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Lake Missoula features. Home Page Not much time? ... Click here for a quick Ice Age Floods summary Ice Age Floods Feature of the month For many years one man understood the clues but no one would listen Glacial Lake Missoula Channeled Scablands of Eastern Washington Temporary Lake Lewis Columbia Gorge Explore the variety of features created by the Ice Age Floods Columbia River Basalt Group The Pleistocene Grand Coulee Dam and the Columbia Basin Irrigation Project Washington Wines Ice Age Floods Institute

Glacial Lake Missoula

The glacial lake, at its maximum height and extent,
may have contained 500 - 600 cubic miles of water

Glacial Lake Missoula viewpoint.

Mission Valley view - Glacial Lake Missoula interpretive display at National Bison Range - Moiese, Montana. - Artwork by Byron Pickering -

Glacial Lake Missoula shorelines above University of Montana campus.

Lake Missoula strandlines (Strandline: "a shoreline above present water level" -Webster) are etched into the hillside behind the University of Montana's Main Hall. Geologists believe the site of present-day Missoula was under about 950 feet of water during the largest lake fillings.

Glacial Lake Missoula Video - Click arrow to play


T.C. Chamberlin explored Montana's Mission Valley in the 1880's.
T.C. Chamberlin
USGS Image

As far back as the 1880's, geologists believed that an immense body of water once occupied the deep mountain valleys of western Montana.

Lake Missoula expert David Alt (geologist/author) believes the first mention of Lake Missoula shorelines was recorded by geologist T.C. Chamberlin in 1886. During a late summer mapping trip through the Mission Valley, Chamberlin noted faint "watermarks" on the surrounding hills. Chamberlin had read reports describing Scotland's "Parallel Roads of Glen Roy" and correctly interpreted the Missiona Valley lake features.

It wasn’t until the 1960's that scientists began to accept the idea that catastrophic floods from Montana were responsible for radically altering the landscapes of eastern Washington.

- Please view at least the first 10 seconds of BBC video above -

Video shows aerial footage of strandlines at Glen Roy. An understanding of these ancient shorelines in Scotland helped Chamberlin realize that a huge body of water once filled the Mission Valley in western Montana.

"So while Chamberlin easily identified the old shorelines, they were tangential to his main duties. The summer was almost over, so he did not pursue the subject. It waited for J.T. Pardee." - David Alt

Glacial Lake Missoula strandlines above the Mission Valley.
Mission Valley strandlines noted by Chamberlin in 1886.

Parallel Roads of Glen Roy.
Parallel Roads of Glen Roy (Scotland)
1876 sketch by unknown artist


The story begins about 20,000 years ago—during the final stages of the Ice Age.

During tens of thousands of years of cooler and wetter climate in North America, huge ice sheets periodically spread southward and then gradually retreated. The final onslaught—known as the Wisconsin glaciation—brought masses of ice to the river valleys of northern Montana, Idaho and Washington. Meanwhile, alpine glaciers formed at the region’s higher elevations.

Glacial Lake Missoula Map showing path of the Ice Age Floods.

Glacial Lake Missoula impounded behind "Ice Dam". Another blockage occured to the west where the Okanogan Lobe plugged the Columbia River's course creating Glacial Lake Columbia (body of water with a 500 square mile surface area - at maximum fill). Glacial Lake Columbia strandline image lower on this page.

Bruce Bjornstad's map shows ice dam that blocked flow of Clark Fork River near the end of the most recent Ice Age.
Purcell Trench Ice Lobe (center) - Illustration by Bruce Bjornstad.

Glacial Lake Missoula shown southeast of ice dam. City of Spokane noted on map. Modern-day Lake Pend Oreille shown covered by ice lobe ("ice dam").

Ice Dam Blocks
Clark Fork River

Originating in British Columbia, one of these frigid tongues of ice lapped down the Purcell Valley [sometimes called the Purcell Trench] beyond the site of present-day Lake Pend Oreille.

The Purcell Lobe blocked the natural outlet of the Clark Fork River. Including its tributaries, Clark Fork represented western Montana’s most important river system. The ice mass that effectively dammed Clark Fork was about 2,000 feet high and extended for at least 10 miles—some people say as much as 30 miles.

Lake Pend Oreille.
Lake Pend Oreille
View east to mouth of Clark Fork River.
Montana's Glacial Lake Missoula.
"Lake Missoula Highest Level" - 4,200 feet.
The Clark Fork River flows into Lake Pend Oreille at 2,062 feet
Trapper Peak 10,157 ft. Bitterroot Range Montanta.
Trapper Peak

The Bitterroot Range formed much of Lake Missoula's western shoreline. The highest point in the range is Trapper Peak (10,157 feet).

