Abstract
Late Pleistocene outburst megafloods, mostly from glacial Lake Missoula, hydraulically ponded behind downstream constrictions in the Columbia River in southeastern Washington State, USA. Optically stimulated luminescence (OSL) ages for flood deposits from the Coyote Canyon Mammoth Site, located in a high (315–320 m asl) distal portion of the transient lake, indicate that at least seven floods ponded high enough to inundate the area during the period 20.9 ± 2.6 ka to 16.3 ± 2.8 ka. This is consistent with a radiocarbon age of about 17.4 ± 0.2 ka cal BP from the middle of the flood sequence. OSL ages from loess deposits overlying a paleosol at the top of the flood sequence range from 14.0 ± 2.3 ka to 10.9 ± 2.0 ka, suggesting a hiatus of about 2.3 thousand years. These datasets are consistent with current understanding that multiple late Pleistocene megafloods occurred between 20 ka and 14 ka and that earlier floods produced higher flood stages than later ones. The lack of flood deposits in the Coyote Canyon area younger than 16 ka supports the hypothesis that younger megafloods did not hydraulically pond in the Pasco Basin above about 230 m asl.
Highlights
Floodwaters from Late Pleistocene outburst megafloods, mostly from glacial LakeMissoula, hydraulically ponded behind downstream constrictions, forming a large transient body of water in southeastern WashingtonState, USA
These floods led to the demise of numerous mammoths and other animals occupying the fertile valleys of southeastern Washington between flood episodes, sweeping their carcasses into the locally quiet waters of Lake
The younger age of these flood deposits compared to the 25.2 ± 0.2 ka cal BP age of the Camelops hesternus metatarsal supports interpretation that the metatarsal was eroded from older strata and redeposited by Missoula floods some 8000 years after its death [33]
Summary
Hydraulically ponded behind downstream constrictions (such as Wallula Gap), forming a large transient body of water (lasting only weeks) in southeastern Washington. These hydraulically ponded flood stages inundated much of the Pasco Basin and backflooded its tributary valleys including the Yakima and Walla Walla valleys This repeatedly formed a transient large body of water has been referred to as Lake Lewis, and sometimes confusedly referred to a “glacial Lake Lewis” (Figure 1). We present the stratigraphy associated with seven optically stimulated luminescence (OSL) ages for Missoula-flood rhythmites that entomb Columbian mammoth (Mammuthus columbi) remains and overlying loess deposits from the Coyote Canyon Mammoth Site (CCMS) and another nearby excavation These data provide key constraints on the timing of geologic events at CCMS, and expand our understanding of the dynamic nature and chronology of Lake Lewis flood stages [6].
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