Cataclysmic Scabland flooding: Insights from a simple depth-averaged numerical model

This work presents numerical assessments of possible routing paths for the reported late Pleistocene Missoula floods, which involved Earth's largest known peak discharges of freshwater. For our numerical model, we adopt the diffusion wave approximation of the shallow-water equation with the empirical Manning coefficient. This simplification is required because (1) most previous studies are one-dimensional or analytical studies based on simple equations, so that our two-dimensional expansion is a natural step towards more sophisticated models; (2) a simple model can easily incorporate the effect of complicated topography; and (3) lower computational costs permit a broader exploration of parameters. Important and new insights obtained from this work include: (1) numerically, we confirm the presence of the Cordilleran Ice Sheet at the time of the Missoula floods; (2) floods with relatively small peak discharges cannot inundate some major reaches, including the Cheney Palouse Scabland Tract; (3) deposits indicative of multiple floods are mostly confined in the locations that can be inundated by relatively lower peak flood discharges; and (4) although Lake Missoula glacial lake failure scenario can reasonably reproduce extensive water coverage, the best fit result to the geological evidence of flooding is obtained when the total water volume is three times larger than that estimated for Glacial Lake Missoula. The above results do not negate previous works, but indicate the need for further detailed field investigation and more sophisticated modeling in order to pursue further understanding of the complex behavior of megaflooding in the Channeled Scabland.