FINALLY, IDENTIFICATION OF FLOOD DEPOSITS WITHIN GLACIAL LAKE COLUMBIA THAT ARE OF A MAGNITUDE WORTHY TO BE ATTRIBUTED TO GLACIAL LAKE MISSOULA OUTBURST MEGAFLOODS

Glacial Lake Columbia was the sediment trap for the debris-loaded waters emanating from numerous glacial Lake Missoula outburst floods. For almost four decades, Missoula floods were identified as sequences of rhythmites containing up to 89 individual flood event layers in the thickness range of a few centimeters in distal settings to several meters along the direct flood path. Recent geologic mapping revealed that a boulder-rich flood gravel sequence occupying the Rathdrum Prairie and Spokane Valley basin to a depth of two hundred meters transitioned into pebbly granule gravel distally. In addition, a two hundred-meter-thick, predominantly sand faces occurred above these gravels.

Could this 400 meter-thick gravel and sand sequence be deposited by single outburst flood early in the history of multiple breachings of the glacial Lake Missoula ice dam? After a decade of detailed geologic mapping and a systematic study of natural and artificial exposures of flood deposits within the upper reaches of glacial Lake Columbia, the answer is yes. This early, between 18 and 16 ka, and greatest Missoula megaflood event filled and elevated glacial Lake Columbia surface to an elevation of 800 m, breaching the southern shoreline in numerous areas, resulting in the Channeled Scablands. It was also apparent that this was dissected by numerous subsequence glacial Lake Missoula outburst floods, resulting in boulder gravel and sand strath terraces. Many of these strath deposits are 10m or more in thickness.

The rhythmite sequences previously attributed to glacial Lake Missoula outburst floods along the upper glacial Lake Columbia are here attributed to other sources. The classic flood sequence exposed along the Latah Creek drainage is much older, based on calcrete and ferricrete development within the pebbly sand flood event beds. The underreported rhythmite sequence exposed along Chamokane Creek and along the Spokane River below Little Falls Dam had its source area near Springdale, where a breached frontal moraine is suggestive of a moraine-dammed supraglacial and proglacial lake source for glacial meltwater floods. The provenance of clasts found within the rhythmite sequences at the confluence of the Spokane and Columbia rivers are more consistent with a source emanating down the flood gravel filled Enterprise Valley.