Ice Age Floods in Washington

  • By Cassandra Tate
  • Posted 3/26/2008
  • Essay 8449

The sound of the great Ice Age floods would have been terrifying: some 530 cubic miles of water bursting through a wall of ice more than 2,000 feet high; roaring over Eastern Washington at speeds of up to 80 miles an hour; drilling deep crevices into ancient basalt, stripping away topsoil in some areas, piling it up in others, flinging boulders around like ping pong balls. The pressure on the earth’s crust probably triggered earthquakes and landslides. The floodwaters pounded down the Columbia Gorge and into the Pacific with enough force to dig channels into the ocean floor. Then a new ice dam formed, impounded more water, and eventually collapsed, unleashing new floods, in a process that was repeated again and again until finally, about 13,000 years ago, the most monumental cycle of hydraulic engineering on earth came to an end.

Pummeled by "Megafloods"

The very size of the floods made it difficult to recognize their impact. Iconoclastic geologist J Harlen Bretz (a former Seattle high school science teacher) first proposed in 1923 that the arid "scablands" of Eastern Washington had been carved out by a rapid, massive flood -- not, as most of his contemporaries believed, by the ordinary processes of erosion by wind and water, over millions of years.

His ideas were ridiculed for decades. Aerial photographs in the 1950s vindicated Bretz to some degree by revealing the outlines of flood features that were too large to be easily identified from the ground. But it was not until the 1970s that satellite photos, taken from a vantage point 570 miles overhead, provided enough of a perspective to convince the last of the skeptics.

Even Bretz underestimated the scale and frequency of the floods. Geologists now believe that the Northwest was pummeled by a hundred or more "megafloods" during the last Ice Age alone, beginning about 20,000 years ago. There were at least a dozen earlier periods of major glaciation, stretching back for 2 million years, and flooding may have occurred during each of them.

The primary source of the more recent floods was Glacial Lake Missoula, a huge inland sea created when a lobe of the Cordilleran Ice Sheet crept south from Canada and blocked the drainage of the Clark Fork River in Idaho. Water backed up behind the dam into western Montana, eventually covering some 3,000 square miles of land to a depth of up to 2,000 feet.

Ice makes a flimsy dam. Studies of ice-jammed lakes in Alaska and elsewhere have shown that when the level of water behind an ice dam reaches a certain point, the ice floats, allowing water to escape from underneath, first in a trickle, then in a torrent. That’s one explanation for the failure of the dam holding back Lake Missoula. Another is that highly pressurized water at the base of the dam permeated cracks in the ice. The cracks widened into tunnels at an exponential rate, and the dam caved in on itself.

Once the dam was breached, Lake Missoula was uncorked as explosively as a bottle of fiercely shaken champagne. An estimated two trillion tons of muddy, ice-laden water -- a volume equal to 10 times the combined flow of all the rivers in the world today -- tore through the remnants of the dam and thundered over parts of four states on a 630-mile journey to the sea. The lake drained in two or three days. Almost as soon as it was emptied, ice blocked its outlet again and the lake began refilling, in a cycle believed to have been repeated every few dozen years or so.

The Pacific Northwest, Rearranged

The Ice Age floods rearranged the dirt, rocks, streambeds, and contours of 16,000 square miles of the Pacific Northwest. They pared land down to bedrock in places, built it up in others; rerouted rivers; scoured out coulees and potholes; and steepened and widened the Columbia Gorge. Enormous boulders, encased in ice, bobbed along on the floodwaters, ending up on hills and in flatlands hundreds of miles from their origins, a puzzle for the farmers who would someday plow around them.

Temporary lakes formed behind natural constrictions along the course of the Lower Columbia River at Wallula Gap, Rowena Gap, and Kalama. Flood-borne sediments settled out of those lakes, becoming part of the rich topsoil that supports the farms and orchards of Yakima, Walla Walla, and the Willamette Valley today.

