Glacial Lake Missoula

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Sediment deposited by the lake with a hammer for scale.

Lake Missoula was a prehistoric proglacial lake in western Montana that existed periodically at the end of the last ice age between 15,000 and 13,000 years ago. The lake measured about and contained about of water, half the volume of Lake Michigan.

The Glacial Lake Missoula National Natural Landmark is located about 110 kilometers (68 mi) northwest of Missoula, Montana, at the north end of the Camas Prairie Valley, just east of Montana Highway 382 and Macfarlane Ranch. It was designated as a National Natural Landmark in 1966 because it contains the great ripples (often measuring high and long) that served as a strong supporting element for J Harlen Bretz's contention that Washington State's Channeled Scablands were formed by repeated cataclysmic floods over only about 2,000 years, rather than through the millions of years of erosion that had been previously assumed.

The lake was the result of an ice dam on the Clark Fork caused by the southern encroachment of a finger of the Cordilleran ice sheet into the Idaho Panhandle (at the present-day location of Clark Fork, Idaho, at the east end of Lake Pend Oreille). The height of the ice dam typically approached , flooding the valleys of western Montana approximately eastward. It was the largest ice-dammed lake known to have occurred.

The periodic rupturing of the ice dam resulted in the Missoula Floods – a series of cataclysmic glacial lake outburst floods (GLOFs) that swept periodically across the area that would become Eastern Washington, Northern Idaho and Northern Oregon, and down the Columbia River Gorge approximately 40 times during a 2,000 year period at the end of the last ice age. The cumulative effect of the floods was to excavate of loess, sediment and basalt from the channeled scablands of eastern Washington and to transport it downstream. These floods are noteworthy for producing canyons and other large geologic features through cataclysms rather than through more typical gradual processes.

In addition, Middle and Early Pleistocene Missoula flood deposits have been documented to comprise parts of the glaciofluvial deposits, informally known as the Hanford formation that are found in parts of the Othello Channels, Columbia River Gorge, Channeled Scabland, Quincy Basin, Pasco Basin, and the Walla Walla Valley. The age of these deposits is demonstrated by the presence of multiple interglacial calcretes interbedded in these glaciofluvial deposits, sequences of sediments with normal and reverse magnetostratigraphy, optically stimulated luminescence dating, and unconformity truncated clastic dikes. Based upon these criteria, Quaternary geologists estimated that the oldest of the Pleistocene Missoula floods happened before 1.5 million years ago. The older Pleistocene glaciofluvial deposits within the Hanford formation are fragmentary in nature because they have been repeatedly eroded and largely removed by subsequent Missoula floods. Because of the fragmentary nature of older glaciofluvial deposits, the exact number of older Missoula floods, which are known as Ancient Cataclysmic Floods, that occurred during the Pleistocene cannot be estimated with any confidence. Although Lake Missoula likely was the source of many of the Ancient Cataclysmic Floods, the fragmentary nature of the older deposits within the Hanford formation makes precise determination of the precise origin of the floods that deposited them very difficult.

Geology

Ice dam on the Clark Fork River

The Cordilleran ice sheet originating in British Columbia expanded out of the mountains and southward. A tongue of ice pushed down the Purcell Valley or Purcell Trench, reaching south beyond Lake Pend Oreille. This 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 deep and extended for at least 10 miles; some say it extended as much as 30 miles. The ice dam reached east up the Clark's Fork to Cabinet, Montana, and southward around the mountain to Bayview, Idaho on the south tip of Lake Pend Oreille in Farragut State Park. Here, the ice sheet stood over and south of Lake Missoula.

Lake levels

The Clark Fork's drainage is a network of valleys among high mountain ranges. Lake Missoula formed through this region of western Montana. It is named for the city of Missoula in the upper reaches of the Clark Fork watershed. The mountains surrounding the city show the strandlines from the lake nearly 20,000 years ago. At its largest extent, Lake Missoula's depth exceeded and may have held of water, as much as Lake Erie and Lake Ontario combined. The surface area covered and the shoreline attained an elevation of .

