula floods dating from late in the Fraser glaciation produced normally graded silt beds that become thinner upsection and which alternate with intervals of progressively fewer varves. The highest three interflood intervals each contain only one or two varves, and about 200–400 successive varves conformably overlie the highest flood bed. This sequence suggests that jökulhlaup frequency progressively increased until Lake Missoula ended, and that Lake Columbia outlasted Lake Missoula. The upper Grand Coulee, Lake Columbia’s late Fraser-age outlet, contains a section of 13 graded beds, most of them sandy and separated by varves, that may correlate with the highest Missoula-flood beds of the Sanpoil River valley. The upper Grand Coulee also contains probable correlatives of many of the approximately 200–400 succeeding varves, as do nearby parts of the Columbia River valley. This collective evidence casts doubt on a prevailing hypothesis according to which one or more late Fraser-age floods from Lake Missoula descended the Columbia River valley with little or no interference from Lake Columbia’s Okanogan-lobe dam.
8. Benito, Gerardo. “Energy Expenditure and Geomorphic Work of the Cataclysmic Missoula Flooding in the Columbia River GGorge, USA.” Earth Surface Processes and Landforms: The Journal of the British Geomorphological Group 22.5 (1997): 457-472.  Cataclysmic releases from the glacially dammed Lake Missoula, producing exceptionally large floods, have resulted in significant erosional processes occurring over relatively short time spans. Erosional landforms produced by the cataclysmic Missoula floods appear to follow a temporal sequence in many areas of eastern Washington State. This study has focused on the sequence observed between Celilo and the John Day River, where the erosional features can be physically quantified in terms of stream power and geomorphic work. The step‐backwater calculations in conjunction with the geologic evidence of maximum flow stages, indicate a peak discharge for the largest Missoula flood of 10 × 10⁶m³s⁻¹. The analysis of local flow hydraulics and its spatial variation were obtained calculating the hydrodynamic variables within the different segments of a cross‐section. The nature and patterns of erosional features left by the floods are controlled by the local hydraulic variations. Therefore, the association of local hydraulic parameters with erosional and depositional flood features was critical in understanding landform development and geomorphic processes. The critical stream power required to initiate erosion varied for the different landforms of the erosional sequence, ranging from 500 W m⁻² for the streamlined hills, up to 4500 W m⁻² to initiate processes producing inner channels. Erosion is possible only during catastrophic floods exceeding those thresholds of stream power below which no work is expended in erosion. In fact, despite the multiple outbursts which occurred during the late Pleistocene, only a few of them had the required magnitude to overcome the threshold conditions and accomplish significant geomorphic work
9. Clague, John J., et al. “Paleomagnetic and tephra evidence for tens of Missoula floods in southern Washington.” Geology31.3 (2003): 247-250.  Paleomagnetic secular variation and a hiatus defined by two tephra layers confirm that tens of floods from Glacial Lake Missoula, Montana, entered Washington’s Yakima and Walla Walla Valleys during the last glaciation. In these valleys, the field evidence for hiatuses between floods is commonly subtle. However, paleomagnetic remanence directions from waterlaid silt beds in three sections of rhythmically bedded flood deposits at Zillah, Touchet, and Burlingame Canyon display consistent secular variation that correlates serially both within and between sections. The secular variation may further correlate with paleomagnetic data from Fish Lake, Oregon, and Mono Lake, California, for the interval 12,000–17,000 ¹⁴C yr B.P. Deposits of two successive floods are separated by two tephras derived from Mount St. Helens, Washington. The tephras differ in age by decades, indicating that a period at least this long separated two successive floods. The beds produced by these two floods are similar to all of the 40 beds in the slack-water sediment sequence, suggesting that the sequence is a product of tens of floods spanning a period of perhaps a few thousand years.
10. Benito, Gerardo, and Jim E. O’Connor. “Number and size of last-glacial Missoula floods in the Columbia River valley between the Pasco Basin, Washington, and Portland, Oregon.” Geological Society of America Bulletin 115.5 (2003): 624-638.Field evidence and radiocarbon age dating, combined with hydraulic flow modeling, provide new information on the magnitude, frequency, and chronology of late Pleistocene Missoula floods in the Columbia River valley between the Pasco Basin, Washington, and Portland, Oregon. More than 25 floods had discharges of >1.0 × 10⁶ m³/s. At least 15 floods had discharges of >3.0 × 10⁶ m³/s. At least six or seven had peak discharges of >6.5 × 10⁶ m³/s, and at least one flood had a peak discharge of ∼10 × 10⁶ m³/s, a value consistent with earlier results from near Wallula Gap, but better defined because of the strong hydraulic controls imposed by critical flow at constrictions near Crown and Mitchell Points in the Columbia River Gorge. Stratigraphy and geomorphic position, combined with 25 radiocarbon ages and the widespread occurrence of the ca. 13 ka (radiocarbon years) Mount St. Helens set-S tephra, show that most if not all the Missoula flood deposits exposed in the study area were emplaced after 19 ka (radiocarbon years), and many were emplaced after 15 ka. More than 13 floods perhaps postdate ca. 13 ka, including at least two with discharges of >6 × 10⁶m³/s. (6,000 years of ice sheet melt). From discharge and stratigraphic relationships upstream, we hypothesize that the largest flood in the study reach resulted from a Missoula flood that predated blockage of the Columbia River valley by the Cordilleran ice sheet. Multiple later floods, probably including the majority of floods recorded by fine- and coarse-grained deposits in the study area, resulted from multiple releases of glacial Lake Missoula that spilled into a blocked and inundated Columbia River valley upstream of the Okanogan lobe and were shunted south across the Channeled Scabland.