The sediment is also the basis for the quality of wines produced in the Mid-Columbia Basin. Wine grapes require soil that is both fast-draining and water-retentive, characteristics provided by the fine-grained sand and silt in the flood deposits. Wheat grows tall in the areas of the Palouse that the floods skipped over, leaving intact the deep layers of windblown silt and soil that once covered most of the Plateau.

Thousands of years after the last drop drained away, the geologic legacy of the floods continues to define the Northwest, from the way the land looks to the uses humans have found for it. The following tour of the state explores the impact of the Ice Age floods on Washington. It was written and curated by HistoryLink senior historian Cassandra Tate. Geologist Bruce Bjornstad, author of On the Trail of the Ice Age Floods, served as guide and technical advisor. HistoryLink contributor Glenn Drosendahl took most of the accompanying photographs. 

1. Spokane Area

The sudden drainage of Glacial Lake Missoula sent avalanches of water, ice, and debris into the Rathdrum Prairie, a broad mountain valley between Sandpoint, Idaho, and Spokane, Washington. Beyond Spokane the flood poured into and overtopped another ice-jammed lake: Glacial Lake Columbia, created when a glacier diverted part of the upper Columbia River. Blocked from its normal riverbed, the river pushed south to begin carving the canyon known today as the Grand Coulee.

Meanwhile, Lake Columbia spread out over 1,500 square miles of the Spokane River Valley. The lake was not large enough to develop the hydraulic pressure needed to float its own ice dam, so it remained in place during most, if not all, of the Missoula floods. This fact would play a critical role in helping geologists unlock some of the secrets of the floods.

The contents of Lake Missoula smashed into the upper end of Lake Columbia with an effect that's been likened to that of a hippo jumping into a swimming pool. The impact sent massive amounts of water over the southern ridge that had contained the smaller lake and out over the Plateau. Moving at freeway speeds and dividing into three major spillways, the floodwaters gouged out the network of interlocking channels, cliffs, and potholes that pioneering geologist J Harlen Bretz called the Channeled Scablands.

Bretz initially thought there had been only one flood. In a seminal paper published in 1923, he called it the "Spokane flood," because the water had seemed to come from the general direction of the present day city of Spokane. By the mid-1950s, he and other geologists understood that there had been more than one flood and that the source had been farther east, in Montana.

But it was not until the 1970s that clues left behind in the sediments of Lake Columbia showed that the floods had occurred regularly. Varves (annual layers of sediment deposited by glacial meltwater) accumulated in Lake Columbia in the years between floods. Studies of these layers suggest that Lake Missoula broke through successive ice dams on an average of every 35 to 55 years, with the floods becoming smaller and more frequent as time went on.

2. Dry Falls (Grant County)

The stark cataracts of Dry Falls are among the most dramatic landscapes carved by the Ice Age floods, but at the peak of the floods, they would have looked like little more than a wrinkle on a roiling sea. Only as the floods began to ebb would Dry Falls have appeared as a magnificent waterfall, twice as high and five times the width of Niagara Falls.

Dry Falls marks the boundary between the upper and the lower portions of the Grand Coulee, a 50-mile-long trench initially cut when glacial ice forced the upper Columbia River from its normal course. Floodwaters from Lake Missoula turned the Upper Grand Coulee into a massive sluiceway, churning over Dry Falls and plunging 350 feet into the lower coulee. The fast and furious flow ripped at the basalt cliffs like jackhammers, deepening and widening the coulee and eating away at the lip of Dry Falls. The edge of the falls ended up 20 miles upstream from its original location.

When the glaciers retreated and the Columbia returned to its present-day course, Grand Coulee dried up, an abandoned riverbed. The upper coulee now contains Banks Lake, a storage reservoir developed in the 1940s as part of the Columbia Basin Irrigation Project. Dry Falls remains as it was, an emblem of the erosive power of water.