View from Trapper Peak summit to Bitterroot Valley.

View from Trapper Peak summit into the Bitterroot Valley. This valley held the huge southern arm of Glacial Lake Missoula.

The Clark Fork’s drainage area includes a network of valleys hemmed in by high mountain ranges. Lake Missoula is named for the Montana city which occupies a central location in the Clark Fork watershed. Missoula’s nearby mountains also contain graphic evidence of the lake’s existence. Given the climate conditions of 20,000 years ago, precipitation and glacial meltwater from most of western Montana’s mountainous regions would have ended in Lake Missoula.

At its largest extent, Lake Missoula’s depth exceeded 2,000 feet and may have held 600 cubic miles of water—as much as Lake Erie and Lake Ontario combined. The surface area covered 3,000 square miles and the shoreline attained an elevation of 4,200 feet. In addition to the Clark Fork Valley northwest of Missoula, arms of the gigantic lake extended south through the Bitterroot; east to near Deer Lodge, Mont.; and north through the Flathead, Thompson, Mission and Clearwater valleys.

Ice Age Lake Missoula probably was a scenic locale, but it also would have been a somewhat forbidding place. The water was deep, dark and murky with sediment. There is no evidence of fish, and scientists speculate that sediment known as “rock flour” [because it was ground down to powder by glacial action] created poor habitat for both fish and the aquatic life-forms that would have nourished them. Alpine glaciers would have intruded upon Lake Missoula’s shores. Although mammoths, mastodons and bison likely roamed the nearby areas, there is no direct evidence of the presence of human beings.

This unfriendly environment for living things was made even worse by the fact that Lake Missoula emptied in dramatic fashion dozens of times over a several thousand years — perhaps as often as once every 50 years or so.


Pardee's clue Lake Missoula's Giant Current Ripples - Camas Prairie, Montana.

Giant Current Ripples - Camas Prairie, MT

Once geologist J. T. Pardee realized the small rolling hills or ridges on Camas Praire were actually giant current ripple marks - he determined they could only have been formed by powerful currents of fast-flowing water. The huge lake had emptied suddenly! Pardee’s new information was presented in 1940 and published in 1942.

Camas Prairie bluebird.
Camas Prairie bluebird

View Larger Map Navigate with Google Map tools to explore Giant Current Ripples on Camas Prairie.
Camas Prairie giant current ripples created when Glacial Lake Missoula suddenly drained.
Rancher's service road crosses giant current ripples on Camas Prairie.

Mission Valley deer.
- Western Montana doe with fawns. -

Click to view book details at
David Alt's book is an excellent resource for anyone interested in exploring Lake Missoula features.
Glacial Lake Missoula varves.
Glacial lake sediments with varves near Missoula, MT.

Geologist David Alt interprets light bands as river silts and dark bands as glacial lake sediments.

Glacial Lake Missoula gulch filling along Flathead River. Glacial Lake Missoula high eddy deposit.
"Gulch fillings" or "High eddy deposits" near Perma, Montana. Accumulated slackwater sediment off main channel.

Ice Age Floods Institute field trips are a great way to learn about Glacial Lake Missoula and the Ice Age Floods

Missoula Chapter field trip Ice Age Floods Institute.
Jim Shelden leads IAFI field trip near Missoula, MT.

Jim is President of the Glacial Lake Missoula Chapter of the Ice Age Floods Institute.

Click to view Missoula area: Hiking & Biking Trail Guide
Rainbow Lake, Montana wildflowers.
Ranbow Lake, MT wildflowers

Rainbow Lake, Montana -Lake Missoula Flood gravel bar.
Boulders near Rainbow Lake

Bedrock was ripped apart in some areas as the lake drained. Angular boulders did not travel far before settling out of flow.

Large bison grazing on the floor of Glacial Lake Missoula at the National Bison Range.

Bison grazes at National Bison Range near Moiese, Montana. Mission Range in the distance.

Bison wallow in Glacial Lake Missoula deposits.
Bison wallow in Glacial Lake Missoula deposits (glacial flour).

