maximum extent of Glacial Lake Missoula (eastern) and Glacial Lake Columbia (western)
areas swept by Missoula and Columbia floods
The Channeled Scablands are a relatively barren and soil-free region of interconnected relict and dry flood channels, coulees and cataracts eroded into Palouse loess and the typically flat-lying basalt flows that remain after cataclysmic floods within the southeastern part of Washington state.[1][2] The Channeled Scablands were scoured by more than 40 cataclysmic floods during the Last Glacial Maximum and innumerable older cataclysmic floods over the last two million years.[3][4][5] These floods were periodically unleashed whenever a large glacial lake broke through its ice dam and swept across eastern Washington and down the Columbia River Plateau during the Pleistoceneepoch. The last of the cataclysmic floods occurred between 18,200 and 14,000 years ago.[6]
GeologistJ Harlen Bretz defined "scablands" in a series of papers written in the 1920s as lowlands diversified by a multiplicity of irregular channels and rock basins eroded into basalt. Flood waters eroded the loess cover, creating large anastomizing channels that exposed bare basalt and creating butte-and-basin topography. The buttes range in height from 30 to 100m (98 to 328ft), while the rock basins range from 10m (33ft) in width up to the 11km (7mi) long and 30m deep Rock Lake. Bretz further stated, "The channels run uphill and downhill, they unite and they divide, they head on the back-slopes and cut through the summit; they could not be more erratically and impossibly designed."[7]
The debate on the origin of the Scablands that ensued for four decades became one of the great controversies in the history of earth science. The Scablands are also important to planetary scientists as perhaps the best terrestrial analog of the Martian outflow channels.[8][pageneeded]
History
Map of the Channeled Scablands
Bretz conducted research and published many papers during the 1920s describing the Channeled Scablands. His theories of how they were formed required short but immense floods–500 cubic miles (2,100km3)–for which Bretz had no explanation. His theories of apparent catastrophism met with vehement opposition from geologists of the day, who tried to explain the features with uniformitarian theories–that they were shaped by processes still acting today.
J. T. Pardee first suggested in 1925 to Bretz that the draining of a glacial lake could account for flows of the magnitude needed. Pardee continued his research over the next 30 years, collecting and analyzing evidence that eventually identified Lake Missoula as the source of the floods (now the Missoula floods) and creator of the Channeled Scablands.
Pardee's and Bretz's theories were accepted only after decades of painstaking work and fierce scientific debate. Research on open-channel hydraulics in the 1970s put Bretz's theories on solid scientific ground. In 1979 Bretz received the highest medal of the Geological Society of America, the Penrose Medal, to recognize that he had developed one of the great ideas in the earth sciences.
The term scabland refers to an area that has experienced fluvial erosion resulting in the loss of loess and other soils, leaving the land barren.[9]River valleys formed by erosional downcutting of rivers create V-shaped valleys, while glaciers carve out U-shaped valleys. The Channeled Scablands have a rectangular cross section, with flat plateaus and steep canyon sides, and are spread over immense areas of eastern Washington. The morphology of the scablands is butte-and-basin.[9] The area that encompasses the Scablands has been estimated between 1,500 and 2,000 square miles (3,900 and 5,200km2), though those estimates still may be too conservative.[10]
They exhibit a unique drainage pattern that appears to have an entrance in the northeast and an exit in the southwest. The Cordilleran ice sheet dammed up Glacial Lake Missoula at the Purcell Trench Lobe.[10] A series of floods occurring over the period of 18,000 to 13,000 years ago swept over the landscape when the ice dam broke. The eroded channels also show an anastomosing, or braided, appearance.