The lake spread through the Clark Fork River basin, reaching east of Missoula, to Gold Creek; northeast up the Blackfoot River to Lake Alva; and east of Ovando . Two large lobes formed to the south and north. To the south the Bitterroot Valley filled as far as Sula, Montana, . To the north the Flathead River basin became an expansive body of water, creating an island of Red Sleep Mountain (in the CSKT Bison Range) and extending north to Polson at the basin of the Flathead Ice Lobe and up the Little Bitterroot River to Niarada some above the Flathead Rivers mouth at the Clarks Fork.

The water was deep (average – ): maximum – ), dark and murky with sediment. Fish fossils have not been found in deposits of Lake Missoula. Possibly, glacial sediment, rock flour, suspended in the turbid lake water which created an hostile aquatic habitat for fish. In addition, fossils of large mammals (megafauna), i.e.; mammoths, mastodons and bison which may have roamed nearby, not been found. Similarly, neither the remains or artifacts of contemporaneous humans have been found associated with Lake Missoula. The Clark Fork River flows into Lake Pend Oreille at .

Basins of Lake Missoula

Reaches of Glacial Lake Missoula

Reach	Riverway	Length	Max depth	Outlet	Features
Clark Fork Canyon	Clark Fork River	from the ice dam at Lake Pend Oreille to Ninemile		Blocked at Lake Pend Orielle in Idaho	Eddy Narrows, St. Regis Notch, Ninemile Rhythmites
Flathead River Basin	Flathead River	from the Clark Fork at Paradise to Polson, where the glacier stood	at Perma	Joins the Clark Fork River at Paradise	Ninepipes Pingo Scars, Paradise Center, CSKT Bison Range, Sloan Bridge Sediments
Little Bitterroot Valley	Little Bitterroot River	from the Flathead River to the glacial front near Niarada	at Camas Prairie	Joins the Clark Fork River near Ninepipe National Wildlife Refuge at Sloan Bridge and crossed the divide between the Clark Fork and the Little Bitterroot Rivers at Rainbow Lake	Gulch Fill, Rainbow "Dog" Lake, Markle Pass Kolk, Camas Prairie Ripples
Missoula Basin	Clark Fork River	from Ninemile to Lolo	at Ninemile	Enters the lower Clark Fork Canyon Reach at Ninemile	Glacial erratic and Strandlines
Bitterroot Valley	Bitterroot River	from Lolo in the Missoula Basin to near Sula	at Lolo	Enters Missoula Basin at Lolo	Features
Blackfoot River Valley	Blackfoot River	from Bonner to Rainy Lake on the Clearwater River and up the Blackfoot River to near Helmsville	at Potomac	Joins the Clark Fork River	Features
Upper Clark Fork	Clark Fork River	from the Bonner Flats to near Gold Creek on Interstate 90	at Bonner	Enters the Missoula Basin of the Clark Fork River west of Bonner	Gold Creek High Water Monument

Clark Fork Canyon

This reach follows Montana Route 200 up the Clark Fork River canyon, to Paradise, then follows the Clark Fork, then through the Paradise-St. Regis Canyon along Montana Highway 135. At St. Regis, the canyon opens out and continues to the east with the river paralleled by Interstate 90 to as far as Ninemile, where it opens out into the Missoula basin. A western branch of this basin runs up the St. Regis River another along with Interstate 90 to near Riverbend.

• Lookout Pass, asl along Interstate 90
• Thompson Falls – Located in the northern or western reach of the basin, the modern river passes through a layer of harder rock, forming a cascade.

  Nine Mile Rhythmites – Located at the eastern end of the basin, near Nine Mile. A light pink sand and silt deposited on the bottom of the lake. The silt deposits exist where the basin was wide, and when the lake drained, the area was not reached by the fast current of the water moving downstream. Each lay represents a period of still water behind the ice dam. The series reflects each period of still water with the intervening draining of the lake. As with "varves", darker layers are winter deposits, comprising fine particles in quiet water, and the lighter layers are coarser particles from the more active summer currents. Here, the number of layers represent 1,000 years of sediment.

Flathead Basin

The Flathead basin abutted the south face of the ice sheet. For most of this period, the glacial ice reached south to Polson, covering the entirety of Flathead Lake. The basin drains from the Polson Moraine at the south end of Flathead Lake, south to Ravalli, with a major lobe up the Little Bitterroot River and a minor basin on Camas Creek near Perma.