The floods also left the Columbia Basin littered with gigantic circular potholes, some so large it was said that rustlers once used them to hide stolen cattle. The deep, fast-moving water formed underwater tornadoes (called "kolks" in German) that drilled down into rock while sucking up the debris, like a combination auger and vacuum cleaner. Potholes that hold water today are called "kolk lakes." With no natural outlet, all are alkaline, and most are fed by water seeping in from nearby irrigation projects.

3. Lake Lenore Caves (Lower Grand Coulee)

The floodwaters plucked huge chunks of rock from the basalt cliffs in the Lower Grand Coulee, leaving shallow caves that Plateau Indians later used for shelter and storage. Among these are the Lake Lenore Caves, located within the 214-acre Lake Lenore Caves State Park.

If there were any human witnesses to the Ice Age floods, Lake Lenore Caves would not have offered a safe vantage point. The caves would have been beneath a 300-foot crest of water that was moving with as much kinetic energy as an erupting volcano. The only dry land near Grand Coulee during peak flooding was above 1,800 feet elevation; farther south, around what is now the Tri-Cities, only areas above 1,200 feet elevation would have escaped the floods. Native Americans normally lived in the lowlands, along the river and its tributaries, close to sources of food and water. People could have waited out a flood in the uplands but only if they were already in the immediate area. The scant half-hour between the first rumbles of the approaching flood and the subsequent inundation would not have been enough time to run for higher ground.

Direct evidence linking man with the floods has not been found, but that may be simply because it was swept away or destroyed by the floods themselves. "When you wash your crime scene clean that many times," says Norman Smyers, a United States Forest Service geologist in Missoula, ''you lose a lot of evidence.'' In the late 1960s, excavators at the Marmes Rockshelter in the Palouse River Canyon found evidence of human activity that has been radiocarbon dated to about 11,300 years. A cache of Clovis spear points, discovered on the top of an Ice Age flood bar at East Wenatchee in 1987, dates from the same period. Some scientists believe it is only a matter of time before other clues are found to support speculation that humans were in the region at least during the last of the floods, 13,000 to 15,000 years ago.

Indirect evidence of a human presence is reflected in Indian legends that tell of colossal floods and of forced evacuations to higher ground. The Sahaptin name given to the 3,500-foot Rattlesnake Ridge near Hanford is "Laliik," which translates as "stands above the water." There is no water around Rattlesnake Ridge today. But during the Ice Age floods, the long peninsula would have stood about a thousand feet above the water, a possible refuge in a frightening sea.

4. Ephrata Fan

After breaking out of the relatively narrow confines of the Lower Grand Coulee, Ice Age floodwaters spread into the Quincy Basin. As they expanded, the floods temporarily slowed, allowing an immense volume of sand, gravel, and boulders to settle out. These deposits formed what is known today as the Ephrata Fan.

In geo-speak, a "fan" is a fan-shaped deposit that develops on the edge of a stream or river. A "bar," in contrast, is an accumulation of gravel, sand, and rocks beneath the surface of running water. Geologist David Alt, author of Glacial Lake Missoula and Its Humungous Floods, argues that the Ephrata Fan should more properly be called the Ephrata Bar. The fan/bar is basically a big debris field, covering about 620 square miles, stretching south from Ephrata and the mouth of the Lower Grand Coulee to Moses Lake.

The floods dumped about 130 feet of sediment and left boulders the size of storage units in this area. One particularly notable (and often photographed) basalt boulder, just east of Ephrata, measures 60 feet in diameter. It was torn out of the Grand Coulee and jounced about six miles to its current resting place. As Alt puts it, the floods plucked rocks from basalt outcroppings the way tornadoes pluck at mobile homes.

"Scour holes" can be found around many of the extremely large boulders. These are depressions created after the rocks stopped moving but the violent, swirling water continued to chew into the sediment around them. The same mechanism produces small cavities around pebbles winnowed by waves on a beach.

