CHRONOLOGY AND HYDROLOGY OF THE BIG LOST RIVER OUTBURST FLOODS AND THE GHOSTS OF MIS 3 GLACIATION IN EAST-CENTRAL IDAHO

The Big Lost River floods exerted major geomorphic impacts on east-central Idaho and provide apparent evidence of extensive MIS 3 and MIS 2 glaciation at the Pioneer Mountains source area. We elucidate the history of the floods through geomorphic mapping, cosmogenic-luminescence geochronology, and hydraulic modeling.

The East Fork Big Lost River was dammed by the Wildhorse Canyon glacier at its maximum extent, forming Glacial Lake East Fork (GLEF). Erratic flood boulders stretch 20 km downvalley from the dam, and also appear 100 km downstream in boulder bars at the Arco Scablands on the eastern Snake River Plain. Because the Wildhorse-East Fork confluence hosts the most distal exposed terminal moraine sequence, ice damming likely occurred during maximum glaciation, with GLEF volume and outburst flood discharge correlative with dam thickness. Ages from new ¹⁰Be CRN and OSL dating in the dam area reveal that GLEF was most recently dammed ca. 20.6 ka, similar to a published ³He chronology of Arco Scabland flood boulders.

HEC-RAS hydraulic modeling and new ³He CRN dating from the Arco Scablands reveal both the longer-term flood chronology and likely flood discharges. ³He boulder ages from distal boulder bars, coupled with the published boulder chronology, indicate two primary boulder age modes, ca. 22 and 35 ka (MIS 2 and 3), with widespread MIS 3 ages, and MIS 2 ages restricted to limited areas near prominent cataracts. HEC-RAS 2-D flood modeling demonstrates the wide variability of flood inundation with discharge, and suggests MIS 3 flooding of ca. 30,000 m³/s and MIS 2 flooding of ca. 10,000 m³/s.

The inferred greater MIS 3 flood inundation strongly suggests glaciation of Wildhorse Canyon extensive enough to create a high ice dam. While the ¹⁰Be CRN and OSL glacial chronology documents only MIS 2 glaciation, geomorphically older moraines generally attributed to MIS 6 may instead reflect MIS 3 glaciation. Published alluvial fan, moraine, and lake chronologies similarly suggest that regional MIS 3 climate was damp and/or cold.