Timing of Filling and Draining Events in Glacial Lake Missoula
The basal sands range in age from 17.6±2.2ka near a late-glacial ice dam (630 m altitude) to 32.3±3.4ka (n=2) at 840 m. The average age of the basal sand at the highest altitude (1170 m) is 20.4±1.4ka (n=4). The lack of interlayering between the basal sands and overlying glaciolacustrine sediments at most locations suggests that these are maximum ages for the lake at different locations.
Ages of a basal sand which is interlayered with GLM deposits at 1170 m (n=4), and of subaerial surfaces within that section (n=10), show that the lake fluctuated around >65% of maximum capacity between 20.4±1.4ka and 13.44±1.3ka. These may have been the deepest stand(s) of the late-glacial lake. The deep-lake stands apparently occurred mostly early in its history, with one or more later.
Catastrophic flooding produced giant gravel bars along the Clark Fork River. The last time cobbles were exposed to sunlight comes from a preliminary age of 14.8±1.8ka for a giant ridge-topping dune near Huson and Ninemile, suggesting the lake drained from a high-pool level late in its history. However, preservation of silty glaciolacustrine sediments throughout the lake basin and all the way to ice dam positions shows that the “last” lake stand likely did not drain catastrophically.
Smith, Larry, "Timing of Filling and Draining Events in Glacial Lake Missoula" (2020). Public Lecture Series. 170.