The northern end of the Ephrata Fan is pimpled with boulders, large and small. Downstream, however, the floods deposited mostly fine-grained sands and silts, much valued today as agricultural land. The difference can be vividly seen in aerial photographs: empty, rocky land to the north, thick clusters of green crop circles to the south.

5. Quincy Basin

It was a topographic map of the Quincy Basin that first piqued J Harlen Bretz's interest in the geology of the scablands of Eastern Washington, in 1910. The erosional features on the margins of the basin -- the maze of interconnecting channels, the huge round potholes, and what looked like the skeletal remnants of waterfalls in an area where there wasn't any water -- puzzled the young high school science teacher. He was intrigued enough to leave his job at Queen Anne High School in Seattle, earn a doctorate in geology, and begin the studies that would lead, in 1923, to his "outrageous hypothesis" that a cataclysmic flood had produced the ragged landscape revealed in the map.

Geologist Bruce Bjornstad says that some of the most compelling evidence supporting Bretz's theory can be found in the Quincy Basin. He points out, for example, a group of granite boulders, clustered at an elevation of 1,200 feet on the north slope of the Frenchman Hills (the ridge that defines the southern edge of the basin). The light-colored boulders stand in sharp contrast to the dark basalt that is the only indigenous rock in this area. These are "ice-rafted erratics" -- rafted in on an iceberg that was cut loose when Glacial Lake Missoula tore through its ice dam in Idaho. Distinctive crystals in the granite match those found at the site of the dam. The fact that the boulders are grouped together indicates that they traveled on a single chunk of ice. Their location on Frenchman Hills is one sign of how high the floods were when they passed through here.

As it moved south from Canada, the Cordilleran Ice Sheet picked up boulders that became embedded in the ice dams that created Lake Missoula and other glacial lakes. When the dams collapsed, the escaping waters carried off huge chunks of the ice, rocks and all. Some of these icebergs collected in valleys and along the margins of basins, where the floods flattened and slowed. Others rode the floods all the way to the Willamette Valley. As the ice melted, the rocks dropped out of the flow. The larger icebergs sometimes left behind piles of rocks, gravel, sand, and silt, which geologists call "bergmounds."

Misfit rocks are found all along the route of the Ice Age floods. Some were simply grabbed up and carried along by the sheer force of the water. When the velocity of water doubles, its load carrying capacity can increase by a factor of 64 -- more than enough to bounce boulders from one place to another, smoothing and rounding their edges along the way.

6. Drumheller Channels

The Ice Age floods swept across Drumheller Channels with the power of a sandblaster stripping weathered paint from an old barn. Gathering momentum after spilling over from the Quincy Basin, the floodwaters peeled off about 100 feet of "loess" (a fertile mixture of windblown soil and volcanic ash) and cut as deeply as 300 feet into the underlying basalt. Left behind were hundreds of isolated buttes, surrounded by a chaotic network of canyons and coulees. J Harlen Bretz considered this to be the most spectacular and bewildering part of the scablands of Eastern Washington. The National Park Service recognized its geologic significance by declaring it a National Natural Landmark in 1986.

Bretz and three of his students got lost in the area in the dark while on a field trip in the 1920s. They tried but could not find Drumheller Ranch, after which the channels are named. "We stumbled, climbed, descended in the darkness for an hour or more, heartily sick of having attempted the traverse at night," he recalled in 1959. "I have since seen a topographic survey and aerial views of Drumheller Channels and unhesitatingly give this area the palm for complexity of all flood-made topography." He also marveled that anyone would try to grow crops in the region. Only a few bluffs had retained topknots of the rich soil that had built up in the centuries before the floods; the lowlands were ribbed with gravel bars; and the native grasses had been depleted by overgrazing by cattle.

