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Posted on: February 16, 2022




‘As ice sheets began melting at the end of the last ice age, a series of cataclysmic floods called the Missoula megafloods scoured the landscape of Eastern Washington, carving long, deep channels and towering cliffs through an area now known as the Channeled Scablands. They were among the largest known floods in Earth’s history, and geologists struggling to reconstruct them have now identified a crucial factor governing their flows. In a study published February 14 in Proceedings of the National Academy of Sciences, researchers showed how the changing weight of the ice sheets would have caused the entire landscape to tilt, changing the course of the megafloods. “People have been looking at high water marks and trying to reconstruct the size of these floods, but all of the estimates are based on looking at the present-day topography,” said lead author Tamara Pico, assistant professor of Earth and planetary sciences at UC Santa Cruz. “This paper shows that the ice age topography would have been different over broad scales due to the deformation of Earth’s crust by the weight of the ice sheets.” During the height of the last ice age, vast ice sheets covered much of North America. They began to melt after about 20,000 years ago, and the Missoula megafloods occurred between 18,000 and 15,500 years ago. Pico’s team studied how the changing weight of the ice sheets during this period would have tilted the topography of eastern Washington, changing how much water would flow into different channels during the floods. Glacial Lake Missoula formed in western Montana when a lobe of the Cordilleran ice sheet dammed the Clark Fork valley in the Idaho panhandle and melt water built up behind the dam. Eventually the water got so deep that the ice dam began to float, resulting in a glacial outburst flood. After enough water had been released, the ice dam resettled and the lake refilled. This process is thought to have been repeated dozens of times over a period of several thousand years. Downstream from glacial Lake Missoula, the Columbia River was dammed by another ice lobe, forming glacial Lake Columbia. When Lake Missoula’s outburst floods poured into Lake Columbia, the water spilled over to the south onto the eastern Washington plateau, eroding the landscape and creating the Channeled Scablands. During this period, the deformation of the Earth’s crust in response to the growing and shrinking of ice sheets would have changed the elevation of the topography by hundreds of meters, Pico said. Her team incorporated these changes into flood models to investigate how the tilting of the landscape would have changed the routing of the megafloods and their erosional power in different channels. Towering cliffs carved by the Missoula megafloods are found throughout the Channeled Scablands in eastern Washington. We used flood models to predict the velocity of the water and the erosional power in each channel, and compared that to what would be needed to erode basalt, the type of rock on that landscape. They focused on two major channel systems, the Cheney-Palouse and Telford-Crab Creek tracts. Their results showed that earlier floods would have eroded both tracts, but that in later floods the flow would have been concentrated in the Telford-Crab Creek system. As the landscape tilted, it affected both where the water overflowed out of Lake Columbia and how water flowed in the channels, but the most important effect was on the spillover into those two tracts. What’s intriguing is that the topography isn’t static, so we can’t just look at the topography of today to reconstruct the past.”The findings provide a new perspective on this fascinating landscape, she said. Steep canyons hundreds of feet deep, dry falls, and giant potholes and ripple marks are among the many remarkable features etched into the landscape by the massive floods. When you are there in person, it’s crazy to think about the scale of the floods needed to carve those canyons, which are now dry,” Pico said. “There are also huge dry waterfalls—it’s a very striking landscape. The oral histories of Native American tribes in this region include references to massive floods. Scientists were not the first people to look at this. People may even have been there to witness these floods.

Modeling floods that formed canyons on Earth and Mars



Western US ‘megafloods’ during last ice age might not have been so mega. Rebecca Dzombak, American Geophysical Union. During the last ice age, flooding water could have spilled off a precipice, like Dry Falls in Washington, pictured here, to carve out the Channeled Scablands, but new research suggests the floods could have been 80% smaller than the canyons’ volume today. After the Last Glacial Maximum in North America, a kilometer-thick ice dam at the toe of a glacier failed, allowing the waters of massive Lake Missoula to rush out and inundate the landscape of what is now eastern Washington. The flooding carved out the scarred and pockmarked landscape of the Channeled Scablands, which cover about 30,000 square kilometers of the northwestern United States. Noted first by Indigenous flood stories and then by geomorphologists in the 1800s, the possibility of massive floods here has long been intriguing; more recently, researchers have turned to the Scablands for insights into Martian flooding, too. But geologists still don’t know quite how big the Scablands floods were. Lehnigk and Larsen turned to the largest of the Scablands’ valleys, Grand Coulee in eastern Washington, for an answer. To start, they didn’t look to water; they looked to rock. Some of the valleys are a peculiar shape; rather than twisting and narrow, they are long and straight, with a uniform width. That shape is thanks to the geometric columns of basalt that make up the bedrock. “You can only maintain that shape, really, if you’re toppling stacks of columns over, so the water is coming over a waterfall,” like Dry Falls, a 3.5-mile-long (5.6-kilometer-long) precipice at the head of the Lower Grand Coulee, said author Karin Lehnigk, a Ph.D. candidate in geosciences at the University of Massachusetts Amherst. “Eventually, it knocks columns at the edge of the waterfall over, and the waterfall retreats back.” Lehnigk trekked around Grand Coulee measuring the size and density of these massive basalt columns. Back in the lab, she set about modeling different flood sizes as they washed over reconstructed, preflood topography in an attempt to reconcile the geological evidence with hydrological constraints. She looked for floods big enough to reach high-water marks on the valley walls and tested whether those floods could knock over basalt columns. Lehnigk and Larsen found that a series of smaller floods that hit far below the high-water marks on today’s postflood topography could have done the job and that considering ancient topography is key. “Imagine your bathtub,” Lehnigk said, “and how much water it takes to fill it up to the rim. But if your bathtub is full of rocks,” like the valley would have been prior to flooding, “you don’t need as much water to reach the edge.” The high-water marks, then, matter only when you consider where the canyon floor was, too. Flood size estimates that use modern topography and rely on the valley’s high-water marks overestimate the discharge by more than 80%, the researchers found. When the floodwaters began to flow and cut into the ground, the canyon’s floor would have been shallower, with relative high-water marks abandoned as the waterfall marched itself back upstream through erosion. The study not only improves our understanding of how the Scablands formed; it also can help estimate how much water once flowed to create Mars’s landscapes, which is critical for knowing what past climates on the Red Planet were like. For flood channels on Mars that likely formed in the same way, such as those in Ares Vallis and Kasei Valles, similar approaches could be used to estimate their paleoflow as well, said Lehnigk.

