DESCRIPTION:
Glacial Lake Missoula and the Missoula Floods

During the last Ice Age, a finger of the Cordilleran ice sheet crept southward into the Idaho Panhandle, blocking the Clark Fork River and creating Glacial Lake Missoula. As the waters rose behind this 2,000-foot ice dam, they flooded the valleys of western Montana. At its greatest extent, Glacial Lake Missoula stretched eastward a distance of some 200 miles, essentially creating an inland sea.

Periodically, the ice dam would fail. These failures were often catastrophic, resulting in a large flood of ice- and dirt-filled water that would rush down the Columbia River drainage, across northern Idaho and eastern and central Washington, through the Columbia River Gorge, back up into Oregon's Willamette Valley, and finally pour into the Pacific Ocean at the mouth of the Columbia River.

The glacial lake, at its maximum height and extent, contained more than 500 cubic miles of water. When Glacial Lake Missoula burst through the ice dam and exploded downstream, it did so at a rate 10 times the combined flow of all the rivers of the world. This towering mass of water and ice literally shook the ground as it thundered towards the Pacific Ocean, stripping away thick soils and cutting deep canyons in the underlying bedrock. With flood waters roaring across the landscape at speeds approaching 65 miles per hour, the lake would have drained in as little as 48 hours.

But the Cordilleran ice sheet continued moving south and blocking the Clark Fork River again and again, creating other Glacial Lake Missoulas. Over thousands of years, the lake filling, dam failure, and flooding were repeated dozens of times, leaving a lasting mark on the landscape of the Northwest. Many of the distinguishing features of the Ice Age Floods remain throughout the region today.

Together, these two interwoven stories of the catastrophic floods and the formation of Glacial Lake Missoula are referred to as the "Ice Age Floods."

During the Pleistocene Epoch Ice Age, beginning about 2.5 million years ago, virtually all of southwestern Canada was repeatedly glaciated by ice sheets that also covered much of Alaska, northern Washington, Idaho, Montana, and the rest of northern United States. In North America, the most recent glacial event is the Wisconsin glaciation, which began about 80,000 years ago and ended around 10,000 years ago. ...

At the end of the last Ice Age, a finger of the Cordilleran ice sheet crept southward into the Idaho panhandle, forming a large ice dam that blocked the mouth of the Clark Fork River, creating a massive lake 2000 feet deep and containing more than 500 cubic miles of water. Glacial Lake Missoula stretched eastward for some 200 miles and contained more water than Lake Erie and Lake Ontario combined. When the highest of these ice dams failed, lake water burst through, shooting out at a rate 10 times the combined flow of all the rivers of the world.

This towering mass of water and ice literally shook the ground as it thundered toward the Pacific Ocean, stripping away hundreds of feet of soil and cutting deep canyons -- "coulees" -- into the underlying bedrock. With flood speeds approaching 65 miles per hour, the lake would have drained in as little as 48 hours.

Over time the Cordilleran ice sheet continued moving south and blocked the Clark Fork River again and again, recreating Glacial Lake Missoula. Over approximately 2,500 years, the lake, ice dam and flooding sequence was repeated dozens of times, leaving a lasting mark on the landscape.

Today we can see how the floods impacted the landscape. They carved out more than 50 cubic miles of earth, piled mountains of gravel 30 stories high, created giant ripple marks the height of three-story buildings, and scattered 200-ton boulders from the Rockies to the Willamette Valley. Grand Coulee, Dry Falls, Palouse Falls -- all were created by these flood waters, as were the Missoula and Spokane ground-water resources, numerous wetlands and the fertile Willamette Valley and Quincy Basin.

In many ways, the story of the Floods is also the story of J Harlen Bretz (1882-1981), who proposed the theory that the Channeled Scablands of eastern Washington, and much of the Northwest as we know it today, were formed by catastrophic flooding. ... Bretz began his field research in the Channeled Scablands of central Washington during the summer of 1922, and it quickly became clear to him that neither glaciation nor ordinary stream erosion explained the Scablands. The following year Bretz made his two presentations to the Geological Society of America on the Scablands.

