USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
About Forty Last-Glacial Lake Missoula Jökulhlaups Through Southern Washington
-- Richard B. Waitt, Jr., 1980,
About Forty Last-Glacial Lake Missoula Jökulhlaups Through Southern Washington:
IN: Journal of Geology, v.88, p.653-679
Abstract
The rhythmic Touchet Beds in the Walla Walla and lower Yakima valleys resulted
from many separate backfloodings by hydraulically ponded glacial Lake Missoula
water. At least once this episodic lake briefly contained half the 2,130 cubic
kilometers of water that catastrophically drained the largest glacial Lakes
Missoula. Evidence that the Touchet Beds rhythmites originated from brief
backfloodings includes upvalley thinning and fining of locally derived bedload,
upvalley paleocurrents, and upvalley transport of erratics derived from
Cordilleran ice. Evidence that a lengthy nonflood environment followed the
emplacement of each of about 40 Touchet Beds rhythmite includes inferred eolian
and slopewash sediment overlying many rhythmites, uncontaminated Mount St.
Helens "set S" tephra couplet atop on rhythmite as much as 220 meters below the
maximum level of backflooding, filled semiconsolidated rodent burrows throughout
the 30 meters of the thickest section, and dispersed skeletons of mammals. The
lack of weathering or soil within the Touchet Beds suggests that all rhythmites
are late Wisconsin. Bottom sediment of glacial Lake Missoula in Montana
consists of rhythmites each interpreted as the record of a gradually deepening
lake. Forty superposed rhythmites record about 40 late-Wisconsin fillings and
emptyings of glacial Lake Missoula. The complementary records of about 40
separate glacial Lakes Missoula and about 40 great floods in southern Washington
and in the Willamette Valley, Oregon indicate that the Missoula floods were
great jokulhlaups. The last several floods were smaller than earlier ones
because the controlling dam of Cordilleran ice thinned during deglaciation.
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04/16/01, Lyn Topinka