Local ranchers used the word "scablands" for this kind of terrain. It was said the raw basalt looked like scabs on the backs of animals. Bretz called it "Channeled Scabland" because of its labyrinth of steep-sided ravines and flat-bottomed coulees. He mapped more than 150 distinct channelways within Drumheller Channels alone, a 50-square-mile piece of the 2,000-square-mile scablands puzzle. The channels repeatedly divide and rejoin, in a braided pattern that geologists call anastomosing. This pattern is produced by water moving so quickly and deeply that it cannot be contained within a normal drainage system. Some of the Drumheller Channels have since been inundated by Potholes Reservoir to the north but what remains is a dramatic testament to the power of the Ice Age floods.

7. West Bar

West Bar, so named because it lies on the western side of the Columbia River, is one of the best places in the Columbia Basin to see "giant current ripples" -- washboard-like formations disguised as long, smooth hills. The ripples at West Bar are an average of 24 feet high and spaced 360 feet apart. J Harlen Bretz suggested as early as 1925 that such areas had been contoured by huge currents of water moving over the land. But it wasn't until decades later that aerial photography confirmed his speculations. Easily overlooked from the ground because of their size, from the air the rippling is exactly like that found along a lake shore or river bed, writ large.

Ordinary hydraulic processes created these features. Silt, sand, and pebbles settle out of moving water in backwaters and eddies; and wave action molds this material into giant waves, visible when the water recedes. The difference is that the water that shaped these ripples was probably more than 600 feet deep and moving at 65 miles per hour.

The direction of the ripples suggests the water came not from Glacial Lake Missoula but from Glacial Lake Columbia, directly north. Lake Columbia, formed when the Okanogan lobe of the Cordilleran Ice Sheet blocked the original course of the upper Columbia River, was too small to build up enough pressure to break through its own ice dam. But the final retreat of the lobe, in the waning stages of the Ice Age, would have unleashed more than 200 cubic miles of water from Lake Columbia -- less than half the size of the biggest of the floods from Lake Missoula, but still large enough to leave indelible marks on the land.

8. Potholes Coulee

The successive collapses of the ice dams that impounded Glacial Lake Missoula unleashed walls of water that rushed down the Grand Coulee and into Quincy Basin. Part of the water drained westward from the basin over low points ("spillover dips") in the Babcock and Evergreen ridges, the boundary between the basin and the Columbia River Valley. The dramatic drop in elevation, from about 1,425 feet at the spillovers down to about 400 feet at the river, less than three miles below, greatly increased the erosive energy of the flood pulse. The water gouged three main channels through the basalt bedrock in its rush to join the river and find a conduit to the sea. The spectacular Potholes Coulee occupies the middle channel, flanked by Crater Coulee to the north and Frenchman Coulee to the south.

Known collectively as the Quincy Cataracts, each of these coulees is a horseshoe-shaped, tiered canyon that steps down to the river. At the head of Potholes Coulee are two parallel alcoves, lined with now-dry cataracts. The alcoves are separated by what J Harlen Bretz described as "a great blade of rock," a mile and half long, 1,000 feet wide and 375 feet high. Deep plunge pools developed at the bottom of some of the cataracts, ground out by the jackhammer force of water pounding into basalt. Bretz realized that these circular basins were left behind by waterfalls that had, in a sense, eaten their own lips; the edge of the falls had receded upstream as the water picked away at the rock.

Potholes Coulee is a geological smorgasbord: recessional cataracts, giant current ripples, rock basins, potholes, pinnacles, pillars, and flood bars that look like the backs of giant whales, rising up from the coulee floor. Flood bars at the upper ends of the alcoves have created natural dams. Lakes, fed by excess water from the Columbia Basin Irrigation Project, have formed behind these dams. The lower coulee itself remains largely undisturbed, with only an occasional power line in the distance to signal the imprint of man.

9. Frenchman Coulee

Like Potholes Coulee, its neighbor to the north, Frenchman Coulee consists of twin cataracts -- the skeletal remains of dry waterfalls -- at the head of a long, wide canyon that slopes down toward the Columbia River. Floodwaters moved down the channel like gargantuan routers, chewing through thick layers of ancient topsoil and deep into the bedrock below. A rib of basalt that survived the onslaught separates the main coulee, to the north, from a smaller basin (called Echo Basin) on the south. The surrounding cliffs are fluted with columns of basalt, standing like sentinels over the canyon below.