The first paper provided a detailed physiographic description of the Scablands; the second suggested that it would have taken a massive volume of water to create the degree of channel erosion that had occurred. Bretz's second paper on the Scablands also discussed the mounded gravel deposits that were scattered throughout the area. He proposed the idea of a catastrophic flood and included the first detailed geological map that included all of the Scablands and showed the extent of the floods. Bretz used the name "Spokane Flood" because he assumed the source of the water for this flood was somewhere near Spokane, Washington.

Bretz was confident that a flood had occurred, but was unable to figure out where the water had come from. Originally, he proposed that the water was the result of increased runoff from melting glaciers. But even Bretz had a tough time imagining any significant volume of water melting rapidly enough to have such devastating impact. Not until 1930 did Bretz consider Glacial Lake Missoula as the possible source of water he was searching for. But the geologic evidence was elusive, and he did not fully embrace the idea until 1956. Unable to provide a clear, scientific argument for the source of flood water, Bretz went on to other activities.

Glacial-outburst waters that crossed the Channeled Scablands during the Spokane floods were channeled through Wallula Gap. For several weeks, as much as 200 cubic miles of water per day were delivered to a gap that could discharge less than 40 cubic miles per day. Ponded water filled the Pasco Basin and the Yakima and Touchet valleys to form temporary Lake Lewis.

Lava Flows:

During late Miocene and early Pliocene times, one of the largest basaltic lava floods ever to appear on the earths surface engulfed about 63,000 square miles of the Pacific Northwest. Over a period of perhaps 10 to 15 million years lava flow after lava flow poured out, eventually accumulating to a thickness of more than 6,000 feet. As the molten rock came to the surface, the earths crust gradually sank into the space left by the rising lava. The subsidence of the crust produced a large, slightly depressed lava plain now known as the Columbia Basin (Plateau). The ancient Columbia River was forced into its present course by the northwesterly advancing lava. The lava, as it flowed over the area, first filled the stream valleys, forming dams that in turn caused impoundments or lakes. In these ancient lake beds are found fossil leaf impressions, petrified wood, fossil insects, and bones of vertebrate animals.

Folding of the Plateau:

With the end of the outpouring of lava, tremendous forces deep within the earth began to warp the plateau in several places. A general uplift of the mountainous region in the north caused the entire plateau to tilt slightly to the south. This tilting and associated stairstep rock folds, called monoclines, in the vicinity of Coulee City and Soap Lake, played an important role in the formation of the Grand Coulee.

The Ice Age:

With the beginning of the Pleistocene time about one million years ago, cooling temperatures provided conditions favorable for the creation of great sheets of moving ice called glaciers. Thus began the Ice Age. Over the centuries, as snowfall exceeded melting and evaporation, a great accumulation of snow covered part of the continent, forming extensive ice fields. This vast continental ice sheet reached a thickness of about 4,000 feet in some areas. Sufficient pressure on the ice caused it to flow outward as a glacier. The glacier moved south out of Canada, damming rivers and creating lakes in Washington, Idaho and Montana. One especially large lake, covering a portion of northwest Montana, played an important role in the formation of Dry Falls. As this lake grew in size, it eventually broke through the ice dam, allowing a tremendous volume of water to rush across northern Idaho and into eastern Washington. Such catastrophic floods raced across the southward-dipping plateau a number of times, etching the coulees which characterize this region, now known as the channeled scablands. As the floods in this vicinity raced southward, two major cascades formed along their course. The larger cataract was that of the Upper Coulee, where the river roared over an 800-foot waterfall. The eroding power of the water plucked pieces of basalt from the precipice, causing the falls to retreat 20 miles and self-destruct by cutting through to the Columbia River valley near what is now the Grand Coulee Dam. The other major cataract started near Soap Lake, where less resistant basalt layers gave way before the great erosive power of this tremendous torrent and waterfalls developed. As in the Upper Coulee, the raging river yanked chunks of rock from the face of the falls and the falls eventually retreated to their present location. Here then is Dry Falls, the skeleton of one of the greatest waterfalls in geologic history. It is three and one-half miles wide, with a drop of more than 400 feet. By way of comparison, Niagara, one mile wide with a drop of only 165 feet, would be dwarfed by Dry Falls.

