EVIDENCE FOR A SINGLE MASSIVE LATE-GLACIAL MEGAFLOOD DURNING FRASER GLACIATION ALONG THE NORTHEASTERN MARGIN OF THE CHANNELED SCABLAND

Glacial Lake Columbia surface waters stood at ~740 meters when a catastrophic failure of the Purcell Trench ice lobe resulted in the glacial Lake Missoula outburst megaflood waters, filling it with granule gravels containing a highly variable amount of larger clasts. Mount St. Helens set S tephra (13.4 ka ¹⁴C BP) was emitted during the waning portion of the Missoula flood and has been widely reported from the upper gravel layers just below the extensive giant current dune fields covering the West Plains near Spokane. There is no evidence for any other Fraser-aged floods in this area, but multiple pre-Fraser floods occur below an erosional disconformity locally.

The 200 m thick granule gravel with rounded vein quartz pebbles, likely derived from the deeply weathered Mio-Pliocene Excelsior quartz gravel, filled the central portion of the river basin beneath glacial Lake Columbia. The Deep Creek expansion bar, up to 30 m thick and covering over 70 sq. km, has a westward prograding sand front overlying an extensive granule gravel with a substantial clast content composed of cobbles and boulders of well-rounded basalt and saprolite clay balls. The provenance of the larger clasts in this distinctive gravel is local, derived from the immediate scouring of the regolith. The quartz pebbles common to the thick basin fill of the lake were mostly derived from the deeply weathered Excelsior quartz gravel and the disinterred basalt corestones and clay balls typifying the delta-form bar scoured from the basalt saprolite.

At the height of the flood, the southern shoreline of glacial Lake Columbia, which stood above 750 meters, was breached, eroding down the head of scabland tracts to an elevation between 700 and 715 meters. South-directed flood sediments at the head of the Cheney-Palouse Scabland tract scoured the southern portion of the just-created, west-prograding Deep Creek expansion bar. There is no indication that any floods prior to or after the one discussed above ever crossed the West Plains. All other individual floods are much smaller magnitude when compared to this one and should probably be explained by non-catastrophic ice dam collapses, such as the accelerated early interglacial storminess resulting in increased ice melt documented elsewhere in the northern hemisphere.