Western U.S. “megafloods” might not have been so mega


Modeling floods that formed canyons on Earth and Mars. by University of Massachusetts Amherst. A new model of canyon-forming floods from UMass Amherst and CalTech researchers suggests that deep canyons can be formed in bedrock by significantly less water than previously thought. Credit: UMass Amherst/Isaac Larsen. Geomorphologists who study Earth’s surface features and the processes that formed them have long been interested in how floods, in particular catastrophic outbursts that occur when a glacial lake ice dam bursts, for example, can change a planet’s surface, not only on Earth but on Mars. Now geoscience researchers Isaac Larsen at the University of Massachusetts Amherst and Michael Lamb at the California Institute of Technology have proposed and tested a new model of canyon-forming floods which suggests that deep canyons can be formed in bedrock by significantly less water than previously thought. They point out that “reconstructing the magnitude of the canyon-forming floods is essential for understanding how floods modify planetary surfaces, the hydrology of early Mars, and abrupt climate change.” Larsen and Lamb apply their new model to the “channeled scablands” in eastern Washington State, an area that, like some on Mars, has very deep canyons cut into fractured basalt bedrock. The researchers say their results suggest “there may be a rich imprint of both the history and discharge of flooding in the morphology of canyons” such as terraces, valley shapes and slope profiles on Earth and on Mars “that warrant further investigation.” Details appear in the current issue of Nature. The researchers say channels in the scablands today, which are up to 650 feet (200 meters) deep and 3 miles (5 km) wide, were likely formed by flood discharges five- to tenfold smaller than brimful estimates, that is by “significantly lower megaflood discharges than previously thought. The channeled scablands are a classic landscape in the history of geomorphology and we’re bringing new views of how it was formed.” Until the 1920s, scientists did not understand what could have formed the tortured landscape of eastern Washington studied for decades by J Harlen Bretz, a giant figure in geosciences, Larsen recalls. Bretz was the first to suggest that they were formed by catastrophic flooding of unknown origin. His views were dismissed for years, but Bretz was later vindicated when glacial Lake Missoula was identified as the floodwater source. As most scientists came to accept the catastrophic flood explanation for the canyons and then tried to estimate floodwater discharges, they assumed that floods filled canyons to the brim, a huge amount of water. But an alternate hypothesis proposed and now tested by Lamb and and Larsen posits that as floodwater cuts into bedrock, the canyon deepens, meaning less water is required to shape it. In areas underlain by fractured bedrock, Larsen says, “our general concept is that the channel floor was being cut and lowered as the floods were happening, and you need to account for that when reconstructing the scenario of flood magnitude. This applies to the scablands, to Mars and other areas where there have been catastrophic outburst floods.” He and Lamb combine numerical flood models with estimates of the force required to erode basalt bedrock to show that for Moses Coulee, a canyon carved by catastrophic Lake Missoula floods in eastern Washington when an ice dam repeatedly broke and reformed around 15,000 years ago, their “threshold shear stress model” explains the shape and depth of currently observed channels better than the brimful model. “We numerically routed floods through the canyon in different states, from current configuration and at four different past scenarios,” Larsen notes. “We predicted the discharge from two models and tested which one is most reasonable, based on the depositional evidence from the current bars seen today in the canyons. The size of floods our model predicts from the basalt erosion better match locations of depositional flood bars in the canyon than the brimful model predicts.” Larsen and Lamb’s new model also works better to explain observed canyon-cutting mechanics and outflow channels observed on Mars, they point out, “supporting the notion of a multi-flood or low-magnitude flood origin for the Mars outflow channels. ” Larsen adds, “There are very similar but larger canyons on the surface of Mars. These outflow channels are much bigger than the ones on Earth, but they look very similar and the assumption is they formed by similar processes. We know in most cases there were not canyons before these floods happened. They had to be carved, so the bottoms were getting lower and lower with each flood. We believe in the final phases of floods, they were not filled to the brim.”