Columnar basalt can be seen in cliff faces and road cuts throughout the Columbia Basin but there are several distinctive examples in Frenchman Coulee, making it a popular destination for rock climbers. The columns were born in the rivers of lava that flooded this region in an earlier geological era. The lava shrank as it crystallized into solid rock and then shrank some more as the rock cooled. The shrinkage pattern creates palisades of multi-sided rock. The shape and structure of columnar basalt makes it particularly vulnerable to erosion by floods.

The water cuts through the surrounding earth, plucks out pieces of the basalt, and sweeps them away. The floor of Frenchman Coulee is littered with such rocks. Some were ripped from the cataracts and carried short distances by the floodwaters. Others simply tumbled off the steep, unstable coulee walls sometime after the last of the floods. They've since been joined by several scattered, rusted-out car bodies, rolled from the edges of Highway 10 (the old Vantage Highway), which follows the length of the coulee.

The most dramatic rock formation in the coulee is a single-file grouping of towering polygonal columns, known locally as "The Feathers" because they resemble a row of crow feathers stuck in the ground. Floodwaters sluiced off the entire hillside that once encased these columns, leaving them exposed and standing alone, a beacon for climbers.

10. Yakima Valley

Yakima was literally a backwater during the Ice Age floods, inundated by water backed up behind Wallula Gap, about 100 miles to the southeast. The gap is the only outlet for water draining from the entire Columbia Basin to the sea. When the contents of Lake Missoula tried to squeeze through the mile-wide opening, it clogged up, like a too-small drain at the bottom of an overflowing sink. A vast, temporary lake -- named Lake Lewis, after the explorer Meriwether Lewis -- spread out behind the bottleneck, surging west into the Yakima Valley, east into the Walla Walla Valley, and north into the Quincy Basin.

Lake Lewis covered some 600 square miles of the Yakima Valley with up to 250 feet of murky water. It took several days for the lake to drain after each successive flood. As the water receded, it left behind thick layers of gravel, sediment, and other debris, like rings on a bathtub. Deposits from successive backwater floods eventually plugged the 15-mile long Badger Coulee, the original course of the ancestral Yakima River, forcing the river to divert to the north to find a new route to its confluence with the Columbia.

Many "erratics" (rocks moved on rafts of ice to locations far from their origins) also settled out of the relatively quiet slackwater in the Yakima Valley. The largest erratic found so far in the Mid-Columbia Basin is a 100-ton granite boulder that lies on a slope in Boulder Coulee.

The deposits also have been a rich source of fossils of mammoths and other prehistoric animals caught up in the floods. All living creatures in the path of the floods were swept to their death. The remains of many came to rest in places like the Yakima Valley where the water temporarily slowed and ponded. Additionally, older fossils may have been pulled from their original burial places and redeposited in slackwater areas. Road building and construction projects have led to significant fossil discoveries at Moxee, Selah, and several other sites near Yakima. It's not clear whether the remains are those of animals who died in the rising waters of Lake Lewis or of animals carried in from other areas. But the sediments that settled out of the backwaters in the Yakima Valley provided a nearly ideal environment for their preservation.

11. Palouse Falls

The Palouse River fits the cataract at Palouse Falls and the massive canyon beyond it as well as the foot of a small boy fits into a size ten shoe. The semicircular plunge pool at the bottom of the falls is absurdly large for the relatively thin thread of water that drops into it. The modern river is too puny to have chewed into the edge of the falls and left the old pool behind. Nor does the river have the hydraulic muscle to have eroded the valley that carries it to the falls or the deep canyon below, with its maze of coulees, abandoned spillways, and serrated buttes. The brunt of this work was done thousands of years ago by Ice Age floods. As geologist Bruce Bjornstad puts it, "The canyon has little to do with the Palouse River and everything to do with the floods."

