Glacial Lake Missoula

LAKE MISSOULA AND ITS CATASTROPHIC FLOODS

As far back as the 1880s geologists believed that an immense body of water once occupied the deep mountain valleys of western Montana. But it wasn’t until the 1960s that scientists began to accept the idea that catastrophic floods from that source were responsible for radical alterations to the landscape of eastern Washington and to the Columbia River’s channel all the way to the Pacific Ocean.

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.

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 Orielle. In doing so it 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 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 held over 500 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 4,300 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 local, 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 a 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 several dozen times over a 2,000 year period—perhaps as often as once every 50 years or so.

THE ICE DAM GIVES WAY

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 500 or more cubic miles of water contained in a lake of 2,000-feet depth will exert a great deal of hydraulic pressure upon the base of the dam.



• All dams leak

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.

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

LAKE MISSOULA’S CATASTROPHIC FLOODS

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 over 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.

THE FLOODS’ PATHWAYS

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.



• 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.


• 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.

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 the 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.

THE FLOODS’ IMPACT

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 scablands and coulees 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.

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 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 horizontal lines 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.