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Channeled Scabland and the Spokane Flood

The landscape of eastern Washington shows some unique landforms that already in the early 20th century fascinated geologists:

"No one with an eye for land forms can cross eastern Washington in daylight without encountering and being impressed by the "scabland." Like great scars marring the otherwise fair face to the plateau are these elongated tracts of bare, black rock carved into mazes of buttes and canyons. Everybody on the plateau knows scabland…[]…The popular name is a metaphor. The scablands are wounds only partially healed - great wounds in the epidermis of soil with which Nature protects the underlying rock…[]…The region is unique: let the observer take wings of the morning to the uttermost parts of the earth: he will nowhere find its likeness."
J Harlen Bretz 1928

American geologist J Harlen Bretz (1882-1981) proposed in 1919, in winter of 1922 and in various papers (here and here and here) in 1923 to 1925 a controversial hypothesis to explain the landscape - a flood episode of very great extant and amount:

"The volume of the invading waters much exceeds the capacity of the existing streamways. The valleys entered become river channels, they brim over into neighboring ones, and minor divides within the system are crossed in hundreds of places.
The topographic features produced during this episode are wholly river-bottom modifications of the invaded and overswept drainage network of hills and valleys. Hundreds of cataract ledges, of basins and canyons eroded into bed rock, of isolated buttes of the bed rock, of gravel bars piled high above the valley floors, and of island hills of the weaker overlying formations are left at the cessation of this episode…[]…Everywhere the record is of extraordinary vigorous sub-fluvial action. The physiographic expression of the region is without parallel; it is unique, this channelled scabland of the Columbia Plateau."
J Harlen Bretz 1928

The fluvial or glacial origin of the scablands was already clear from the observed features - deep incised gorges, steep cliffs or former waterfalls and cataracts, hanging valleys, eroded bedrock and widespread erratic boulders.
In 1838 Reverend Samuel Parker explained the scablands as the former river valleys of the Columbia River and in 1882 during a topographic survey Lieutenant T.W. Symons proposed that the river changed direction by a former ice-shield blocking its path. In alternative Thomas Condon in 1902 imagined a flood coming from the sea with the erratic boulders transported by drift ice.

Fig.1. Bristow, H.G. (1872) "The world before the deluge by Louis Figuier", showing the transport of boulders enclosed in drift ice.

Bretz in 1919 proposed a freshwater flood coming from the interior of the North American continent and following in part the path of the late Pleistocene Columbia River. However he couldn't explain the origin of the water - there were two possibilities, a rapid warming of the climate causing the melting of the Laurentian ice shield or a "jokulloup" caused by volcanic eruptions under the ice.

Fig.2. Hypothesized submergence map of the lower Columbia River system, after BRETZ 1919.

The proposed hypothesis arouse so much interest that on 12, January 1927 a meeting of the Geological Society of Washington with the title "Channeled Scabland and the Spokane Flood" was organized.
The main criticism in the following discussion concentrated on the problem of the estimated huge amount of water required (not explainable by the proposed mechanisms) and the short interval involved in the formation of the scablands (the various mapped spillways could have formed at various moments).
Research done and published some years previously by geologist Joseph T. Pardee (1871-1960) helped to solve this mystery. Pardee had mapped the outlines of a gigantic ice-dammed lake, comprising an area extending from today's north-western Washington to Idaho and Montana, which he named Lake Missoula (in fact there were various lakes dammed up by various ice lobes and named today Lake Missoula and Columbia/Spokane).
In 1933 the International Geological Congress field trip led to the Channeled Scablands, dividing the community in supporters and opponents of the flood-hypothesis.
In 1940 the American Association for the Advancement of Science met in Seattle, in the session "Quaternary Geology of the Pacific Northwest" most contributions were against the flood, but again Pardee proposed an interesting paper entitled "Ripple marks (?) in glacial Lake Missoula", where he described extraordinary large gravel ripples (15m high and with a wavelength of 150m) found in a basin of Montana. These ripple marks were a strong evidence supporting a catastrophic flood draining Lake Missoula, providing the necessary great amounts of water to explain the erosion of the scablands.

In the following decades Bretz continued to collect geologic information about the extant of the Spokane flood and finally in the decade of 1960 to 1970 the flood hypothesis convinced definitively the geologic community.


Bibliography:

BAKER, V.R. (2008): The Spokane Flood debates: historical background and philosophical perspective. Geological Society, London, Special Publications Vol. 301: 33-50

BAKER, V.R. (2009): The Channeled Scabland: A Retrospective. Annu. Rev. Earth Planet. Sci. Vol.37(6): 1-19
BRETZ, J. H. (1928): Channeled Scabland of eastern Washington. Geographical Review, 18: 446–477

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