HIGH-ENERGY MEGAFLOODS AND ICE SHEETS

High-energy megafloods are planetary-scale phenomena, associated with glacier outburst settings, glacial lake spillways, and the immense Martian outflow channels. Paleohydraulic analyses show that peak discharges for these paleofloods, like those of ocean currents, are measured in millions of cubic meters per second. Ice-marginal lakes sourced the largest, well-documented (and non-controversial) terrestrial glacial megafloods. The best-studied example is Glacial Lake Missoula, impounded by a lobe of the Cordilleran Ice Sheet that extended into northern Idaho. Other examples are associated with the Laurentide Ice Sheet of central and eastern Canada, whose retreat from its late Quaternary maximum extent resulted in immense melt-water lakes filling troughs that had been created by ice loading of Earth’s crust. Lowland glacial lakes also developed during the expansion and retreat of the huge Quaternary ice sheets that covered what are now shallow marine shelves north of Eurasia. The resulting blockage of north-flowing rivers, notably the Ob, Irtysh and the Yenesei, produced a network of megaflood spillways that ultimately delivered meltwater to the Mediterranean. These ice sheets also seem to have been associated with sub-glacial lakes, similar to those occurring today in Antarctica.

The most controversial hypotheses for terrestrial megaflooding involve subglacial flooding in explaining assemblages of landforms that include drumlins, Rogen moraines, large-scale bedrock erosional flutings and streamlining, gravel sheets in eskers, hummocky terrain, pendant bars, and tunnel channels (valleys). There are similarities between this assemblage and the anastomosing channel complexes eroded by Pleistocene megaflooding into the basalt bedrock and overlying sediments of the Columbia Basin region of eastern Washington state, U.S.A., where the cataclysmic flooding produced the erosional and depositional macro- and meso-form features of the Channeled Scabland.