MORE about the Columbia Plateau

Cordilleran Ice Sheet:

During the Fraser (Late Wisconsin) Glaciation, the Cordilleran ice sheet advanced southward from source areas in British Columbia and terminated in the United States between the Pacific Ocean and the Continental Divide. The ice sheet extended farthest along major south-trending valleys and lowlands that traverse the international boundary; it formed several composite lobes segregated by highlands and mountain ranges. Each lobe dammed sizable lakes that drained generally southward or westward along ice margins and across divides. ...

Glacial Lake Missoula and Jökulhlaups:

Glacial Lake Missoula, an immense water body dammed in the upper Clark Fork valley by the Purcell Trench lobe, was the source of floodwater that catastrophically swept across the Channeled Scabland (Bretz, et.al., 1956; see also Baker, 1982). Shorelines near and west of Missoula (Pardee, 1942) record successive stands of the lake as much as 300 meters below its highest level. The many sparsely developed shorelines argue that no one level records a lengthy stand of the lake; consistent with this relation is the absence of evidence for a lake spillway around the ice dam (Bretz, et.al., 1956).

Until recently there were thought to have been only a few Fraser-age fillings and emptyings of the lake (Pardee, 1942; Bretz et.al., 1956; Richmond, et.al., 1965; Bretz, 1969; Baker, 1973, 1978b), but Chambers (1971) and Waitt (1980a) inferred as many as 40 successive fillings and drainings. Lake Missoula bottom sediment is rhythmically bedded (Chambers, 1971; Curry, 1977, Waitt, 1980a). Thin-bedded silt at the base of a typical rhythmite passes upward to progressively thinner varves, the record of a gradually deepening lake. Complementary rhythmic slack-water sediment reveals evidence for about 40 separate catastrophic backfloodings of Columbia River tributaries in southern Washington and northwestern Oregon (Waitt, 1980a). These stratigraphic and sedimentologic relations indicate that glacial Lake Missoula was hydrostatically controlled by the thickness of the ice dam; the lakes periodically emptied as great jokulhlaups that discharged down the Columbia River valley and Channeled Scabland. Just south of Spokane, 16 beds of gravel deposited by floods that ascended Latah Creek valley are each capped by a bed of varved clay at least as high as altitude 610 meters, evidence that at least 16 of the jokulhlaups that swept into the Spokane area from the Rathdrum valley emptied into glacial Lake Columbia I (or Lake Spokane ?). In several places far north of Spokane, the intercalation of similar flood deposits with thin clay beds at least as high as altitude 690 meters indicates that some of the floods that swept through the highlands south of the Pend Oreille River valley also disgorged into glacial Lake Columbia I.

Relation of Glacial Lake Missoula to Alpine Glaciers

Alpine glaciers terminated in and near the east and south margins of glacial Lake Missoula. Lake sediment interfingers with Fraser-age alpine-glacial terminal moraines in the southern Bitterroot Range (Weber, 1971), and shorelines are cut into them. These relations indicate that the moraines were contemporaneous with high stands of glacial Lake Missoula and perhaps predated them. The maximum stand of the alpine glaciers therefore was roughly contemporaneous with the maximum stand of the Purcell Trench lobe. Lake Missoula shorelines etched across the sharp Fraser-age terminal moraines of alpine glaciers that flowed from mountains on the east side of the lake (Alden, 1953; 197-13) similarly indicate that the alpine-glacial maximum there occurred before or during the higher stands of the lake.

