Pleistocene glacial outburst flooding along the Big Lost River, east-central Idaho

Abstract

Cataclysmic flood features including scabland topography, streamlined hills, a loess scarp, and flood transported boulders were mapped along Box Canyon, lower Big Lost River, eastern Snake River Plain. These features are similar to landforms within the Cheney-Palouse scabland tract, eastern Washington, formed by the great Missoula floods. Step-backwater hydraulic modeling of flow through the 11 km-long Box Canyon gorge indicates that a discharge of 60,000 m^3sec^-1 was required to produce the geologic paleostage evidence. Maximum stream power per unit area of bed locally attained values of 26,000 Wm^-2, which is comparable to the more extensive late Pleistocene Bonneville and Missoula flows. Flood power, estimated to exceed 1000 Wm^-2 , induced plucking of the jointed-basalt channel banks of Box Canyon. Tracts of scabland with networks of anastomosing channels migrated headward, driven by unit stream power values in the 600-1000 Win^-2 range. Deposition of the largest flood boulders occurred above a limiting unit stream power of 1400 Wm^-2. This ceiling on boulder deposition indicates that entrainment of these largest boulders probably took place under maximum unit stream power conditions (26,000 Wm^-2). The irregular volcanic rift topography along Box Canyon was the dominant control on removal and accumulation of flood boulders, however. Paleoflooding along Box Canyon may in part, although probably not solely, be attributed to outbursts from a glacial lake in the headwaters region located in the Pioneer Mountains.