Paleohydrology and sedimentology of lake missoula flooding in Eastern Washington

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Abstract

The Missoula floods probably involved the largest discharges of fresh water that have been documented in the geologic record. The Columbia Plateau of eastern Washington contains evidence for flooding in pre-Bull Lake, Bull Lake, and early Pinedale time. Stratigraphic relationships between flood deposits and loess, relict soils on the flood gravel, and radiocarbon dates provide the means for the relative dating of these events. The early Pinedale flood was the most extensive and left considerable high-water mark evidence in the form of (1) eroded channel margins, (2) highest flood gravel, (3) minor divide crossings, and (4) ice-rafted erratics. The water-surface gradients and channel geometry provide input data for the slope-area and contracted-opening hydraulic calculation procedures. Maximum discharges through scabland channels ranged from 752 × 106 cubic feet per second in the Rathdrum Prairie to 17.5 × 106 cubic feet per second in Rocky Coulee. At these discharges, the ponding of flood water by the Wallula Gap constriction required about a day. The flood's duration was amazingly short. Even with gradually waning flows, the early Pinedale flood probably lasted only a week or two. The Greenacres reach, near Spokane, provides the closest approximation to an alluvial reach required for the Einstein bedload calculation. The predicted unit rate of sediment transport, 3,300 lbs/sec/ft-width, is probably too high. This may arise because the laboratory flume studies, on which the Einstein procedure is based, cannot be adequately extrapolated to the huge depth-to-grain-size ratios which characterized the scabland channel geometries. Flood sediments occur as channel deposits and slack-water deposits. The channel deposits include three main types: 1. Pendant bars-streamlined mounds of relatively well sorted fore-set-bedded flood gravel; often associated with giant current ripples. 2. Eddy bars-mounds of poorly sorted flood debris deposited by eddies which formed in alcoves and tributary mouths. 3. Expansion bars-fore-set-bedded gravels deposited by the decelerating flows just downstream from a constriction; often showing armoring and scour features. The slack-water deposits occur as a sequence of silts and sands in valleys tributary to the main scabland channels. Slack-water deposits in the Tucannon River Valley show primary sedimentary structures which suggest deposition by turbidity flows initiated by transient surges up the tributary valley. Sixty trains of giant current ripples are recognized in the study area. Mean ripple chords vary from 60 to 425 feet. Mean ripple heights vary from 1.5 to 22 feet. Ripple dimensions correlate to depth, depth-slope product, maximum size of armor, mean flow velocity, and stream power. Correlation coefficients generally exceed 0.7. Correlation coefficients greater than 0.9 occur for the regressions of ripple chord versus depth-slope product and ripple chord versus stream power. These empirical relationships only allow the prediction of hydraulic parameters for the narrow range of flow conditions which characterized the flood reaches containing the giant current ripples.

Original languageEnglish (US)
Pages (from-to)1-73
Number of pages73
JournalSpecial Paper of the Geological Society of America
Volume144
DOIs
StatePublished - 1973
Externally publishedYes

ASJC Scopus subject areas

  • Geology

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