The presence of Middle and Early Pleistocene Missoula flood deposits have been documented within the Channeled Scabland as other parts of the Columbia Basin, e.g. the Othello Channels, Columbia River Gorge, Quincy Basin, Pasco Basin, and the Walla Walla Valley. Based on the presence of multiple interglacialcalcretes interbedded with glaciofluvial flood deposits, magnetostratigraphy, optically stimulated luminescence dating, and unconformity truncated clastic dikes, it has been estimated that the oldest of these megafloods flowed through the Channel Scablands sometime before 1.5 million years ago. Because of the fragmentary nature of older glaciofluvial deposits, which have been largely removed by subsequent Missoula floods, 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.[3][4] As many as 100 separate, cataclysmic Ice Age floods may have occurred during the last glaciation.[11] There have been at least 17 complete interglacial-glacial cycles since about 1.77 million years ago, and perhaps as many as 44 interglacial-glacial cycles since the beginning of the Pleistocene about 2.58 million years ago. Presuming a dozen (or more) floods were associated with each glaciation, the total number of cataclysmic Ice Age Missoula floods that flowed through the Channeled Scablands for the entire Pleistocene Epoch could possibly number in the hundreds, perhaps exceeding a thousand Ancient Cataclysmic Floods.[5]
There are also immense potholes and ripple marks, much larger than those found on ordinary rivers. When these features were first studied, no known theories could explain their origin. The giant current ripples are between 3 and 49 feet (1 and 15m) high and are regularly spaced, relatively uniform hills.[9] Vast volumes of flowing water would be required to produce ripple marks of this magnitude, as they are larger-scale versions of the ripple marks found on streambeds that are typically only centimeters high. Large potholes were formed by swirling vortexes of water called kolks scouring and plucking out the bedrock.[10]
The Scablands are littered with large boulders called glacial erratics that rafted on glaciers and were deposited by the glacial outburst flooding. The lithology of erratics usually does not match the rock type that surrounds it, as they are often carried very far from their origin.[10]
See also
Wallula Gap– Large water gap of the Columbia River through basalt anticlines in the U.S. state of Washington
Grand Coulee– Ancient river bed in the U.S. state of Washington
Moses Coulee– Canyon in the Waterville plateau region of Douglas County, Washington
Lake Lenore (Washington)– Lake formed by the Missoula Floods in the lower Coulee in Washington state
Glacial lake outburst flood– Type of outburst flood that occurs when the dam containing a glacial lake fails
Lake Lewis– lake in United States of AmericaPages displaying wikidata descriptions as a fallback
Notes
↑ Bjornstad, B.; Kiver, E. (2012). On the Trail of the Ice Age Floods: The Northern Reaches: A geological field guide to northern Idaho and the Channeled Scabland. Sandpoint, Idaho: Keokee Books. ISBN978-1879628397.
↑ Neuendorf, K.K.E., J.P. Mehl, Jr., and J.A. Jackson, eds. (2005) Glossary of Geology (5th ed.). Alexandria, Virginia, American Geological Institute. 779 pp. ISBN0-922152-76-4
1 2 Medley, E. (2012). Ancient Cataclysmic Floods in the Pacific Northwest: Ancestors to the Missoula Floods (Thesis). Portland, Oregon: Unpublished Masters thesis, Portland State University.
1 2 Spencer, P. K.; Jaffee, M. A. (2002). "Pre-Late Wisconsinan Glacial Outburst Floods in Southeastern Washington—The Indirect Record". Washington Geology. 30 (1/2): 9–16.
1 2 Bjornstad, B. (2002). Standardized Stratigraphic Nomenclature for Post-Ringold-Formation Sediments Within the Central Pasco Basin. Report DOE/RL-2002-39 Rev. 0. Richland, Washington: Pacific Northwest National Laboratory, US Department of Energy.
1 2 Baker, Victor (2010). Migoń, Piotr (ed.). Channeled Scablands: A Megaflood Landscape, in Geomorphological Landscapes of the World. Springer. pp.21–28. ISBN9789048130542.
↑ Carr, M. H. (2006), The Surface of Mars. Cambridge Planetary Science Series, Cambridge University Press.
↑ Waitt, RB, Jr. (1994) Scores of Gigantic, Successively Smaller Lake Missoula Floods Through Channeled Scabland and Columbia Valley, in Geologic Field Trips in the Pacific Northwest: 1994 Geological Society of America Meeting, Chapter 1K, D. A. Swanson and R. A. Haugerud (eds.), Geological Society of America, Boulder, Colorado.
Additional references
Allen, John Eliot; Burns, Marjorie; Sargent, Sam C. (1986). Cataclysms on the Columbia. Portland, Oregon: Timber Press. ISBN0-88192-215-3.
Bretz, J.; Smith, H. T.; Neff, G. E. (1956). "Channeled Scabland of Washington: New Data and Interpretations". Geological Society of America Bulletin. 67 (8): 957–1049. doi:10.1130/0016-7606(1956)67[957:CSOWND]2.0.CO;2.
