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 g orge of the lower Big Lost River. Geomorphic mapping along Box Canyon indicates that a cataclysmic flood completely inundated the gorge, res ulting in large-scale erosional and depositional features on the adjac ent basalt upland. Step-backwater hydraulic modeling indicates that a discharge of 60,000 m(3) s(-1) was required to produce the geologic pa leostage evidence. Maximum stream power per unit area of bed locally a ttained 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 m floods. The spatial distribution of unit stream power ind icates that bedrock erosion and boulder deposition on the basalt uplan d adjacent to Box Canyon were governed primarily by decreasing unit st ream power and/or fluctuating unit stream power gradients. A prelimina ry depositional threshold for the largest flood boulders defines a low er limit of flood power required to sustain boulder transport along th is bedrock fluvial system. Ultimately, hydrodynamic controls on Box Ca nyon flood erosion and deposition derive from the irregular volcanic r ift topography of the eastern Snake River Plain. Outburst floods from a glacial lake in headwater regions during the late Pleistocene may ha ve induced the torrential discharges within Box Canyon.