Regional, not global, processes led to huge Martian floods. Gigantic groundwater outbursts created the largest flood channels in the solar system on Mars, 3.2 billion years ago. For many years it was thought that this was caused by the release of water from a global water table, but research led by J. Alexis P. Rodriguez of the Planetary Science Institute reveals regional deposits of sediment and ice emplaced 450 million years earlier to be the source. “The flooding is due to regional processes, not global processes,” said Rodriguez, a Senior Scientist at the Planetary Science Institute and lead author of “Martian outflow channels: How did their source aquifers form, and why did they drain so quickly?” that appears in Nature’s Scientific Reports. “Deposition of sediment from rivers and glacial melt filled giant canyons beneath a primordial ocean contained within the planet’s northern lowlands. It was the water preserved in these canyon sediments that was later released as great floods, the effects of which can be seen today.” The canyons filled, the Martian ocean disappeared, and the surface froze for approximately 450 million years. Then, about 3.2 billion years ago, lava beneath the canyons heated the soil, melted the icy materials, and produced vast systems of subterranean rivers extending hundreds of kilometers. This water erupted onto the now-dry surface in giant floods. “Our investigation suggests that early Mars sedimentation could have buried and trapped enormous volumes of surface water, perhaps triggering the transition into the frozen world that Mars has been during most of its history,” Rodriguez said. “Evidence for ancient environments capable of sustaining Earth-like life forms could be present in subsurface materials that are now exposed.” “Because the process of deposition, freezing, heating and eruption were regional, there may be vast reservoirs of water ice that are still trapped beneath the Martian surface along the boundaries of its ancient northern ocean as well as within the subsurface of other regions of the planet where contemporaneous seas and lakes formed,” he said. “This could be critical to the future of human activity on Mars.”

Tracing the origin of ancient water flows on Mars in the lab


Were eruptions of pressurised goundwater once commonplace on Mars? Credit: ESA, CC. BY Building our own copy of Mars in the laboratory was hard work. We had to shift 15 tonnes of sand to create a swimming-pool-sized model of the red planet. But the effort was well worth it as our experiments shed light on a much-debated issue: the origin of ancient water on the planet. The model suggests water erupted from large subsurface lakes creating enormous volcano-like eruptions. Back in the 19th century Giovanni Schiaparelli peered through his telescope and spied networks of channels on the planet. The similarity to the watercourses on Earth is striking. Ever since this discovery, scientists suspected that Mars could have had liquid water on its surface. Now, it is widely accepted that Mars did have surface water once upon a time. But all the contents of those channels have long since gone. Mars’ atmosphere is now very thin (about 1% of that on Earth), which means that any exposed water essentially boils off into space. The tiny fraction that remains on modern Mars is locked away as either ice at the poles or within abundant water-rich materials, such as clay. So we can’t observe any ongoing water erosion on the red planet’s surface. Shovel ready. I normally work on the formation of rivers and channels on our own planet. But the Martian versions are no less fascinating and it turns out that all the tools that I use to study Earth-bound rivers are pretty good for extraterrestrial ones as well. Unfortunately there are a couple of major problems with studying channels on Mars. First, and most obvious, I can’t go there yet (and I don’t think my wife would let me go even if I could). Second, there really isn’t much water on Mars any more. So to answer the question of how water channels on Mars formed, it was obvious what we had to do. We had to build Mars here. Or at least a model of it. Historical map of Mars based on Giovanni Schiaparelli’s observations. So a team of international scientists and I grabbed our shovels and set about constructing a mock-up scaled version of the red planets surface. After shifting all the sand we had our model of Mars’ sediments housed in a chamber the size of a swimming pool all within our Total Environment Simulator. Pressurized Groundwater Eruption Experiment from Wouter Marra on Vimeo. Groundwater eruption in an experiment (left) and the resulting landscape of such event on Mars (right). Author provided Let it rain! We used the model to add water in a variety of ways. We let it rain on our model, we trickled and flooded water over our mock Martian surface and we forced water up from beneath. Then we compared the features on the Martian surface with those that we made in our simulator. The scaled experiments, that took us three months to complete, suggest that the most important water flows on ancient Mars came from massive outburst floods of pressurised groundwater. It is tough being a scientist. We think the water may have erupted from large subsurface lakes creating vast volcano-like eruptions with maximum flood volumes that could be over 10,000 times bigger than the Amazon River on Earth. Our results suggest that this groundwater repeatedly flowed up to the surface, albeit very sporadically and in ever-decreasing volumes over time, carving out the channels that provide us with the evidence of these mega-floods from the past. It is quite possible that our experiments now explain the formation of these channels that caught the eye of Giovanni Schiaparelli as he peered through his 19th century telescope. Perhaps most importantly they suggest that water was not around for very long on the surface of Mars, which makes the hunt for extra-terrestrial life that bit more complicated. If water was not around in liquid form for very long then the chances that life existed on mars diminish significantly.




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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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). 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. 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. 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. 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. 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. 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.
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. 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?



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  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.


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