Soennichsen, John (2008). Bretz's Flood: The Remarkable Story of a Rebel Geologist and the World's Greatest Flood. Seattle, Washington: Sasquatch Books. ISBN978-1-57061-505-4.
The Missoula floods were cataclysmic glacial lake outburst floods that swept periodically across eastern Washington and down the Columbia River Gorge at the end of the last ice age. These floods were the result of periodic sudden ruptures of the ice dam on the Clark Fork River that created Glacial Lake Missoula. After each ice dam rupture, the waters of the lake would rush down the Clark Fork and the Columbia River, flooding much of eastern Washington and the Willamette Valley in western Oregon. After the lake drained, the ice would reform, creating Glacial Lake Missoula again.
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 7,770 square kilometres (3,000 sq mi) and contained about 2,100 cubic kilometres (500 cu mi) of water, half the volume of Lake Michigan.
In geomorphology, an outburst flood—a type of megaflood—is a high-magnitude, low-frequency catastrophic flood involving the sudden release of a large quantity of water. During the last deglaciation, numerous glacial lake outburst floods were caused by the collapse of either ice sheets or glaciers that formed the dams of proglacial lakes. Examples of older outburst floods are known from the geological past of the Earth and inferred from geomorphological evidence on Mars. Landslides, lahars, and volcanic dams can also block rivers and create lakes, which trigger such floods when the rock or earthen barrier collapses or is eroded. Lakes also form behind glacial moraines or ice dams, which can collapse and create outburst floods.
Grand Coulee is an ancient river bed in the U.S. state of Washington. This National Natural Landmark stretches for about 60 miles (100 km) southwest from Grand Coulee Dam to Soap Lake, being bisected by Dry Falls into the Upper and Lower Grand Coulee.
J Harlen Bretz was an American geologist, best known for his research that led to the acceptance of the Missoula Floods and for his work on caves.
Dry Falls is a 3.5-mile-long (5.6 km) scalloped precipice with four major alcoves, in central Washington scablands. This cataract complex is on the opposite side of the Upper Grand Coulee from the Columbia River, and at the head of the Lower Grand Coulee, northern end of Lenore Canyon. According to the current geological model, catastrophic flooding channeled water at 65 miles per hour through the Upper Grand Coulee and over this 400-foot (120 m) rock face at the end of the last glaciation. It is estimated that the falls were five times the width of Niagara Falls, with ten times the flow of all the current rivers in the world combined.
Joseph Thomas Pardee was a U.S. geologist who worked for the U.S. Geological Survey, and contributed to the understanding of the origin of the Channeled Scablands. He discovered the trail of evidence left by Glacial Lake Missoula, a lake created by an ice dam 23 miles (37 km) wide and 0.5 miles (0.8 km) high during the most recent ice age. He discovered that when the dam broke, the water flowed towards the scablands, supporting J Harlen Bretz's theory of the cataclysmic floods.
The Touchet Formation or Touchet beds consist of well-bedded, coarse to fine sand and silt which overlays local bedrock composed of Neogene basalt of the Columbia River Basalt Group in south-central Washington and north-central Oregon. The beds consist of more than 40 to 62 distinct rhythmites – horizontal layers of sediment, each clearly demarcated from the layer below. These Touchet beds are often covered by windblown loess which were deposited later; the number of layers varies with location. The beds vary in thickness from 330 ft (100 m) at lower elevations where a number of layers can be found to a few extremely thin layers at the maximum elevation where they are observed.
Lake Lewis was a large transient lake in the Pacific Northwest region of North America, formed by periodic Missoula Floods along the Columbia River paleocourse between 21,000 and 16,000 years ago.
Drumheller Channels National Natural Landmark showcases the Drumheller Channels, which are the most significant example in the Columbia Plateau of basalt butte-and-basin Channeled Scablands. This National Natural Landmark is an extensively eroded landscape, located in south central Washington state characterized by hundreds of isolated, steep-sided hills (buttes) surrounded by a braided network of numerous channels, all but one of which are currently dry. It is a classic example of the tremendous erosive powers of extremely large floods such as those that reformed the Columbia Plateau volcanic terrain during the late Pleistocene glacial Missoula Floods.