Purcell Trench Lobe:

As the Purcell Trench lobe retreated to north of the Bitterroot Range, glacial Lake Missoula was succeeded by glacial Lake Clark. During further retreat of the ice margin, Lake Clark merged to the north with the expanding glacial Lake Kootenay. Alden (1953) recognized no significant recessional moraines built during retreat of the lobe.

Glacial Lake Missoula:

Superposed rhythmites of individual floods in southern Washington and the absence of weathering between the rhythmites indicate that the huge jokulhlaups from glacial Lake Missoula occurred repeatedly during the entire episode of ponding (Waitt, 1980a). Because the lake was ponded near the terminus of the Purcell Trench lobe, the long interval of ponding implies that the lobe maintained its near-maximal position for millennia. As the ice dam eventually began to thin, Lake Missoula filled to successively lower levels after each discharge (Waitt, 1980a). During the glacial maximum and initial retreat, glacial Lake Missoula lapped against tongues of the West Kootenai and East Kootenai glaciers. The paucity of sand and silt in the upper Lake Missoula rhythmites suggests that these glaciers receded from the north margin of the lake while the Purcell Trench ice dam was thinning (Waitt, 1980a). As these glaciers retreated, the successively lowering Lake Missoula eventually became separated from the lake basins to the north by the emerging divide.

Varved clay and silt deposits of moraine-dammed glacial Priest Lake are regularly interrupted by 14 beds of sand having upvalley-directed paleocurrents, apparently the record of 14 large Lake Missoula jokulhlaups that backflooded into a long-lived glacial lake (R.B.Waitt, Jr., unpublished report). Varved lake deposits near Spokane are interpreted by many as 16 beds of bed-load flood gravel (E.P.Kiver and D.F.Stradling, personal commun., 1981; R.B.Waitt, Jr., unpublished report), evidence that at least 16 of the last Lake Missoula jokulhlaups discharged into the eastern end of glacial Lake Columbia I. As many as 11 of these floods in southern Washington postdated the 13,000 years B.P. (Mullineaux, et.al., 1978) Mount St. Helens set-S tephra (Waitt, 1980a, 1980b). Lake Missoula drained for the last time when the Purcell Trench lobe at last withdrew from the north end of the Bitterroot Range. The Clark Fork drainage remained ponded, as glacial Lake Clark by the retreating Pend Oreille River sublobe. Glacial Lake Clark drained with the combined Colville River sublobe and Columbia River lobe withdrew a few kilometers north of the international boundary and thus deglaciated the lower Pend Oreille River valley.

Glacial Lake Missoula:

The only outlet of the 2,500-cubic-kilometer 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 Grimsvotn, Iceland, Lake Missoula remained sealed as long as any segment of the glacial dam remained grounded; when the lake rose to a critical level around 600 meters 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 jokulhlaup were thus re-established scores of times during the 2,000- to 2,500-year episode of last-glacial damming. ...

Relation to Ice Sheet, Ash Layers, and Radiocarbon Dates:

Proof that only one graded backflood rhythmite is produced per Missoula flood permits revisions in the inferred age of the floods. Earlier speculations such as the notion of an "early Pinedale" flood 18,000 to 20,000 years ago (Richmond, et.al., 1965; Bretz, 1969, Baker, 1973), are now suspect because of limiting ages on the Cordilleran ice sheet, Lake Missoula, and the Missoula floods. Glacial Lake Missoula could have existed only within the broad limits between preglacial Carbon-14 dates as young as approximately 17,200 years B.P. and postglacial dates as old as 11,000 years B.P. in southernmost British Columbia (Clague, 1981). If the Purcell Trench lobe took a millennium to advance 100 kilometers from there to the Bitterroot Range and a millennium to retreat back into British Columbia, then Lake Missoula existed only between about 16,000 and 12,000 years B.P. The lake probably existed for only about half this period, for the known varves within Lake Missoula bottom sediment or between flood-laid beds in other glacial lakes sum up to less than 2,500 (Chamber, 1971; Atwater, 1983).

