Көне Миссула мұздық көлі

Prehistoric proglacial lake in Western Montana

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 300 feet (91 m) 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 – cataclysmic floods that swept across eastern Washington and down the Columbia River Gorge approximately 40 times during a 2,000 year period. 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.

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 2000 feet (610 m) 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 2000 feet (610 m) and 25 mile (40 km) south of Lake Missoula.

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 3620 feet (1103 m) asl and for the Camas Prairie basin when the lake level exceeded 3680 feet (1122 m) asl. At the maximum depth of Lake Missoula, the valley was a 520 feet (158 m) waterway. Rainbow Lake is thought to be a cataract retreat lake, formed by a 100 feet (30 m) waterfall. The Clark Fork River dropped 1700 feet (518 m) 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, 25 feet (8 m) height and 100 feet (30 m) 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 2770 feet (844 m) asl. As the water in the Lake Missoula Basin rose, this basin gained a second outlet through Rainbow Lake at 3588 feet (1094 m) asl; Willis Gulch 3349 feet (1021 m) asl; Markle Pass 3352 feet (1022 m) asl; and Big Gulch 3435 feet (1047 m) 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.

The basin extends from south of Conner to Lolo, 57 mile (92 km) to the north. The Bitterroot Mountains form the west shore and the Sapphire Mountains the east.

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 394 mile (634 km) from Seeley Lake, eastward to Browns Lake along Montana Route 83 and Montana Route 200.

The Clark Fork of the Columbia River has its headwater near Butte, 130 mile (209 km) east of Missoula. Lake Missoula reached up the valley, about 55 mile (89 km) to the east along I 90 to just east of Gold Creek. Smaller reaches formed along the tributary valleys of Gold Creek, 13.5 mile (22 km) up Flint Creek, forming an 8 mile (13 km) basin, up Lower Willow Creek, and 20 mile (32 km) up Rock Creek.