2-D HYDRAULIC MODELING OF GLACIAL LAKE MISSOULA DRAININGS

Using full two-dimensional, depth-averaged hydraulic modeling (TUFLOW 2D), we calculated paleoflow conditions (velocities, bed shear stresses) that developed throughout the basin of Glacial Lake Missoula (GLM) during its largest known late Pleistocene drainage events. Values are calculated for previously estimated drainage events (2.6; 13; 17 x 106 m3 s-1) on a 1 km grid of the complex bathymetric basin. Consistent with regional mapping by Smith (2006, Quat. Res., v. 66, p. 311-322) and with the original interpretation by Pardee (1942, GSA Bull., v. 53, p. 1569-1600), the modeling shows that lake silt sequences in the basins and narrows areas of GLM must postdate the most highly energetic outburst events from the lake. This is because the flows generated in the lake by megaflood outflows are sufficiently energetic to erode any accumulated silt deposits. In contrast, the gravels underlying the silts include boulder-sized clasts, large-scale cross stratification, and 70-100 m-high bars forms, all of which indicate very high-energy flood-flow conditions. The modeling also shows that paleohydraulic conditions developed in the lake basin during the largest megaflood outflows would be capable of accounting for the observed bedrock scour at Rainbow Lake Pass, various “high-eddy deposits” (eddy bars of flood gravel) in narrows zones, and the “giant current ripples” (subaqueous gravel dunes) that were classically described by Pardee in his 1942 paper. Previous studies interpreted the glacial-lake silt/rhythmites and related deposits, such as the Ninemile Section, as evidence for numerous fillings and drainings of GLM. Our modeling suggests that these inferred events must have been relatively low in energy relative to the earlier outburst(s) that emplaced the flood gravels, giant current ripples, and scabland-like erosion surfaces. Late-lake erosive flow was along inner-valleys, mostly near the modern rivers.