The age of the later Missoula floods into southern Washington is limited by the intercalated 13,000-years B.P. Mount St. Helens ash (Mullineaux and others, 1978), which overlies at least 28 flood rhythmites and underlies at least 11 (Waitt, 1980b). A new radiocarbon date from a shelly dune at the top of the third Missoula-flood rhythmite below the as bed at Mabton is 14,060 +/- 450 years B.P. (USGS-684). The 11,250 +/- 250-years Glacier Peak ash-layer G (Mehringer and others, 1984) postdates ice-sheet retreat in Washington and Montana (Waitt and Thorson, 1983); it overlies deposits of one huge flood but not those of small floods that followed deglaciation of the Columbia valley (Waitt, 1982a, 1982b). All considered, the various limits on the ice sheet and floods suggest that glacial Lake Missoula existed for 2,000 to 2,500 years between 15,300 and 12,700 years Before Present.

Number and Periodicity of Missoula Jökulhlaups:

From stratigraphic successions of approximately 40 rhythmic beds at exposures in Montana, Washington, and Oregon, Waitt (1980b) inferred that approximately 40 great jökulhlaups had escaped last glacial Lake Missoula. Only one of these sections has both base and top exposed. There may be unexposed beds beneath the other sections; still other sections are too high and distant from the main floodways to have received sediment from small floods. The number "40" is a minimum; there were at least that many huge floods during the last glaciation.

More than 50 beds are exposed near Cummings Bridge in the Walla Walla valley; Bjornstad (1980) counted 62 beds at Touchet. Atwater (1983, 1984) counted 80 or more beds, each of which he inferred as having been laid by a separate Missoula flood into glacial Lake Columbia. Although problems remain on the number and correlation of events attributed in various areas to successive Missoula floods, regionally scattered sections indicate that there were more than 40 colossal last-glacial floods, probably more than 60.

Along the entire flood route between Glacial Lake Missoula and the Pacific Ocean, the great floods spread out over wide areas and then funneled through constrictions. The narrowest constriction of all was near Crown Point in the Columbia River Gorge, where all 500 cubic miles of water were forced between tall cliffs only 1.5 miles apart. Like a jet of water exiting a nozzle, the flow accelerated from the narrow opening, probably reaching speeds of 80 miles an hour before slowing and spreading out over the Portland Basin.

Crown Point State Park at the west entrance to the Columbia Gorge stands at 700 feet above the river and is an interesting Floods landmark. It was inundated during the time of the peak floods.

Grand Coulee and Moses Coulee to the west largely were formed by the Missoula Floods. A lobe of the Cordilleran Ice Sheet descended into the Okanogan Valley, blocked the Columbia River, and covered 500 square miles of the Waterville Plateau west of Grand Coulee. The south terminus of the Okanogan lobe is clearly marked by an abrupt south limit of lumpy, rocky moraines. The ice-dammed Columbia River backed up to form Glacial Lake Columbia, a huge version of the lake now ponded by Grand Coulee Dam. Lake Columbia's overflow -- the diverted Columbia River -- occupied Grand Coulee between Ice Age Floods events.

Missoula flood deposits form a late Pleistocene capping of sand and silt as thick as 30 meters in some parts of Hood River valley.

Before the great floods, the Palouse River flowed down Washtucna Coulee to join the Columbia River in the Pasco Basin. The huge torrents of the Floods filled and overflowed Washtucna Coulee and swept across the divide down into the adjacent Snake River Valley. This enormous overflow carved back the divide in one place enough to capture the Palouse River, diverting it south to the Snake. The lower 10 miles of the Palouse now flow through a deep spillway. Its last descent is over a 200-foot cliff: Palouse Falls.

