Timing of Filling and Draining Events in Glacial Lake Missoula

Document Type


Publication Date

Spring 3-12-2020


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.

Streaming Media


Dr. Smith is a leading expert on Glacial Lake Missoula (GLM). To determine when and how often GLM filled and drained, he has obtained 17 new optical ages for basal sands at 7 sites, a number of dates on cobbles deposited during high-velocity drainage events, and about 10 other dates bracket lake-lowering episodes at one site. After explaining the concepts and processes of optical dating, he will present his results and interpretations.