A LITTLE ICE AGE OUTBURST FLOOD AT MAMMOTH GLACIER, WIND RIVER RANGE, WYOMING

Multiple sediment cores from Pleistocene moraine dammed lakes (Upper and Lower Green River Lakes) in Wind River Range, Wyoming, preserve a detailed record of late Holocene glaciation, including an unusual flood event linked to glacier dynamics during the Little Ice Age. The primary source of glacier outwash to the lakes is from Mammoth Glacier, the largest (~1.9 km²) glacier on the western slope of the Wind River Range. As a result, these lakes trap the majority of the rock flour produced by the glacier, preserving a proxy record of fluctuations in size and activity of the glacier. Our cores include a long core (3.5 m) from Upper Green River Lake and numerous shorter, higher resolution cores from both lakes. Although we do not have numerical dates yet, the long cores likely span the last several thousand years.

Scott Lake is the only additional lake between Mammoth Glacier and the Green River lakes. It is a tarn that shows clear evidence (stranded beaches, delta, and oxidation staining) of a higher relict shoreline, ~10 m above modern lake level. The stranded delta in particular indicates this lake lowering occurred relatively rapidly, as a result of incision of a boulder-rich sediment dam. Preliminary analyses of organic content (OC) and magnetic susceptibility (MS) in the cores demonstrates that the most recent peak in rock flour flux to the lakes is also the largest in both lakes. MS and visual stratigraphy in cores from the Upper Green River Lake record a younger secondary spike that has a gradational lower boundary and a sharp upper boundary. Based on these observations, we interpret the main portion of the peak to be rock flour changes related to glacier growth and retreat during the Little Ice Age (LIA), whereas the smaller, younger spike appears to record the dam failure at Scott Lake. We speculate that this failure may have originated as an outburst at Mammoth Glacier related to retreat at the end of the LIA.

Ongoing analyses (e.g., radiometric dating) of the cores should help resolve the timing of these events as well as possible earlier Neoglacial advances.