Glacial flour: (USGS Glossary)
  • Glacial flour is the fine-grained sediment carried by glacial rivers that results from the abrasion of rock at the glacier bed. Its presence turns lake water aqua blue or brown, depending on its parent rock type.
  • Rivers originating beneath glaciers are choked with glacial flour, the silty fine-grained sediment produced by the abrasion of rocks at the glacier bed.
Mission Valley dropstone - Glacial Lake Missoula.
Mission Valley Dropstone.

Glacial ice grinds rock into glacial flour (aka rock flour).
Glaciers grind against bedrock.

Glacial or rock flour carried by glacial meltwater.
Glacial meltwater carries rock flour into valley lakes and streams.


Scientists don’t completely agree on the precise sequences or nature of actions that led to the periodic failures of the Lake Missoula ice dams, but several general principles contributed to the event.

  • Ice has less density than water, and when the water gets deep enough the ice will try to float.

  • The weight of hundreds of cubic miles of water contained in a lake with up to a 2,000-feet depth will exert a great deal of hydraulic pressure upon the base of the dam.

  • All dams leak

Eddy Narrows

Joseph Pardee estimated the discharge rat of Glacial Lake Missoula at Eddy Narrows.

The pressure of water against the ice dam’s base, coupled with any dams tendency to suffer minor leakage, probably caused a weakening of both the dam’s base and the rock and other material immediately beneath it. Meanwhile, the dam itself was being tugged upward by its natural buoyancy, since ice is lighter than water.

At some point the dam was breached, probably through a combination of undercutting by water and multiple fractures of the ice mass itself. Whatever happened, it wasn’t a minor break. Enough ice was bashed out of the way in a very short period of time to make room for a flood torrent whose volume has been calculated at 8 to 10 cubic miles per hour—a rate that amounts to 10 times the combined flows of all the rivers on the planet Earth.

Eventually the failed ice dam would be replaced by a new one as the Purcell Valley glacial lobe continued its southward extension. This process was repeated until the glaciers finally retreated northward.


The cycle of floods that reshaped the landscape from the Idaho panhandle all the way to the Pacific Ocean represented a series of events—each one of them ranking as one of nature’s truly catastrophic occurrences. Many scientists believe that about 40 catastrophic floods originated at Lake Missoula. Others believe the number could be substantially higher.

There is evidence of flood episodes from other sources during the same period—glacial Lake Columbia, for example. There are indications of flood events at much earlier times during the Ice Age. Since subsequent glaciation and flooding would have obliterated evidence, we may never know for certain.

Glacial Lake Columbia strandlines near Grand Coulee Dam.
Glacial Lake Columbia strandlines high on hillside near Grand Coulee Dam.
Glacial Lake Columbia varves.
Glacial Lake Columbia varves

- An eyewitness to one of these events would have been terrified -

Depending upon the viewer’s vantage point, the oncoming torrent would appear as a huge wave from 300 to 1,000 feet high. Actually, it would look more like a cross between a wave and a mudslide because the flood would be carrying tons of earth, boulders, chunks of ice, trees and any other moveable debris that got in the way. Depending upon the location, the torrent could move at a speed ranging between 30 and 80 miles an hour. The noise would be deafening.


Outflows from Lake Missoula raced southward down the Purcell Valley and then made a right turn through Rathdrum Prairie and headed for the site of present-day Spokane, Wash. There it would have encountered the eastern extensions of another body of water—Lake Columbia. The Lake Missoula torrent would have filled Lake Columbia to overflowing, while at the same time it continued to race through the landscape of eastern Washington—carving flow channels as it traveled.

The initial Lake Missoula flood would have begun the process of reshaping the terrain of eastern Washington. Subsequent floods would have continued the process but generally would have followed the pathways established at the beginning. These floods, which are known as “scabland” floods because of the unique terrain features they created, were concentrated along three routes.

  • Grand Coulee floodwaters surged through that coulee into the open area known as the Quincy Basin, which is centered at the present-day cities of Quincy, Ephrata and Moses Lake.

    Lower Grand Coulee.

  • Telford/Crab Creek floods flowed southwesterly in a broad channel from the area between Davenport and Wilbur. They joined the Grand Coulee outflows in Quincy Basin. The combined flows move southward to the Pasco Basin, either by way of the Columbia River channel through Sentinel Gap or through the Othello region. The main flows generally bypassed the obstructing ridges of the Frenchman Hills and Saddle Mountains, but some waters flowed westerly through the valley of lower Crab Creek and joined the Columbia River near Sentinel Gap.

    Telford - Crab Creek Ice Age Floods tract pothole.

