PALEOHYDRAULIC RECONSTRUCTION OF THE LARGEST GLACIAL LAKE MISSOULA DRAINING(S)

      While many studies of the Channeled Scabland ascribed its origin to cataclysmic megaflood(s) from Glacial Lake Missoula (GLM), the GLM basin has attracted less interest.  GLM developed in the late Pleistocene on the Clark Fork drainage basin in northwestern Montana, USA.  Recent studies (1) clarify the various water levels and volumes for GLM, and (2) document previously unrecognized catastrophic lake-drainage gravel units lying beneath long-known glacial-lake silt/rhythmite units.  The gravel alluvium includes boulder-sized clasts, large-scale cross stratification, and 70-100 m-high bars forms, all of which indicate a high-energy flood environment. Previous work interpreted the glacial-lake silt/rhythmites and related deposits as evidence for numerous fillings and drainings of GLM.

    In this study, we reconstruct, using 2D hydraulic modelling (TUFLOW 2D), the paleoflow conditions (velocities, bed shear stresses and unit power values) that developed throughout the lake basin during the largest known draining(s) of GLM.  Values are calculated for 3 GLM drainage events (2.6; 13; 17 x 10⁶ m³ s^-1) that previous studies had estimated based on downstream geomorphologic evidence and/or glaciological considerations. Topographical data for our lake-basin model from the USGS 30 m DEM was transformed to a 1-km grid in order to expedite basin-wide computing time.  Our model results show that all three postulated GLM drainage hydrographs coincide rather well with known geomorphological and sedimentological features of Camas Prairie and Rainbow Lake Pass. However, the 2.6 x 10⁶ m³ s^-1 peak discharge was not capable of generating the high eddy deposits of Eddy Narrows.  Since these and other GLM high eddy deposits would be able to form with the two larger hydrographs used in our flow reconstructions, we conclude that the highest outflow(s) of GLM must have been > 13-17 x 10⁶ m³ s^-1.   These results have implications for the mode of GLM ice-dam failure(s).