GEOLOGICAL IMPLICATIONS FROM HYDRAULIC MODELING OF THE LARGEST MEGAFLOODING ON EARTH

The Channeled Scabland and various Columbia River pathways of the northwestern U.S., as impacted by the Late Pleistocene “Missoula Floods,” provide the best-studied examples of megaflood-generated landscapes. Combining a 1/3 arc-second DEM with the GeoClaw and HEC-RAS 2D hydraulic models, we analyzed the effects on megaflood routings of various boundary conditions dictated by current understanding of the region’s geological history. Our modeling results confirm the long-recognized issue of a mismatch between the regional high-water marks and the megaflooding discharge peaks associated with current understanding of the boundary conditions. We find that the minimum megaflood-generating water volumes required to match the geological evidence is about two times the maximum that can be achieved through cataclysmic outflow from Glacial Lake Missoula (GLM). Among the factors that might explain this discrepancy, we consider the addition of non-GLM megaflooding sources to be a reasonable working hypothesis. Nevertheless, regardless of the source issue, we find that simulated local flow strengths are basically comparable with previous work about Channeled Scabland megaflooding and with results obtained for many other megaflood landscape on Earth and Mars. These results are inconsistent with recent proposals that the causative megaflooding responsible such morphologies needs to be “downsized.” Our study also indicates that an iterative simulation modeling approach needs to be employed, involving both geological and hydraulic analyses, to properly understand the role of late Pleistocene megaflooding in creating the Channeled Scabland and similar megaflood landscapes.