ONLY THE FACTS: PLEISTOCENE GLACIAL OUTBURST MEGAFLOOD OR FLASH FLOOD DEPOSITS, A DEDUCTIVE APPROACH

I propose that a new paradigm is needed to interpret the genesis of Pleistocene flood event beds within the confines of eastern glacial Lake Columbia. The research presented here is based on observations of each flood event bed and matching it with the most compatible hydraulic regime to reconstruct the type of flood event responsible. Thus, all the observations of the stacked sequences of flood event beds, i.e., lithologies, bedforms, channel orientations, and scale; dictate that these so-called “rhythmites” occurring within the confines of glacial Lake Columbia are the result of flash floods, not glacial outburst floods.

Exposures along the shoreline of FDR reservoir, from the confluence of the Spokane River to Hawke Creek, contain numerous sublacustrine hybrid flood event beds, separated by varved rhythmites These event beds are characterized by a basal clast supported angular basalt layer overlain by a pebbly sand within a depositional context that is only found in a flash flood induced debris flow deposits. Lithified Late Wisconsin flood deposits have not been reported. However, these deposits often are lithified.

Late Wisconsin subglacial outburst boulder laden hyperconcentrated density flows, now masquerading as strath terrace deposits, attest to multiple valley fill events decreasing in magnitude as the surface of glacial Lake Columbia decreased from just over 800m to 700m in the first outburst event ~18,000 cal yrs ago and by ~30m increments in each of the half dozen or so subsequent events. The Late Wisconsin outburst fill deposits of the Rathdrum Prairie consist of a subaqueous prograding clinoformal boulder gravel in the proximal area of the ice dam to a basal boulder gravel layer overlain by an angular basalt clast-rich sand, reaching an elevation of 800m, with subsequent outburst events preserved as remnant terraces. The ~100 pre-Wisconsin flood event beds preserved along the main flood course down the Spokane and Columbia rivers and their tributaries, can now be attributed to periodic flash flood intervals at the transition between glacial and interglacial stages. Fewer than a dozen glacial outburst breachings of glacial Lake Missoula occurred in the Late Pleistocene and the 60m thick laminated basalt clast sand deposit suggest a much longer flood flow period was necessary to drain glacial Lake Missoula.