Pleistocene cataclysmic flooding along the Big Lost River, east central Idaho

Relationships between cataclysmic flood-generated landforms and flood hydraulics were investigated along Box Canyon, an 11 km long bedrock gorge of the lower Big Lost River. Geomorphic mapping along Box Canyon indicates that a cataclysmic flood completely inundated the gorge, resulting in large-scale erosional and depositional features on the adjacent basalt upland. Step-backwater hydraulic modeling indicates that a discharge of 60,000 m 3 s~ was required to produce the geologic paleostage evidence. Maximum stream power per unit area of bed locally attained values of 26,000 W m-2 during the peak, ranking the Big Lost River flood third, in terms of power, behind the famous Missoula and Bonneville floods. The spatial distribution of unit stream power indicates that bedrock erosion and boulder deposition on the basalt upland adjacent to Box Canyon were governed primarily by decreasing unit stream power and/or fluctuating unit stream power gradients. A preliminary depositional threshold for the largest flood boulders defines a lower limit of flood power required to sustain boulder transport along this bedrock fluvial system. Ultimately, hydrodynamic controls on Box Canyon flood erosion and deposition derive from the irregular volcanic rift topography of the eastern Snake River Plain. Outburst floods from a glacial lake in headwater regions during the late Pleistocene may have induced the torrential discharges within Box Canyon.