Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 19-12
Presentation Time: 12:00 PM


WAITT, Richard B., U.S. Geological Survey, Cascades Volcano Observatory, 1300 SE Cardinal Ct., Ste 100, Vancouver, WA 98683

Dozens of late Wisconsin Missoula-flood beds, each separated in places by varved beds, prove that each gigantic Missoula flood lays but one major graded bed. Skeptics for years billed evidence of repeated Missoula floods as reflecting only low-energy settings along low-level routes. Yet stratigraphy of high-energy gravel bars and backflood deposits shows that many great floods also swept down high scabland tracts like the Cheney-Palouse.

At least 25 separate gravel-bearing floods backwashed from the Cheney-Palouse into Willow Creek. Rhythmic beds in Tucannon valley record at least 25 giant backfloods that could only have routed down the Cheney-Palouse. Rhythmic beds record at least 21 separate giant backfloods far up Snake valley that could only come via the Cheney-Palouse. An intercalated 16 ka Mount St. Helens ash shows these floods to have been the same as backflooded lower-level Walla Walla and Yakima valleys.

At least 14 floods invaded high Priest valley. Pine Coulee exposes at least 12 flood beds near the upper limit of Missoula flooding into the Cheney-Palouse. Of at least 62 backfloods into lower Yakima and Walla Walla valleys, 15 or more require more than 500 km3 of water ponded hydraulically behind kilometer-wide Wallula Gap. At least 22 mighty floods swept from broad Portland basin over Lake Oswego trough into Tualatin valley. At least 40 débâcles from Portland basin backflooded tens of kilometers up Willamette valley.

Thick Purcell Trench ice dammed early Lakes Missoula that released its largest many floods once every several decades. During this period Okanogan ice to the west closed three huge floodways before reopening one. This changing geography shifted routings and discharges of successive floods down different distributaries.

New 2-D hydraulic models largely concur that successive Missoula débâcles under changing geographic conditions flood all high scabland tracts. Missoula megafloods under the different model settings sweep the Channeled Scabland and Columbia valley nearly up to the limits of field evidence. Channels blocked in succession by ice lobes made different channels experience their peak discharges during different floods. No one Missoula flood, no matter how immense, could inundate all floodways at once up to the limit of evidence.