CHRONOLOGY OF MISSOULA FLOOD DEPOSITS AND THE DEMISE OF THE COYOTE CANYON MAMMOTH, BENTON COUNTY, WASHINGTON

Late Pleistocene outburst megafloods, mostly from Glacial Lake Missoula, repeatedly formed a temporary lake, Lake Lewis, in southeastern Washington. Fine-grained graded beds (rhythmites) found in slackwater areas of the temporary lake provide evidence for repeated flooding. Ice-rafted erratics and spillover channels indicate that some of these floods exceeded an elevation of 380 m asl. However, the distribution of erratics, mammoth remains, and other debris left stranded on the shoreline of Lake Lewis suggest that most of the floods reached elevations of only 180 – 300 m asl.

The timing and relative flood stage of these temporary lakes remain poorly constrained. Here we report optically stimulated luminescence (OSL) ages for a sequence of at least seven flood beds found in a high (315 m asl) backwater portion of Lake Lewis, impacted by only the largest flood stages. The oldest flood deposits exposed at the Coyote Canyon Mammoth Site (CCMS) in Benton County, WA (46.1587°, -119.2648°) yielded an OSL age of 20.9 ± 2.6 ka, while the youngest yielded an OSL age of 16.8 ± 2.7 ka. OSL dates from a second section located at a similar elevation (~314 m asl) approximate 300 meters to the west-northwest (46.1592°, -119.2686°), yielded similar dates (17.4 ± 2.6 ka, 16.9 ± 3.2, and 16.3 ± 2.8 ka) for the Missoula flood deposits. OSL dating of loess deposits overlying the flood deposits at the mammoth site, yielded ages of 14.0 ± 2.3 ka and 10.9 ± 2.0 ka. Radiocarbon dating of the left humerus of a Columbian Mammoth (Mammuthus columbi) found within the middle of the sequence of flood deposits, yielded Accelerator Mass Spectrometry (AMS) conventional radiocarbon ages of about 14.4 ± 0.05 ka BP and 14.2 ± 0.06 ka BP with a pooled calibrated age of death of about 17.5 ± 0.2 ka cal BP.

These data suggest that at least seven Missoula floods occurring between 20.9 ka and 16.3 ka created a high-stand of Lake Lewis that exceeded 315 m asl and impacted the Coyote Canyon Mammoth Site. The relationship of flood rhythmites with the concentration of mammoth bones indicates that a flood at 17.5 ka led to the demise of the Coyote Canyon Mammoth.