| The source, size, and chronology of Big Lost River floods in central Idaho have remained enigmatic despite previous mapping, modelling, and dating efforts. Twenty new 10Be cosmogenic exposure ages from the Pioneer Mountains confirm the timing of recent glaciation and the source of the most recent ca. 22 ka flood, Glacial Lake East Fork (GLEF). Six new 3He ages from distal flood boulders at the Arco scablands augment previously dated deposits at Bone and Lost Moon Cataracts near Box Canyon, producing a more complete chronology of flooding. Coupled with HEC-RAS flood extent modelling, results suggest at least one prior flood event ca. 35 ka of larger size and impact; previously recognized ca. 22 ka flood evidence at the Arco scablands is proximal to the Big Lost River and may have been produced by a flood as small as ~10,000 m3/s, while new ca. 35 ka ages from distal flood boulders require ~30,000 m3/s. This higher discharge estimate is also required to produce undated flood evidence at the northern Arco scabland highlands, thus is an estimate of maximum historical peak discharge and confirms the recurrence potential of GLEF. This approach reconciles many of the interpretive issues introduced from poor preservation potential within the geomorphically active Lost River corridor and informs best practices for studying similar paleofloods of modest scale. In addit ion, age data informs the first cosmogenic glacial chronology from the Pioneer Mountains, thoroughly described with a new surficial geologic map of tributary junctions and potential flood source ages. Key Words: Big Lost River Flood, Cosmogenic Dating, Glacial Chronology, Pioneer Mountains, Fall Creek Lake, Glacial Lake East Fork, Glacial Outburst Flood, Central Idaho, Idaho National Laboratory |