Moses Coulee is a canyon in the Waterville plateau region of Douglas County, Washington. Moses Coulee is the second-largest and westernmost canyon of the Channeled Scablands, located about 30 kilometres (19 mi) to the west of the larger Grand Coulee. This water channel is now dry, but during glacial periods, large outburst floods with discharges greater than 600,000 m3/s (21,000,000 cu ft/s) carved the channel. While it's clear that glacial floodwaters passed through and contributed to the erosion of Moses Coulee, the age of those waters, thus the origins of the coulee are less clear. No clear connection between the head of the coulee and major flood routes to the north, east, or west is known. Some researchers propose that floods from glacial Lake Missoula formed Moses Coulee, while others suggest that subglacial floods from the Okanogan Lobe incised the canyon. The mouth of Moses Coulee discharges into the Columbia River.
Crab Creek is a stream in the U.S. state of Washington. Named for the presence of crayfish, it is one of the few perennial streams in the Columbia Basin of central Washington, flowing from the northeastern Columbia River Plateau, roughly 5 km (3.1 mi) east of Reardan, west-southwest to empty into the Columbia River near the small town of Beverly. Its course exhibits many examples of the erosive powers of extremely large glacial Missoula Floods of the late Pleistocene, which scoured the region. In addition, Crab Creek and its region have been transformed by the large-scale irrigation of the Bureau of Reclamation's Columbia Basin Project (CBP), which has raised water table levels, significantly extending the length of Crab Creek and created new lakes and streams.
The Saddle Mountains consists of an upfolded anticline ridge of basalt in Grant County of central Washington state. The ridge, reaching to 2,700 feet, terminates in the east south of Othello, Washington near the foot of the Drumheller Channels. It continues to the west where it is broken at Sentinel Gap before ending in the foothills of the Cascade Mountains.
Umtanum Ridge Water Gap is a geologic feature in Washington state in the United States. It includes the Yakima Canyon, and is located between the cities of Ellensburg and Yakima in central Washington. Washington State Route 821 was once the main route between Ellensburg and Yakima. The old highway still runs close to the river through the canyon, with Interstate 82 currently carrying most traffic between Ellensburg and Yakima on large bridges nearby.
The Columbia Plateau State Park Trail is a 130-mile-long (210 km), 20-foot-wide (6.1 m) corridor in eastern Washington state maintained as part of the Washington State Park system. The rail trail runs along the abandoned right-of-way of the former Spokane, Portland and Seattle Railway.
Boulder Park National Natural Landmark, of Douglas County, Washington, along with the nearby McNeil Canyon Haystack Rocks and Sims Corner Eskers and Kames natural landmarks, illustrate well-preserved examples of classic Pleistocene ice stagnation landforms that are found in Washington. These landforms include numerous glacial erratics and haystack rocks that occur near and on the Withrow Moraine, which is the terminal moraine of the Okanogan ice lobe.
The Ice Age Floods National Geologic Trail is a network of routes connecting natural sites and facilities that provide interpretation of the geological consequences of the Glacial Lake Missoula floods of the last glacial period that occurred about 18,000 to 15,000 years ago. It includes sites in Washington, Oregon, Idaho, and Montana. It was designated as the first National Geologic Trail in the United States in 2009.
Lake Condon was a temporary lake in the Pacific Northwest region of North America, formed periodically by the Missoula Floods from 15,000 to 13,000 BC. It was named after Oregon geologist Thomas Condon.
Giant current ripples, giant gravel bars, gravel dunes or GCRs are a form of subaqueous dune. They are active channel topographic forms up to 20 m high, which occur within near-thalweg areas of the main outflow routes created by glacial lake outburst floods. Giant current ripple marks are large scale analogues of small current ripples formed by sand in streams. Giant current ripple marks are important features associated with scablands. As a landscape component, they are found in several areas that were previously in the vicinity of large glacial lakes.
A pendant bar is a large, streamlined, fluvial bar that is typically composed of gravel which occurs just downstream of a bedrock obstruction within a river channel or floodway that has been scoured by either an outburst flood, megaflood, or jökulhlaup. They are often associated with giant current ripples. Malde introduced this to refer to streamlined mounds of gravel deposited by the Bonneville Flood that lie downstream of bedrock projections on the scoured valley floor of the Snake River. They are most common type of bar found within the Channeled Scablands created by the Missoula floods. The obstruction for the initiation of pendant bars in the Channeled Scablands is typically either a knob of basalt or the relict bend of a pre-flood meandering valley.
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