• Little Money Creek Gulch Fill – Exit 96, north on US 93 to Ravalli. The coarse materials filled the side gulches on the narrow valley as Lake Missoula drained; eddy currents in tributary gulches deposited debris.
• Rainbow "Dog" Lake – Drains into the Clark Fork near Plains. During the existence of Lake Missoula, it was a drain for the Little Bitterroot basin when the lake level exceeded asl and for the Camas Prairie basin when the lake level exceeded asl. At the maximum depth of Lake Missoula, the valley was a waterway. Rainbow Lake is thought to be a cataract retreat lake, formed by a waterfall. The Clark Fork River dropped near Plains, creating a current through Boyer Creek. A weaker layer of rock beneath a more resistant layer was removed, causing the lip of the falls to retreat backward. Evidence is provided by the debris that lines the valley bottom.
• Camas Prairie Mega ripples – Camas Prairie is a small basin on Camas Creek, north of Perma. At the maximum water levels of the Hwy 382 through the prairie to information sign at mile marker13. Multiple long ridges of sediment, height and apart. Average height between . Formed during the outflow of water during a break in the ice dam. The Camas Prairie Basin filled when Lake Missoula reached asl. As the water in the Lake Missoula Basin rose, this basin gained a second outlet through Rainbow Lake at asl; Willis Gulch asl; Markle Pass asl; and Big Gulch asl.
• Markle Pass Kolks – Montana Highway 382 travels through Markle Pass between Camas Prairie and the Little Bitterroot Valley. The Kolks were carved out of the bedrock by strong underwater vortices created as Lake Missoula quickly drained during the great floods. When the tornado-like currents reached the bottom of the waterway, rocks were pulled out of the bottom surface. This debris can be found downstream towards Camas Prairie and Burgess Lake.

Missoula Basin

The basin extends from Missoula, west to Ninemile and up the Ninemile Creek valley. This valley broadens from at Ninemile to at Missoula. The central part of this basin around Missoula is wide east–west and north-south. The basin is bordered by Rattlesnake Ridge on the north and Petty Mountain on the south(west). Features: strandlines along the valleys east flank.

• Glacial erratic on the grounds of the University of Montana
• Strandlines can be seen on the slopes of both Mount Jumbo ( asl) and Mount Sentinel ( asl), east in the Missoula Valley. These lines represent lake levels as the ice dam in the Purcell Trench failed and a lower lake level formed.
• Nine Mile Rhythmites on the bottom of the lake are visible.

Hamilton Basin

The basin extends from south of Conner to Lolo, to the north. The Bitterroot Mountains form the west shore and the Sapphire Mountains the east.

Blackfoot River Basin

The valleys of Potomac, Greenough, and Ovando-Helmville are linked by the Blackfoot River east of Missoula. A second reach, up the Clearwater River, joins the Blackfoot River at Clearwater. This basin joins the Clark Fork at Bonner. Upper valleys of the Clearwater-Blackfoot River basins run from Seeley Lake, eastward to Browns Lake along Montana Route 83 and Montana Route 200.

Upper Clark Fork

The Clark Fork of the Columbia River has its headwater near Butte, east of Missoula. Lake Missoula reached up the valley, about to the east along I-90 to just east of Gold Creek. Smaller reaches formed along the tributary valleys of Gold Creek, up Flint Creek, forming an basin, up Lower Willow Creek, and up Rock Creek.

See also

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  Cordilleran Ice Sheet

  maximum extent of Glacial Lake Missoula (eastern) and Glacial Lake Columbia (western)

  areas swept by Missoula and Columbia floods

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External links

• USGS Site on Glacial Lake Missoula
• US Park Service Site for Glacial Lake Missoula National Natural Landmark
• PBS's NOVA (TV series): Mystery of the Megaflood for information on the Missoula Floods
• The Seattle Times' Pacific NW magazine – "Trailing an Apocalypse" – 30-Sep-2007
• The Ice Age Floods Institute
• U of Montana publication, The Montanan, "Sedimental Journey: Following the Path of Glacial Lake Missoula's Flood Waters."