  • The Cheney/Palouse floods flowed southward into the western sections of the Palouse Hills region, eventually reaching the Pasco basin via the Palouse and Snake rivers, or by way of Washtucna Coulee.

    Streamlined Palouse Hills, Palouse tract - Ice Age Floods.

The tremendous volume of water collecting in the Pasco Basin overtaxed the capacity of the Columbia River’s channel through Wallula Gap to handle it. A temporary impoundment—known as Lake Lewis—filled to depths nearing 900 feet in the Pasco Basin for several days until this “retention pond” drained. The Columbia Gorge created another choke point, which resulted in Lake Condon, another temporary impoundment. A final series of obstacles west of Portland caused extensive backflooding into the Willamette Valley.


Along the floodways more than 50 cubic miles of earth and rock were removed and deposited downstream. The rich Palouse soils were scoured to depths as great as 250 feet, and prime farmland was transformed into scabland. Gravel bars, some of them 400 feet high, were created. Large boulders carried by ice rafts were deposited hundreds of miles from their origins—as far as Oregon’s central Willamette Valley. Much of the eroded material was carried all the way to the Pacific Ocean, where extensive deposits of flood sediment have been found hundreds of miles from the mouth of the Columbia River.

The most prominent visible remains of the late Ice Age Floods are the Channeled Scablands of eastern Washington.

The scabland region dominate a huge tract of the central Columbia Basin, although in the northeastern sections scablands are intermixed with higher hills on which the rich Palouse soils survived the floods and today are valuable cropland. In the region’s valleys the floods scoured everything down to the upper basalt layers—and even dislodged huge chunks of basalt. This led to further erosion from later floods. One result was the creation of numerous lakes, ranging from small ponds to larger impoundments such as Sprague Lake and Rock Lake. The scablands also featured numerous buttes, knobs, and other basalt projections. The most visually impressive scabland area is the Drumheller Channels, where floodwater spilled southward out of the Quincy Basin.

The Grand Coulee is a remarkable legacy of the Ice Age floods

The Grand Coulee created by the Ice Age Floods stores Columbia Basin Irrigation Project water.

And for "him who in the love of Nature holds communion with her visible forms" the majestic coulee tells a heroic tale of vanished power and glory far transcending that of Niagara and beggaring the leisurely story of the Yellowstone, the Yosemite, or even the Grand Canyon of the Colorado.

- J Harlen Bretz (1932)

Grand Coulee is the most spectacular of the valleys created by the floods, but 40-mile-long Moses Coulee also ranks as an impressive remnant, as do several smaller coulees which provided direct outflows to the Columbia River—Frenchman, Potholes and Crater coulees. Crab Creek Coulee, coupled with Coal Creek Coulee, extends from near Harrington to west of the town of Wilson Creek, and was the primary conduit for the Telford/Crab Creek floods. Cheney/Palouse flooding utilized numerous major coulees, including Lind, Hatton, Providence, Washtucna and Esquatzel. The coulee containing lower Crab Creek between the Saddle Mountains and Frenchman Hills was an important alternate route to the Columbia River for Quincy Basin floodwaters.

The Lake Missoula floods also diverted the Palouse River to its present confluence with the Snake River from a former course which led it to the Columbia River near Pasco. In doing so, the magnificent Palouse Falls and Palouse River Canyon were created.

Certain terrain features ultimately served as proof that catastrophic flooding had indeed been responsible for fashioning the region’s landscape. The floods created immense streambed ripples of sand and sediment at Camas Prairie [near Hot Springs, Mont.], and at West Bar on the Columbia River across from Crescent Bar, Wash. Huge deposits of flood sediment are visible at Lake Sacajawea Bar and Walker Bar on the Snake River. A well-defined pattern of strandlines on mountains at Missoula, Mont., establishes the shoreline levels of Lake Missoula at various times in its existence.

Among the debris scattered by the floodwaters are rocks—ranging in size from small stones to large boulders—whose composition identifies them as originating in western Montana or British Columbia. Known as “erratics”—because they are not native to the locales they were deposited in—these rocks could only have been transported by monster floods as part of ice rafts. The source of these floating ice chunks would have been the glacial lobe in Purcell Valley, the Lake Missoula ice dams, and alpine glaciers on the Lake Missoula shoreline.

David Alt describes many interesting Lake Missoula features in his book -Glacial Lake Missoula and its Humongous Floods.

Wikipedia Lake Missoula Page

USGS: Glacial Lake Missoula and the Missoula Floods Page

All photos by Tom Foster unless otherwise noted.

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