Created by the Lake Missoula floods, Palouse Falls is located in the Palouse Falls Natural Area, this 198-foot waterfall is spectacular to view, particularly in spring and early summer. ... Created by the Lake Missoula floods, Palouse Falls is the only major waterfall left along the glacial flood path of 15,000 years ago. ... The park was dedicated June 3, 1951. For many years the falls were called "Aputapat." Later, the name was changed to commemorate the Palouse Indian culture. According to a story of the Palouse tribe, the Palouse River once flowed smoothly into the Snake. But four giant brothers, in pursuit of a mythic creature called "Big Beaver," speared the great creature five times. Each time Big Beaver was wounded, he gouged the canyon walls, causing the river to bend and change. The fifth time he was speared, he fought the brothers valiantly and tore out a huge canyon. The river tumbled over a cliff at this point to become Palouse Falls. The jagged canyon walls show the deep marks of Big Beaver's claws.

The park is located in the area of the West known as "the Scablands." The terrain was formed by large lava flows, followed by huge floods (known as Missoula floods) and winds. Large sand dunes, coulees and lava flows can be visited near the park.

Separating the tandem canyons of upper and lower Grand Coulee is Dry Falls (now within the Sun Lakes - Dry Falls State Park). Dry Falls serves as a stark reminder of the magnitude of the Ice Age Floods. At the height of the Floods, water nearly 400 feet deep poured over the lip of the Falls, showing but a wrinkle on the floodwater's surface. Today, the 350-foot cliffs, plunge pools and lakes show that the Falls were over three miles wide or five times the width of Niagara Falls. At the height of the Floods, water poured not only over the Falls, but also over cataracts farther east.

Sun Lakes-Dry Falls State Park is a 4,027-acre camping park with 73,640 feet of freshwater shoreline at the foot of Dry Falls. Dry Falls is one of the great geological wonders of North America. Carved by ice-age floods that long ago disappeared, the former waterfall is now a stark cliff, 400 feet high and 3 1/2 miles wide. In its heyday, the waterfall was ten times the size of Niagara Falls. Today it overlooks a desert oasis filled with lakes and abundant wildlife.

Glacial-outburst waters that crossed the Channeled Scablands during the Spokane floods were channeled through Wallula Gap. For several weeks, as much as 200 cubic miles of water per day were delivered to a gap that could discharge less than 40 cubic miles per day. Ponded water filled the Pasco Basin and the Yakima and Touchet valleys to form temporary Lake Lewis.

What is a hydraulic dam? It is the restriction of the rate of water flow caused by a narrowed reach in a river valley. During a valley-filling flood, the narrows restrict flow, thus causing water upstream to pond partly and temporarily. The most spectacular example of a hydraulic dam during the Ice Age Floods was Wallula Gap, which restricted nearly 200 cubic miles of water in a huge, temporary pond in Pasco basin. On the lower Columbia, a narrows near Kalama also briefly ponded floodwater. This narrows thus helped to back up floodwater upstream, flooding not only the Portland-Vancouver basin but also the Willamette Valley to beyond Eugene.

What is a hydraulic dam? It is the restriction of the rate of water flow caused by a narrowed reach in a river valley. During a valley-filling flood, the narrows restrict flow, thus causing water upstream to pond partly and temporarily. The most spectacular example of a hydraulic dam during the Ice Age Floods was Wallula Gap, which restricted nearly 200 cubic miles of water in a huge, temporary pond in Pasco basin. On the lower Columbia, a narrows near Kalama also briefly ponded floodwater. This narrows thus helped to back up floodwater upstream, flooding not only the Portland-Vancouver basin but also the Willamette Valley to beyond Eugene.

Return to:
[Glaciers and Glaciations Menu] ...
[Glaciations, Ice Sheets, and Glacial Lakes Menu] ...