Geomorphic and sedimentologic impacts of the Big Lost River Floods, east-central Idaho, USA
Authors: Thackray Glenn , Warner Braedon 
Published: March 2022

&lt;p&gt;The Big Lost River Floods impacted the Basin and Range landscape of east-central Idaho during Late Pleistocene time, exerting geomorphic and sedimentologic effects preserved primarily in the flood source and sink areas.&amp;#160; The floods resulted from ice dam rupture in the Pioneer Mountains, traversed the wide tectonic basin of the Big Lost River valley, and terminated in a closed lacustrine basin on the Eastern Snake River Plain. We elucidate the history of multiple floods, their magnitudes, and their timing through surficial geologic mapping, cosmogenic radionuclide dating, and hydraulic modeling.&amp;#160;&lt;/p&gt;&lt;p&gt;The East Fork Big Lost River was dammed by the Wildhorse Canyon glacier at its maximum extent, forming Glacial Lake East Fork (GLEF). Flood-transported boulders extend ca. 20 km downvalley from the dam.&amp;#160; Distinct boulder bars and meso-scale cataracts cover several hundred hectares of basalt plains landscape in the Arco Scablands, 100 km downstream from the source, with isolated boulders from the source area.&amp;#160; Very little flood evidence has been identified in the intervening segment of the floodway.&amp;#160;&lt;/p&gt;&lt;p&gt;In the source area, ice damming occurred only during near-maximum ice extent, with GLEF volume and outburst flood discharge assumed to be correlative with dam thickness. Ages from new &lt;sup&gt;10&lt;/sup&gt;Be CRN and OSL dating reveal that GLEF was most recently dammed ca. 20.6 ka.&amp;#160; This age is similar to a published 3He chronology from Arco Scabland flood boulders. However, we have conducted additional dating in the Arco Scablands, and a second age mode of 35 ka is clear from the combined &lt;sup&gt;3&lt;/sup&gt;He datasets, suggesting extensive glaciation of the flood source area at that time. A closed-basin lake in the river-terminating basin further downstream has also yielded unpublished results from other workers, demonstrating correlative MIS 3 and 2 lake highstands.&lt;/p&gt;&lt;p&gt;HEC-RAS 2-D hydraulic modeling constrains likely flood discharges in the Arco Scablands. The results suggest MIS 3 flood discharge of ca. 30,000 m&lt;sup&gt;3&lt;/sup&gt;/s and MIS 2 flood discharge of ca. 10,000 m&lt;sup&gt;3&lt;/sup&gt;/s.&lt;/p&gt;&lt;p&gt;The concentration of apparent flood evidence likely reflects the variability of stream power along the floodway.&amp;#160; In the upstream reach, floodwaters were confined within a 1 km-wide valley, concentrating stream power. Erratic boulders mantle outwash terraces throughout this reach.&amp;#160; Downstream, the valley widens to 3-10 km; the wide valley would have dramatically reduced stream power and, thus, limited the capacity for geomorphic work. &amp;#160;Flood deposits in that reach were presumably either eroded or buried. In this context, it is surprising that flood evidence is dramatic in the Arco Scablands, which occupy low-relief, basalt-mantled Eastern Snake River Plain landscape.&amp;#160; Despite the overall low relief, two factors appear to have focused floodwaters into the Scablands.&amp;#160; First, simple topographic variability amongst individual basalt flows and monogenetic shield volcano slopes appear to have been sufficient to limit the floodway width and concentrate stream power, despite the general low relief.&amp;#160; Second, a ca. 1 km wide structural graben at the mouth of the Big Lost River valley appears to have focused the floodwaters into that low-relief floodway.&lt;/p&gt;