The river once ran west through Washtucna Coulee on its way to join the Columbia in the Pasco Basin. Floodwaters racing down from the scablands overfilled the coulee and surged across the Palouse-Snake River Divide, creating a shortcut to the Snake. The Palouse River still uses this flood-carved shortcut. Turning abruptly to the south just past Hooper, it drops over a 198-foot cliff and then flows 11 miles to the Snake. Geologists believe that the earliest of the Ice Age floods plunged directly into the Snake. Successive floods eroded the lip of the original waterfall, forcing it to migrate upstream to its present position. The Palouse River Canyon is the result of that retreat.

Floodwaters rammed into the Snake with enough force to reverse its flow, backing it upstream for more than 100 miles, to Lewiston, Idaho, and beyond. These "backwards floods" heaped gravel into bars several hundred feet high and two to three miles long, at intervals all along the Snake and into the valley of the Clearwater River. One of the largest of the bars is located at the mouth of the Palouse River Canyon, near Lyons Ferry Park (formerly a state park, now privately managed). Nearly a half-mile wide, it fills more than half the width of the Snake River Valley floor.

The floods skipped most of the Palouse Prairie in Eastern Washington and north central Idaho, leaving in place ancient deposits of windblown sand and silt called "loess." Layers of loess, up to 250 feet deep, are the basis for the rolling hills and fertile topsoil that characterize the Palouse. Known as "hills of gold," they have made the region one of the most productive agricultural areas in the world.

12. Wallula Gap

Within hours after breaking through the ice dam in Idaho, the contents of Glacial Lake Missoula began to converge at Wallula Gap, an opening cut by the Columbia River through the Horse Heaven Hills near Pasco. The gap -- the only outlet to the sea for water draining from the Columbia Plateau -- is about two miles wide. Two miles is more than wide enough for normal flows, but not for an Ice Age flood. When the ice dam failed, some 530 cubic miles of water rushed toward a chute that could discharge no more than 40 cubic miles a day. The result was a bottleneck that has been compared to a dime-sized drain at the bottom of a very large hot tub: the plug may be pulled, but the water still takes a while to trickle out.

Water backed up for more than 2,000 square miles behind Wallula Gap, creating a temporary lake called Lake Lewis. The lake stretched west into the Yakima Valley, east into the Walla Walla Valley, and north into the Quincy Basin. The maximum depth was 1,200 feet, enough to turn several large hills into what seemed to be islands, poking out of the water like whales at rest. The present day site of the Tri-Cities was buried under more than 900 feet of water; the backwaters were 250 feet deep at Yakima and Walla Walla. It took nearly a week for all the water to drain and the ephemeral, intermittent lake to disappear.

Left behind were layers of fine-grained sediment that have proven to be ideal for the production of wine grapes. Ninety percent of the vineyards in Washington state are planted in areas that have been blanketed by sand and silt deposited during Ice Age floods. The most prolific and renowned regions are those that were once at the bottom of Lake Lewis.

Lake Lewis also provided important clues about the total number of floods. In 1926, an irrigation ditch went out of control and cut a deep ravine, known as Burlingame Canyon, near Walla Walla. Some 50 years later, geologist Richard Waitt noticed that the ravine had exposed sedimentary deposits called "rhythmites," composed of sand and silt laid down by successive flood pulses. Geologists have counted 40 separate layers here, leading to the conclusion that the lake filled and drained at least that many times.

13. Columbia River Gorge

Ice Age floodwaters accelerated in places where the flow was constricted and slowed where they could spread out, just like the stream from an ordinary garden hose when the nozzle is tightened or loosened. They boomed through the long, narrow chute of the Columbia Gorge at speeds approaching 80 miles an hour. The canyon walls, which had sloped gently down to the river before the floods, were planed to almost vertical surfaces. Many tributary streams on the south (Oregon) rim were left hanging, creating the waterfalls and cascades that thread that side of the Gorge today. Walls on the north (Washington) rim were over-steepened, making them vulnerable to landslides.

