GLACIAL MEGAFLOODS

Megafloods (terrestrial water flows with discharges exceeding one million cubic meters per second) are the largest known freshwater floods, with flows comparable in scale to (though of shorter duration than) ocean currents. Although there are no modern examples of megafloods, such flows occurred during major periods of glaciation. A prominent example is the paleoflooding caused by late Pleistocene outbursts from Glacial Lake Missoula, which formed when the Purcell Lobe of the Cordilleran Ice Sheet extended south from British Columbia to the basin of modern Pend Oreille Lake in northern Idaho. The ice thereby impounded the Clark Fork River drainage to the east, forming a lake extending into western Montana with a water volume of about 2500 cubic kilometers and a depth of 600 m at the dam. The largest Lake Missoula outbursts were in the range of 10 to 20 sverdrups (one sverdrup equals one million cubic meters per second) and involved flows that lasted for several days. The Missoula Floods were responsible for generating the Channeled Scabland of east-central Washington state -- a complex of anastomosing rock-cut fluvial channels, cataracts, loess “islands,” rock basins, broad gravel deposits, and immense gravel bars. These flows deeply inundated the Columbia Gorge and the Willamette Valley before discharging into the eastern Pacific Ocean. Other late-glacial megafloods occurred along the margins of the great ice sheets that formed during the Pleistocene in North America, Eurasia, and southern South America (Patagonia). River basins connected to these ice sheets were impacted by the flooding such that invasions of water changed drainage patterns, enlarged valleys, and delivered huge fluxes of water and sediment to the oceans. Numerous spillways were flood-integrated into temporary rivers in parts of North America (the St. Lawence, Mackenzie, Yukon, Mississippi, and Columbia Rivers) and Eurasia (the Ob, Irtysch, Yenesei, Volga, Dneiper Rivers). The late Pleistocene megafloods are associated with a broad range of hydrologic and climatic changes that are only now being fully understood.