35-10 New Constraints on the Timing of Glacial Lake Missoula Floods

Session: Ice sheets, glaciers, and landscapes, oh my! (Posters)

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Abstract:

Repeated formation and failure of an ice dam formed by the Purcell Trench Lobe of the Cordilleran Ice Sheet caused glacial Lake Missoula to form and drain more than 100 times. The catastrophic drainage of glacial Lake Missoula carved the Channeled Scabland of eastern Washington, and left behind an extraordinary record of rapid, ice-marginal landscape change. The floodwaters scoured bedrock and deposited iceberg-rafted erratics that were entrained in the ice dam and now serve as indicators of flood inundation. Despite the immense discharge, extreme bedrock erosion, and the intense debate regarding catastrophic versus uniformitarian views of landscape evolution that followed Bretz’s interpretation of the Channeled Scabland a century ago, we have only recently been able to precisely quantify the timing of flows in the various paths of the Missoula floods. Existing age constraints for the earliest Missoula flood(s) through the Columbia Valley, prior to blockage by the Okanogan Lobe of the Cordilleran Ice Sheet, are limited to three ¹⁰Be exposure ages from a single location on the Columbia River near Wenatchee, WA. To better constrain the timing of the first flood, we are analyzing 37 new ¹⁰Be exposure ages of materials collected from flood-scoured granitic bedrock and iceberg-rafted granitic erratics. The samples are from two key locations along the path of the initial floods: (1) near Spokane, Washington/Coeur d’Alene, Idaho just downstream from the former ice dam, and (2) along the Columbia River near Vantage, Washington where Sentinel Gap produced extensive back-flooding and left erratics perched at high elevations, which potentially record the timing of the largest floods. Resolving the timing and magnitude of the initial Missoula outburst floods strengthens our understanding of megaflood recurrence, ice-dam failure processes, and landscape evolution at the margins of Pleistocene ice sheets.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2, 2026

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