The result is a place of such surpassing beauty that Congress designated it the nation's first National Scenic Area, in 1986. With its flood-scoured walls and sleek waterfalls, the Gorge inspires visitors to reach for adjectives: awesome; spectacular; breathtaking.

The Gorge is the only passageway through the Cascade Mountains for water draining from the entire Columbia Basin. It is about 90 miles long (extending from Maryhill almost to Vancouver), as deep as 4,000 feet in places, and varies in width from one and a half to three miles. The Columbia River made the initial cut. The floods chiseled out the rest. Floodwaters as deep as 900 feet tore through the Gorge with ferocious power. They scrubbed soil from the lower canyon walls, sliced into the bedrock, and swept away huge piles of gravel.

The strong currents also ripped through the flanks of an extinct volcano on the Washington side of the river. The volcano had been composed largely of a kind of rock called andesite, which is high in silica and particularly vulnerable to erosion. "If there were a building code for mountains," says geologist David Alt, "andesite volcanoes would never pass inspection." Only the more resistant, 848-foot-high lava core of the volcano remained standing. The explorers Meriwether Lewis and William Clark gave it the name it has today: Beacon Rock.

14. Pacific Ocean

Echoes of the Ice Age floods can be imagined in the sound of the waves breaking on the beach at Cape Disappointment, like aural footprints lingering in the air. It's been calculated that the release of some 530 cubic miles of debris-laden water from an elevation of 4,200 feet in Montana down to the sea produced 4,500 megatons of energy: more than 10 times the energy produced by the 1980 eruption of Mount St. Helens. The combined energy of all the floods may have been double that of the impact of the comet that is believed to have wiped out the dinosaurs 65 million years ago. The floodwater -- still carrying much of its load of rocks, gravel, topsoil, vegetation, animal carcasses, and ice -- would have smashed into the sea on a shock wave of noise.

It took about a week for all the water from Glacial Lake Missoula to reach the sea. At several points along the route, the flow backed up to form temporary lakes behind hydraulic dams, created when water poured in faster upstream than it could drain downstream. The last squeeze came at the Kalama Narrows below the present city of Portland, where the Columbia River channel is a little less than two miles wide. About a third of the flood surge was diverted into the Willamette Valley. Icebergs bobbed along on the spillover, carrying boulders that would end up in fields as far south as Eugene. The ponded water dropped up to 100 feet of nutrient-rich Palouse sediment on the valley floor. "Think of all the barren bedrock in the scablands as you pass the lush fields and the flourishing groves of fruit and nut trees in the Willamette Valley," says geologist David Alt.

The main flood pulse slammed into the Pacific -- which was 300 feet lower than now, and about 50 miles west of the current coastline -- with enough violence to plow an underwater trench into the continental shelf. An estimated 600 million cubic yards of debris ended up on the ocean floor. For all the sediment and rocks that settled out of backwaters in the Quincy and Pasco Basins and in the Willamette Valley, the floods still hit the sea with most of their solid material intact.

Licensing: This essay is licensed under a Creative Commons license that encourages reproduction with attribution. Credit should be given to both and to the author, and sources must be included with any reproduction. Click the icon for more info. Please note that this Creative Commons license applies to text only, and not to images. For more information regarding individual photos or images, please contact the source noted in the image credit.
This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License
Major Support for Provided By: The State of Washington | Patsy Bullitt Collins | Paul G. Allen Family Foundation | Museum Of History & Industry | 4Culture (King County Lodging Tax Revenue) | City of Seattle | City of Bellevue | City of Tacoma | King County | The Peach Foundation | Microsoft Corporation, Other Public and Private Sponsors and Visitors Like You